PH SPH RU FIX I IN I DIAN S ILS

A REVIEW

J.S.KANWAR International Crops Research Institute for the Semi· Arid Tropics, Hyderabad

and J.S.GREWAL

Central Potato Research Institute Shimla

¥ ~3r..!,q leAR

PUBLISHED BY

PUBLICATIONS AND_INFORMATIONDIVISION INDlAN COUNCn.. OF AGRICULTURAL RESEARCH

kRrsHI ANUSANDHAN BHA VAN, PUSA, NEW DELHI 110012

FIRST PRINTED 1971

REVISED EDITION OcrOBER 1990

Chief Editor Editor

Assistant Editor

Chief Production Officer Production Associate

Chief Artist Senior Artist

S.N. TATA

Dr (Mrs) A.M. W ADHWANI

SATYA PAL

S.G. Prasad V.K. Bahl

M.K. BARDHAN

B.C. MAzuMDER

All Rights Reserved ©1990. by th~ lndi&n Council of Agricullural Research, New Delhi

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Laser typeset at Computics. 339 Somdatt Chambers II, 9 Bhikaji Cama Place, New Delbi-ll0066 Printed at Power Printers. 2l8-A. Ansari Road, Daryaganj, New Delhi-llOOO2 and Published by G.S .. Agrawal Under-Secretary, for the Publications and Information Division, Indian Council of Agricultural Research. Krishi Anusandhan Bhavan, Pusa, New Delhi-llOO12.

PREFACE

Phosphorus is one of the major essential plant nutrients and without its adequate supply the plant can neither reach its yield potential nor it can complete a normal reproductive process. P deficiency in Indian soils is very serious and is assuming alarming proportions in many situations, particularly, with the use of high-yielding varieties under intensive agriculture and specialized cropping system with heavy use of nitrogenous fertilizers. The use of P fertilizers is increasing at a very rapid rate. According to F AI 1987 estimates the average compound rate of growth of P fertilizers per annum during the period 1978-87 was 8.4%. It is expected that the consumption of P fertilizers (Pps) in India will increase from 2.1 million tonnes in 1986-87 to 2.5 million tonnes in 1990 and to 4.4 million tonnes by 2000. We can appreciated the problem ofP fixation in soils when we consider this along with the fact that a large amount of raw material for the manufacture of P fertilizer in India is imported and the percentageofutilizalion of the P fertilizer by the crop is very low, ranging from 10·20%. The phenomenon of fixation of added P in soils considerably affects the availability and efficiency of applied P. Thus, it is of paramount importance to understand this phenomenon and to develop the practical techniques of making the best use of fixed P or reduce its fixation so as to ensure greater

_availabililY ofP to the crop. This revised edition brings the information about P fixation in Indian soils up-to·

date. We hope, that this publication will be of considerable use.

1.S. KANWAR l.S.GREWAL

CONTENTS

Page

Introduction

1. Phosphorus Status of Indian Soils 3

2. Phosphorus Fixation Mechanism and Transformations in Soils 12

3. Factors affecting Phosphorus Fixation in Soils 19

4. Phosphorus Fixation Capacity of Indian Soils 29

5. Availability afFixed Soil Phosphorus and Methods to Determine 33 Available Soil Phe>sphorus

6. Methods to Cheek Phosphdrus Fixation in Soils and Practical 39 Significance of Phosphorus Fixation in Soils

7. References 50

INTRODUCTION

THE P fixation is a process whereby the readily soluble forms ofP are changed to sparingly soluble forms by reacting with inorganic and organic component') of soils or by other means thus restricting the availability of P to the plants. Most of the researchers have used the terms, precipitation,retention' and a.dsorption interchangeably for describing P fixation. In the real sense, these denote different reactions. The precipitation involves the chemical reaction of cations in solution with LheP. TheretenLionrefers to Lhatportion of P which is loosely held by the soil apd which can be extracted with dilute acids. This form ofP is considered to be largely available to the plants. The adsorption refers to that P which is adsorbed by the colloid complex of the soil. The P is also fixed by micro­organisms in soil but this type of fixation accounts only for a small portion of the fixed P and is considered not of much practical significance. Thus the fixed P refers to that portion ofP which is not easily extractable with dilute acids and is not readily available to the plants.

Way (1850) was one of the pioneers who observed thatP could be adsorbed by the soils. In India. Harrison and Das (1921) were among the first to observe that alkaline earths were extremely active for the retention of P in the. calcareous soils. The real pace in this direction was achieved only after 1940 when Raychaudhuri and Mukerjee (1941) reponed that laterite soils fixed considerable amounts ofP. Patel and Viswanath (1946) showed that black soils had high fixation capacity. Kanwar and Grewal (1960) found that alluvial soils of erstwhile Punjab also fixed appreciable quantities ofP. Mukerjee et al.(1946) and Chatterjee and Datta (1951) studiedP fixation in clay minerals. Later many studies were carried to understand the P transformation in the different soils and the relative availability of different forms of P to plants. De (1963) reviewed the India's contribution to the study of P fixation by soils, clay minerals, hydrous oxides and lime. Later MandaI (1975) reviewed the information on P management in Indian soils. The phenomenon ofP fixation is of world-wide imponance and is being studied extensively throughout the world.

Because of economic considerations and scarcity of the fertilizers, the studies relating to the efficient use of fertilizers and improving the availability of native and fertilizer P have drawn the attention of soil scientists and agronomists. The efficient and economic use of P fertilizers is possible only if the r~ctions which the fertilizers undergo in different soils (mechanism ofP fixation) are well-understood. P fixation leads to the low recovery of the P applied to the crops. Nearly 80% of the applied P remain un utilized by the current crop. The availability of the unutilized P is further governed by their reaction products in a particular soil. Many studies are being carried on to explore the possibility of use of the comparatively cheap and indigenous sources ofP besides water solublePfertilizersin crop production. The purpose of this review is to critically examine the research work done on P fixation in India: and to identify the gaps in the knowledge,and suggest future lines of work. The research work done on vario(ls aspects of P fixation and its availability in Indian soils is discussed in this publicatiOiT;.

1. "PHOSPHORUS STATUS OF INDIAN SOILS

Knowledge regarding the amount and chemical nature ofP compounds present in different types of soils is considered a pre-requisite to understand and interpret the behaviour of. P added to the soils. The P in soils is present in inorganic and organic forms. The inorganic P mainly occurs as fluor-apatite carbonate-apatite, hydroxy-apatite and oxy-apatite; mono, di- and' tri-Ca-P and; Fe-P and AI-P, such as dufrenite, wavellite, strcngite and variscite. Organic P compounds are present mainly as phytin and its derivatives, nucleic acid and phospholipids.

Total Phosphorus Total P in soil depends primarily on the parent material and management practices.

The P status of the Indian soils has been summarized in Table 1. According to Raychaudhuri and Datta (1964) the total P in Indian soils varied from 130 to 1310 ppm. In acidic hill soils of Assam, it varied from 175 to 1220 ppm and in soils from Bengal it ranged from 131 to 1120 ppm but most of the soils contained around 436 ppm. AUuvial soils of Bihar had a range of 175 to 436 ppm total P with a mean value around 350 ppm. Most of the black soils of Maharash tra had around 1000 ppm P. Semi-arid soils of Rajasthan

- were ramer poor in total P. Alluvial soil ofU ttar Pradesh, on an average, contained 350 ppm P. Dhir (1956) reported that Indian soils contained 165 to 1377 ppm total P with an average value of 474 ppm. The average values for black, alluvial, forests, desert and red soils were 419,415, 1268,240 and 376 ppm P.respectively. The HCl-solubleP fraction of these soils comprised 46.1 to 94.5 % of the total P with a mean value of83.5 %. In all soils except forest soils, very high amount of total P was in HC} soluble fonn. Organic P of these soils had a higher variation not only from one soil group to another but also within the group. On an average, forest, red, black, alluvial and desert soils contained 28.5, 23.3, 16.6, 8.8 and 6.5%, respectively of theirtotal P in organic form. With the exception of, forest and some alluvial soils, most of the soils were deficient in available P. Yadavand Pathak (1963) reported that in forest soils, coastal sands were the poorest in both total and available P. Other coarse-textured and intensively leached soils also lacked in Preserve. The soils of temperate hilly regions contained the maximum amoun t of total P. The desert, alluvial. red loam, red earths, black and laterite soils possessed a fair amount of total P but, . in general, showed low P availability. The soils containing high quantities of clay and organic matter were characterized by great retention of P and were deficient in available P, The soils which were non-calcareous, leached, near neutral in reaction -.md developed' from basalt, exhibited the highest level of P availabilily. Only a very -small percentage of total P w~ present in available fonn in all these soils. Total P in' acid brown hill soH of Shimla district ranged from 850 to 1800 ppm with an average value of 1175 ppm and organic P comprised about 23 % of total f (Shanna et al. 1979).

In alluvial soils of Kanpur (Uttar Pradesh)· total P decreased with maturity of soil (Agarwal and Gael, 1960). The highest content of total P was in the clay fracuon and lowest in the sand fraction. The immature soils with maximum concentration ofP in clay

4 PHOSPHORUS FIXATION IN INDIAN SOILS

size fraction were better supplier ofP to crops than mature soils with maximum P in the sand size fraction. Total P in sugarcane growing soils of Bihar and Uttar Pradesh decreased with depth. except in heavy clay soils of Bihar and in some immature soils of Uttar Pradesh, where the total P increased with depth. indicating the effect of parent material (Gupta, 1965). Major portion of the total P was in inorganic fonn which ranged between 75 and 95% of the total P. Organic P was low and decreased with the depth of the profile. The availability of P was maximum either in slightly acidic or in slightly alkaline soils. Total P ranged from 100 to 760 ppm with an average value of 350 ppm, in alluvial tarai. Bundelkhand and Vindhyan soils of Uttar Pradesh (Bhan and Tripathi, 1973). The available P wa'l positively correlated with organic matter and total P content of the soils and negati vely with pH and calcium carbonate. Total P in soils from Kamataka ranged from 61 to 982 ppm with an average value of 415 ppm (prakash, 1975). Acid soils were generally richer in total P than alluvial soils. Ninety-five per cent of the total Pin acinic soils and 75 % of total P in alkaline soils was in the inorganic fonn. These studies revealed that total P in Indian soils vary greatly in relation to 'Parent material, soil group, climatic condition, texture and management practices. MQst of the P in soils is present in the inorganic forms but forest soils contain quite high amounts of P in organic forms.

FRACTIONATION OF SOIL PHOSPHORUS

Methods Different chemical extractants have been suggested to determine different forms of

Pin soils. Ghani (1943a) reported thatCa-Psuchas mono; di-andtri-Ca-Pcan be extracted with acetic acid; Fe, Al and organic P with sodium hydroxide arid apatite with sulphuric acid. Reconsidered that mona; di-and tri-Ca·P is easily available to plants. Fe-P and Al­p probably constitutes the P fraction that is also available to plants. The organic P of soils is available to plants only through decomposition. Phosphates of apatite type are probably not available to plants. Insoluble P which is an integral part oftbe clay complex is not available to the plants. Kanwar (1953) sl.lggested the extraction of mono· and di­calcium and magnesium P with borate buffer (Borax+E.D.T.A.) at pH 8.95; andFe, AI and organic P with decinormal sodium hydroxide. He considered that mono-and di­calcium and magnesium P extracted from the soils were readily available to plants but Fe, Al and organic P were slowly available to plants. Chang and Jackson (1957) developed a procedure for fractionation of soil P into the discrete chemical forms such as Ca-P, AI-P, Fe-P, reductant soluble (iron oxide coated) Fe-P and occluded AI-Fe-P, based on the selective solubility of soUF fractions in various extractants. According to their procedure, different P fractions are determined by treating the same soil sample successively with normal ammonium chloride for loosely bound or saloid bound P, O.5N ammonium fluoride for AI-P, O.1N sodium hydroxide for Fe-P, O.SN sulphuric acid for Ca-P, O.3N sodium citrate sodium dithionite for reductant soluble Fe-P and then with O.1N sodium hydroxide for occluded P. They considered that P precipitated upon soil surfaces is readily available to plants. It includes Ca-P precipitated on calcium carbonate, AI.p precipitated upon alumino-silicates and Fe-P precipitated upon iron oxides. The slightly available P, include Ca~P such as variscite, Fe-P such as strengite and AI·Fe-P such as barrandite. The occluded P is not available. It is Ca-P occluded in calcium carbonate, At·P occluded in iron oxides and reductant soluble Fe-P also occluded in iron oxides. Peterson and Corey

PHOSPHORUS STATUS OF INDIAN SOILS 5

(1966) modified Chang and Jackson's (1957) procedure for routine fractionation of inorganic soil P. They recommended the use of constant suction pipettc.'l and an isobutyl alcohol extraction for the determination of reductant-soluble P, which greatly increased the speed ofP determination. Prakash (1975) suggested the use of alkaline ammonium fl uoride (PH 8.5) instead of neutral ammonium in Chang and Jackson method for extracting AI-P fraction from soils containing high amounts iron oxides. The above procedures are being used to differentiate the soil phosphorus into different discrete forms, to know their chemical nature and availability to plants.

Forms of phosphorus in soils The fractionation ofP of some soils from Bengal, Assam, Bihar and Madras by Ghani

and Aleem (1943) indicated that contents of acetic acid soluble (non-apatite Ca-P) were extremely low in acid soils over a pH range of 4.2 to 5.8. Under neutral conditions its amount was the highest which again decreased as the pH increased towards alkalinity. Fe­P and Al-P were relatively high in acid soils and decreased as the pH increased. Nearly 10% of the total P was present in apatite forms which increased with an increase in pH. The organic P compounds occurred in the highest amount under acidic conditions and decreased with the increase in pH up to neutral reaction and then increased with the increase in pH. The insoluble P which is considered the integral part of the clay minerals comprised nearly 25% of the total P and had no correlation with pH and sesquioxide contents of the soils. In majority of the soils, it, however, decreased as the organic P increased. Khanna and Datta (1968) observed that Ca-P dominated in the alluvial soils with pH 7.0 and above while AI-P and Fe-P dominated in acidic and red soils of India.

Punjab, Haryana, Himachal Pradesh and Delhi Kanwar and Grewal (1959) reported that in alluvial soils of the erstwhile Punjab with

a pH 7.3 to 8.4 the total P was comparatively higher than acidic hill soils of Kangra with a pH 4.9 to 6.5. The Ca and Mg compounds ofP which are soluble in borate buffer were twice as much in soils of higher pH as in acidic soils. The acidic soils had more AI, Fe and organic compounds ofP which are soluble in sodium hydroxide. About three-fourth of total P was present in the apatite and other resistant forms in these soils. The amount of this fraction increased with an increase. in soil pH. The inorganic P soluble in borate buffer + E.D.T.A. (Ca and Mg compounds ofP) increased with increase in soil pH, whereas, Al­P, Fe-P and organic P decreased WiLh increase in pH. In neutral range, both fonns of inogranic P were equally balanced. The borate buffer + E.D.T.A. soluble fraction of P had positive and significant correlation with pH in acidic soits as well as in alkaline soils, .the correlation coeffiCients were 0.866 and 0.603, respectively. The sodium hydroxide-

. soluble inorganic p. had negative and poor correlation with pH. The organic P had positive significant correlation with organic matter.

In representative profiles of alluvial soils ofHisar (Haryana) Ca-P was the predomi­nant which comprised about 75 to 85% of the total of At, Fe and Ca-P (Khanna and Singh, . 1966). A1-Pwas slightly higher thanFc-P. There was some relationship between calcium carbonate and Ca-P contents of these soils. In soil profiles of the Punjab, Haryana and Himachal Pradesh, total P, organic P, and Al and Fc bound P were high in acidic soils of Himachal Pradesh .and Ca-bound P was more in alkaline soils of plains (Randhawa, 1967). There was gradual decrease in total and organic P V(ith depth. The acidic group of soils

6 PHOSPHORUS FlXA nON IN INDIAN SOILS

had the highest amount of Fe and AI-P. The sum of both Fe and AI-P was lower than Ca­p in moderately alkaline to alkaline soil group and was almost equal in neutral soils. There· was a considerable decrease in the content of AI-P with increase i~ soil depth. The decrease in Fe-P was relatively gradual. Moderately alkaline to alkaline soils were the richest in Ca-P. Like other forms of P, Ca-P also tended to decline with depth. In sandy alkaline soils of Punjab and Haryana, organic P ranged from 8 to 22 % in surface soils and 5 to 10 % in lower layers (Singh and Arora. 1969). The organic P was directly related to the organic matter. Inorganic P in the alluvial soils of Ludhiana, Jullundur. and Hisar increased with depth but in Abohar and Sirsa soils it decreased with depth. The CIP and NIP ratio ranged from 37 to 46 and 6.0 to 10.15, respectively in the surface soils. The C/ P ratio was related to the organic matter distribution whereas the NIP ratio did not follow the same trend.

According to Sharma(1967) total P in hill soils of Kangra and Kulu districts of Himar.;hal Pradesh increased with the increase in clay content (1-0.87). The organic matter and organic P were highly correlated (r=0.61). Organic P constituted 36 to 66 % of total P and decreac;ed with depth. Fe-P and At-P together constituted 45 to 68 % and Ca-P constituted 21 to 50 % of inorganic P. At-P, Fe-P and ammonium chloride-soluble forms of P decreased with depth and Ca·P increased with depth. Available P had a significant correlation with Al-P but no association with soil pH. Khanna and Mahajan (1971) also observed that Olsen's and Bray's available P were highly correlated With AI-P in acidic soils of Palampur (r=O.91) and alkaline soils of Hisar (r=0.98). Singh and Bahaman (1976) reported that in acidic soils of Pal am pur, different Pfractions were in the order: Fe-I» Al-P> Ca-I» SaIoid-P. Negi (1976) found that P status of Kinnaur soils was high. The available P ranged from 2.5 to 319.5 ppm. The average total P content of the soil was 550 ppm. Acidic soils of Shimla district (Himachal Pradesh). on an average contained 66 ppm AI-P, 17 ppm Fe-P and 189 ppm Ca-P which constituted 5.6, 1.5 and 16.1 % oftota! P (Sharma et al., 1979). Organic P content ranged between 90 and 690 ppm and comprised 11 to 34 % OflOta! P, with an average value of23 %. The Fe-P. Al-P and organiC P were positively associated with low pH, silt, clay and organic carbon content of soils, whereas Ca-P was associated with high pH and sand fraction.

In alluvial soils of Delhi Ca-P was the most abundant P fraction, it varied from 185 to 1050 ppm with a mean valueof391 ppm (Datta and Khera, 1969). The saloid boundP varied from 0.6 to 28.5 ppm with a mean value of 7.6 ppm. The Al-P ranged from 8.0 to 38.0 ppm with a mean value of 21.8 ppm. The Fe-P varied from 16.0 to 39.4 ppm with a mean value of22.3 ppm. The occluded Al and Fe-P varied from 4.2 to 16.0 ppm with a mean value of7.9 ppm. The Fe-P and AI-P explained most of the variations in available P in Delhi soils, although Ca-P was the most abundant fraction in these soils. From above studies it can be concluded that alkaline soils of plains of these states contains most of the inorganicP in the fonn ofCa-P and in acidic hill soils, Fe-P and AI-P are the dominant P fractions. Organic P is quite high in the hill soils but very low in the soils of plains due to their low organic matter content. Uttar Pradesh

Eroded aHuvi&l soils of Uttar Pradesh were very low in available P, Ca-P and organic P; and had greater proportion of Fe-P and AI-P (Gupta and Khanna, 1964). Very high responses to P fertilization were found on these soils.

PHOSPHORUS STATUS OF INDIAN SOILS 7

In upland and lowland soils of Mirzapur Ca-P was 58.5 and 70.1 %, AI-P 5.4 and 5.l %, occluded AI-P 5.3 and 3.5 %.Fe-P 1.3 and 0.7%. occluded Fe-P 22.1 and:;lS.7% of the inorganic P, respectively (Singh and Gangwar, 1968). In the typical soil profiles of Jaunpur in Indo-Gangetic alluvium, the amount of total P varied from 300 to 487.5 ppm and 0.49 to 6.85 % of total phosphorus was in the organic form (GuPta and Misra, 1968). The inorganicP principally consisted of occluded Fe-P (80-86 %) and Ca-P (3.1-13 %). The percentage contribution of AI, Fe and occluded AI-P to total soil P was 0.5 to 1.4, 1.2 to 2.5 and 0.5 to 0.9%, respectively.

In representative 22 surface soils of Uttar Pradesh with pH 6.2 to 8.8 and clay content 9.2 to 58.2 %, Ca-P dominated over AI-P and Fe-P (Srivastava and Pathak, 1968). Ca-P ranged from 22.6 to 308.0 ppm with an average value of 117.5 ppm; AI-P ranged from 2.1 to 59.S ppm with an average value of25.1, Fe-P ranged from 16.9 to 61.0 ppm with an average value of 34.3 ppm and; Olsen's P ranged from 3.8 to 35.8 ppm with an average value of 11.0 ppm. The slightly alkaline alluvial soils of Uttar Pradesh contained 212.9 to 493.8 ppm total P with an average value of 323.5 ppm (Srivastava and Pathak, 1972). The average value of saloid bound was 9.0 ppm, Ai-P 35.2 ppm, Fe-P 42.0 ppm, reductant soluble p77.2 ppm, occluded Fe-P 12.2 ppm, Ca-P 105.7 ppm and organic P47.1 ppm. Singh and Pathak. (1972) reported that Bhat soils of Deoria district were poor in total P. The Ca-Pwashigherthanotherf0rmsofP.Onanaverageof13soi}s, Ca-Pwas430ppm, saloid bound-P 34.1 ppm, AI-P 54.5 ppm, Fe-P 42.1 ppm, reductant soluble Fe-P 24.0 ppm, occluded AI·P 14.8 ppm, organic P 68.4 ppm and total P 667.7 ppm. The Bangar soils of Deoria and Gorakhpur district on an average contained 847.5 ppm total P, 82.6 ppm organic P, 20.5 ppm saloid bound P, 72.9 ppm AI-P, 109.7 Fe-P, 134.3 ppm reductant soluble Fe-P, 13.9 ppm occluded AI-P and 412 ppm Ca-P (Singh and Pathak,,1973a). Reductant soluble Fe-P and organic P had significant positive correlations with pH and organic mauer, repectively. Significant positive correlations were observed between available P and AI-P, reductant soluble Fe-P and.Ca-P.

In sub-mountaineous soil profiles of Uttar Pradesh total P content varied from 180 to 275 ppm and decreased wIth increasing soil depth (Tyagi and Dass, 1969). Nearly 28.2 to 54.9 % of the total P was in inorganic combination and the rest in organic combination. In surface soils, Fe-P was the highest whereas in subsoils insoluble Ca-P was dominant while the soils contained very low amount of water-soluble and AI-P. In soils of Agra district, AI, Fe, reductant soluble and Ca-P fractions were in the ratio of 1 : 1: 5 : 10, respectively (Tripathi et al .• 1970). The Ca-P fraction constituted 41 % of the total P but made insignificant contribution to labile P. Most of the Ca-P was occluded within calcium carbonate matrices. Most of variations in Olsen-P and Bray-P was accounted for by Al­P whereas, Fe-P accounted for most of the variation in isotopically exchangeable P. In selected soils of Uttar Pradesh total P content of soils ranged from 19.0 to 82.5 mg P/l00g soil (Bhan and Shankar, 1973a). Organic, available, saloid, AI, Fe and Ca-P comprised 32.7,4.3, 2.9, 8.8, 8.2 and 40.4 % of total P. ·Total and organic P, in general, decreased with depth. whereas inorganic P increased. AI-P and Fe-P decreased with rise in pH, whereas Ca-P showed direct relationship with pH. Ca-P decreased with the increase in organic matter content and Fe-P had negative correlation with Ca-P. Organic P had significan t correlation with AI-P and Pe-P, indicating that organiC P migh t be mineralised into AI-P and Fe-P. In surface alluvial soils of Uttar Pradesh total P ranged from 405 to

PHOSPHORUS FIXATION IN INDIAN SOILS

530 ppm nntl it decreased vrogrcssively with depth (Misra and Verma,1979). Organic P constituted 11.5 to 23.7 % of total Pam] dccrclL'{Ccl with depth, synchronizing with organic carbon. Ca·P \vas the most dominant P fmction, constituting about half of lOla I P, Fe-P was the second most abundant P fraction, followed by AI·P. The raLio of Ca·P, Fe­P amI AI-P in the surface soil samples was 4.0, 1.1, 1.0 and their distribution pattern with depth was similar to towl P. The amount of reductant soluble P was very Jow, ranging from 8 to 32 ppm. In cultivated and forest Molisols ofUltar Pradesh, total P ranged from 270 to 905 ppm and organic P from 79 to 529 ppm (Ghosh et al., 1981). The quantity of LOIaI ac; well as organic P was more in the 0-15 em layer as compared to 15-30 cm layer. About 26 to 60% of total P were in the organic form. Forest soils had more organic-P than cultivat('d soils. Organic P was highly correlat{~d wilh organic malter, total P and lOLal N content of the soils. The above studies indicate thaL Ca-P is the most abundant P fraction in soils of UUill Pradesh. At and Fe-P is more in soils with low pH and high organic matter. The organic P was high in mountaineous, sub-mountainous and forest soils which contain higher amounts of organi):: matter. The tolal and organic P decreased with increase in the soil depth.

3ihttr Total Pin rcd .'oils of upper Damodar Valley WlL~ medium La low and inorganic forms

or P ut:counled for 94 to 96 % of the total P (Chaudhuri, 1964). The amount of Fc and Al bound Pwas high ascomparcd with Ca-P. The Fe-P and AI-Pvariedfrom 362.7 t0656.2 ppm (89.4 to 94.6 % of the total P). The Ca-P was only 10 to 40.1 ppm and accounted for 1.44Lo 5.95 (10 of the total P. Inorganic P constituted nearly 70 % ofthc Lotal Pin soils from Bihar (Singh e/ at. 1968). Fractionation of soil inorganic P indicated that various P fractions decreascU in thcfollowingorder: occluded Fe-P, Ca-P, Fe-P, AI-P, occludcdAI­P in the sedentary soils anti Ca-P, occludedFe-P,Fe-P, occluded AI-P, AI-P in the alluvial soils. Fe-P and AI-P were nearly 40 % of the total P in both the soils. The variation in Ca­p was from lO % in sedentary soils to 40 % in alluvial soils. Ca-P had significant correlation with total inorganic P; occluded Fe H.!Id AI-P and available P, indicating an eqUilibrium ofCa-P with these forms of soil P. Acidic sandy loam to clay loam soils of Bihar contained 186 to 1150 ppm total P, tracer ofsaloidP, 0 to 177 ppm AI-P, 0 to 257 ppm Fe-P and 0 to 394 ppm Ca·P (Dhua and Joshi, 1972). Neutral and alkaline soils comained300to1320ppmtotalP, traces ofsaloid-P, 0 Lo77ppm AI-P, 0 to 123 ppm Fe-P and 18 to 346 ppm Ca-P. Ai and Fe·P were negatively correlated and Ca-P was positively correlated with soil pH. In acidic soils of Bihar 70 to 90 % of the total P was in inorganic form (Challopadhyay . and Kar, 1973). The inorganic P was distributed among the P fractions in the following decreasing order: reductant soluble P, Fe-P, Ca-P, AI-P, occluded AI-P. Reductant soluble P accounted for about 50 % of the total inorganic P. These studies indicate that most of the P in soils of Bihar is present in inorganic form. Fe and Al-P arc the dominant P form in red and sedentary soils and Ca-P in alluvial soils.

Rajasthan In well-developed, moderately developed and young soils of Mewar Ca-P on an

average was 21.5, 28.2 and 39.2 % of the lOUlI inorganic P, respectively (Deo and Ruhal, 1970). There was no definite pallern in the distribution of Fe-P and AI-P in Lhe soil profiles

PHOSPHORUS STATUS OF INDIAN SOILS 9

of Mewar. In calcareous soils of Rajasthan, Ca-Pconstituted the major portion i.e. 44 to 52 % of the tolal inorganic P, followed by AI-P i.e. 9 to 17 % and Fc-P i.e. 3 to 8 % of \ the total soil P (Deo and Ruhal, 1972). The total P ami other P fractions increased with the depth of the normal, alkali and saline soil profiles of Western RajasLhan (Mehta el

al., 1971). Organic P was found to be positively correlated with organic carbon and clay content of soils. The inorganic P was found to be nearly 53.3 to 90.7 % of total P. The Ca-p was found to be predominant in soil profiles as compared with oLher fractions. The available P showed a negative correlation with pH and calcium carbonate. The inorganic fractions (AI-P and Fe-P) were found to affect the availability of the P. Medium black soils of Chambal command area, on an average, contained 79 % inorganic P as Ca-P, 10 % as AI-P and occluded AI-P, 1 % as Fe-P and 9 % a., occluded Fe-P (Vijay et al., 1972). Inorganic P accounted for 61 to 99 % of total P in soils of Rajasthan, representing three physiographic regions, namely Chappan plain, the Bana., basin and Aravalli hilly region (Choudhari and Jain, 1972). Ca-P and dithionate soluble-P were more in these soils than AI-P and Fe-P. Representative soil from Rajasthan on an average contained 456.6 ppm total P,30.2 ppm absorbed-P, 62.2 ppm AI-P, 36.9 ppm Fe-P and ] 83.8 ppm Ca-p (Choudhari et al., 1974). In some soil orders of Rajasthan, total and organic P decreased with the weathering of soils and there was a shift of inorganic P from Ca-P to reductant P with maturity of profile (Choudhari et ai .• 1979).

Surface soH samples (155) from major soil groups of Rajasthan contained 9.2 p.pm saloid boundP, 24.3 ppm AI-P, 15.5 ppm Fe-P. 193.8ppmCa-Pand17.0 ppm reductant soluble and occluded Al and Fe-P (Sacheti and Saxena, 1973). Saloid bound and Fe-P had negative correlations with pH but Ca-P was positively co-related with pH. Calcium, reductant soluble and occluded P was positively correlated with calcium carbonate. Saloid bound P, AI-P, reductant soluble and occluded P were also relaled in different types of soils. In north western Rajasthan flood plain soils contained the highest proportion of inorganic and organic P fractions (Talati et al., 1975). Ca-P varied from 52.6 to 71.9 % was the highest in alkaline desert plain. Distribution ofCa-P, AI-P and Fe-P fractions in profiles indicated effective movement of soil P to subsoil. Organic P was the high«:;st in surface layers and ratios of organic carbon and total N to organic P were narrow, indicating high P mineralization in these soils. In nine profiles from south·east semi-arid regions of Rajasthan organic P ranged from 10 ppm in lower horizon to 158 ppm in the surface horizons (Bhandari and Saxena, 1968). Organic P decrca'lcd with depth in most of the soils. It was related to organiC carbon and total nitrogen. Rallo of organic carbon to organic P was narrow. indicating a high ratc of organic P mineralization in these soils. In typical soil profiles of Rajasthan, organic-P was associated with non-humic fraction, followed by fulvic acid . and humic acid. Humic and fulvie acid bound P increased with organic carbon and clay + silt content of soil (Joshi and Ghonsckar, 1981). There was significant correlation between humic and fuMe acid and between organic-P and non­humic P fraction of soils.

The studies indicate that mostofthe P in soils of Rajasthan is present in the inorganic form and Ca-P is the major inorganic P fraction. Organic P is related to organic carbon and decreased with depth of soil profiles. AI·P and adsorbed P arc related to available P in soils.

PHOSPHORUS FlXA TION IN INDIAN SOILS

Gujarat The lOp layers were richer in total ami available P than subsoil layers in soil profiles

of Gujarat (Palel and Mehla, 1962). The available P in top layer was sufficient in Goradu soil of Anand, black soil of Athwa arid greyish soil of Padhiarka and low in medium black soil of Chharoli and deep black soil of Upleta. Factors, such as pij. organic matter and finer fractions of soil had no significant relation with available or total P. The soils of Gujarat contained 338 to 1488 ppm total. P with a mean value of 665 ppm (Mehta and Patel, 1963).The Ca-P varied from 17 to 403 ppm with a mean value of 228 ppm. Adsorbed P ranged from 49 to 259 ppm with a mean value of 105 ppm. Fe and AI-P varied from 64 to 280 ppm with a mean value of 129 ppm. Insoluble P varied from 306 to 533 ppm with a mean value of187. Organic Pranged from 3.1 to 28.4 ppm with a mean valueoflO.9ppm. Average available P in 14 districts of Gujmal ranged from 2.2 to 24.5 ppm.

Maharashtra Surface layers of all the soil profiles from Vidarbha were high in total P (Bapat et

al .• 1965). Inorganic P comprised the major portion of the total P. The organic P was low amI generally decreased with depth. The soils containing sufficiently high amount of free calcium carbonate and total calcium oxide were rich in Ca-P and .others were rich in Fe and AI-P. The reductant soluble P was more than 25 % of the total P and was related to the sesquioxide or Fe content. All the soils were poor in available P. In calcium-rich soils, Ca-P had significant correlation with available P and in other soils, Fe and AI-P were related to available P. Ai-P,· occluded P and total P were positively correlated with available P in soils of Vidharbha and Ca-P was negatively correlated with available P in black soils but positively in other soils (Puranik and Bapat, 1973).

Tamil Nadu Laterite soils of Nilgiri hills contained 104.7 to 1515 ppm total P with an average

valueof620ppm (MathanandDurairaj, 1967a). AvailablePrangedfrom 0.54 to 39.l ppm with a mean value of2.82 ppm. Available P was positively correlated with ea-P, occluded AI-P and total P. Mostly P was present as Ca-P in the black soil~, AI-P in red soils, Fe-P in latosols and Fe, At and Ca-P in alluvial soil profiles of Tamil Nadu (Raja, 1967). The organic P fraction was more in alluvial soil, followed by latosols as compared with black and red soil~. The Olsen's P was positively correlated with AI-P fraction in red and latosol soils, whereas in alluvial soils, the Fe-P was negatively correlated with Olsen's P. The lime and P potential values indicated the presence of octa-calcium phosphate in black soils and hydtoxy apatite in red soils and both octa-calcium and hydroxyapatite in the alluvial soiTs. The Ca-P fraction dominated in the black, alluvial and red soils . and constituted 51.3 % oftotal P (Balasubrarnanium and Raj, 1969). In laterite soils, Fe and Al­p occupied the major position among these fractions whereas Ca-P was found in traces. On an ~verage, alluvial soils contained 331 ppm total P, 15.3 ppm Al·P~ 16.5 ppm Fe-P and 209.6 ppm Ca·P; red soils contained 154.8 ppm total P, 4.4 ppm AI-P, 11.9 ppm Ca~P; bWtek-sQils contained 2925 ppm total P, 203 ppm AI-P, 4.6 ppm Fe-P and 202.1 ppm Ca­p; ·andlateritesoiIscontained468.7ppm total P, 43.2 ppm AI·P, 72.0 ppm Fe-Pand 13.7 ppm Ca-P. The typical south Indian acidic soils contained higher amounts of Fe-P and occluded Fe-P than Ca-P and AI-P (Vijayachandran and Raj, 1973). Organic P constituted

PHOSPHORUS STATUS OF INDIAN SOILS 11

four-fifth of the total P content of these soils. The inorganic P forms were positively correlated with P fixing capacity of the soils and with total and individual scsquioxides.

Kerala In soils of_Travancore-Coehin, acetic acid-soluble P was extJremely low in acid and

neutral soils but increased as the reaction shifted toward alkalinity, reaching a maximum at pH 8.0 (Brito-Mutunaryagam and Koshy, 1951). Its amount decreased with the depth of the profile in the alkaline soils. Fe and AI-P comprised a major fraction in acid soils but decreased as pH increased. In majority of the soils this fraction diminished steadily with depth. Apatite P, on an average, formed 11 % of the total soil P in acid soils. It oecmed in appreciable amount only under alkaline conditions and decreased with depth in most of the soils. Organic P constituted one of the largest fraction iii acid soils and generally decreased with depth. The insoluble P fractions accounted for 31 % of total P. It was equally high in acid and alkaline soils. In majority of the profiles, there was downward decrease in this fraction.

West Bengal The soils of Nadia district (West Bengal) are mostly calcareous in nature and the P

of these soils in intermediate in composition between oeta-Ca-P and hydroxyapatite (BaggiandChakravarty, 1972). The rice soils of West Bengal contained 177.1 to 543.7 ppm total P with a mean value of 311.6 ppm. The organic P constituted about34.7 % of the total P (Khan and Mandal, 1973). Both the total and organic P were significantly correlated with organic maUer. AI-P. Fe-P, Ca·P, reductant soluble Fe bound P and occluded AI·P fractions constituted about 7.2, 27.8, 46.6,16.2 and 2.2 %, respectively of the total inorganically bound P. All these P fractions were significantly related to total Fe-content. Troug's and Bray's P were significantly correlated with AI-P and Ca-P.

Assam Total P content in soils from Assam ranged from 58.0 to 2360 ppm and organic P

formed 51.1 and 86.4 % of the total P (Chakravarty and Tewari, 1970). The mineralized P had positive correlation with organic P and soil pH. Total P in alluvial, forest, hill and bheel soils of Assam was high and dependent on maturity and acidity of the soil (Chakravarty and Majumdar, 1970). A large proportion of total Pwas in the organic fonn. Available P was low. In major soil types of A~sam, the total and inorganic P content decreased in the following order: new alluvial, old alluvial, forest and hill soils (Chakravarty and Majumdar, 1971). The acid soil of Shillong contained 1500 ppm total P and about 60 % P was in organic form and Al-P dominated among the inorganic fractions. (Swaminathan and Sud, 1976). .

The literature reveals that the P content of soil varies with the parent material. In general, Ca-P is predominant in calcarepus and alkaline soils while AI-:P and Fe-P are predominant in acid soils. Organic P is low in soils of plains but fairly high in hilly and forest soils. The total, organic and available P decreased with the depth of soils profiles. The available P is related to saloid bound and AI-P but in most of the native soil P is unavailable to the plants.

2. PHOSPHORUS FIXATION MECHANISM AND TRANSFORMATONS IN SOILS

Fixation ofP by soils has long been recognized. Differences of opinions have, however, been expressed from time to time by various workers regarding the man\ler in which P a fixed by the soils. It is suggested that probably three separate mechanisms, which possibly overlap each other, are responsible for P fixation. At pH 2 to 5 the retention of P is chiefly due to the gradual dissolution of Fe and Al oxides which are reprecipitated as phosphates. At pH 4.5 to 7.5, P is fixed on the: surface of the clay particles and at pH 6 to 10, P is precipitated by the divalent cations. No single mechanism is responsible for P fixation in all soils. Different theories have been postulated to explain the mechanism of P fixation and are briefly discussed below. '

Precipitation Theory Probably the oldest theory pertaining to the m,echanism ofP fixation is that phosphate

ions in solution are precipitated, thus, becoming a part of the solid phase. The term precipitated P is limited to those compounds which areformed as chemically homoge­neous particles from ions in solution. This definition does not include chemically precipitated layers on surface of soil constituent~. In acid soils, Fe and Al appear to be the most likely soil constituents to fix P by chemical precipitation (Dean, 1949). When Fe and P are combined in equivalent quantities, minimum solubility occurs between pH 2 and 3. In the presence of excess of Fe, however; there is a tendency to extend the range of minimum solubility to pH 4. When Al and P are combined in equivalent quantities minimum solubility occurs atpH4 but whele excess of Al is present, the range of minimum solubilitY' extends from pH 4 to 7. The Fe and Al silicates and sesquioxides are the primary sources supplying Fe2 and AP ions leading to the formation of chemically pre­cipitated Fe and Al phosphates in acid soils. Som~ of the workers, however, reported that such compounds do not exist in large quantities in soils except in highly acid soils.

In alkaline and calcareous soils, ea forms a series of compounds with P, ranging from mono-calcium phosphate to hydroxyapatite. ,P added to calcareous soils, is converted to di-calcium phosphate, then to tri-calcium phosphate, octo-phosphate and finally to hydroxyapatite. The last one is the only stable compound. It is suggested that part of the calcillm phosphate combinations existing in soils are of unknown composition and that phosphate and lime exist in a series of combinations, having an apatite structures. The relation of one with another form of calcium phosphate when subject to pH changes. hydrolysis. and carbonation are shown in the phosphate cycle by Kardos (1964).

PHOSPHORUS FIXATION MECHANISM AND TRANSFORMA"nONS IN SOILS 13

CARBONATE APATITE

It is considered that in calcareous soils the P of low solubility is a carbonate phosphate compound in which one mole of calcium carbonate is combined with three moles of the calcium phosphate. Some other workers held that some of the superphosphate incorporated into limed soils will ultimately be reverted to fluorapatite similar to rock phosphate in characters.

Adsorption Theory According to this theory. P is fixed by adsorption between the liquid and solid phase

of the soil system. The phosphate ions penetrate the liquid solid interface to form new compounds with the hydrated minerals (Dean. 1949). The. phosphate ions are held tightly by the minerals and non-diffusable structural units are named colloid bOund P. The phosphate ions in the diffusable ion atmosphere held as compensation to ions of opposite charges are considered saloid bound P. These two forms of bindings are named as miCellar binding in contrast to extra-micellar bindings in precipitation theory, both being out side the soil micelles. Another classification of adsorption reaction of P by the soil is that of chemical adsorption and physical adsorption. In the chemical adsorption, the phosphate ions react mostly with Fe, Ai and Ca on the clay surface and Conn Fe; Al and Ca hydroxy phosphates, The adsorption of P on the surfaCe of the clay minerals without involving any chemical reaction is considered as physical adsorption. Both typeIof P adsor.ptions may be characterized by Freundlich adsor i'uon isotherms or by Langmuir adsorption equation (Kardos, 1964). Fixation ofP by kaolinite· from dilute P solutions obeyed the Freundlich adsorption iSotherm and increru;.ed with temperature (Low and Black, 1950). The P adsorption by red loam, laterite, sh'illow black, alluvial and coastal saline soils of rice growing areas of Orissa followed Freundlich isotherm s (Gaikward and . Patnaik. 1969). In coastal alluvium, laterite,. red. mixed red and black soils ofKarnataka ...

14 PHOSPHORUS FIXATION IN INDIAN SOILS

Pwas fixed by surflccadsorption in which Fe and Al played imporIant role in acidic laterite soils and amorphous AI oxide and exchangeable ea in alkaline soils (Prakash, 1975). The P fixation within 30 minutes was attributed to exchangeable Al and ea ions. The slow P fixation was due to non-exchangeable Al in the acid soils. There was more than one active site for P fixation by these soils. P at low concentration was fixed at the first site and P at high concentration was fixed at the second site in non-calcareous soils. In acidic soils of Himachal.pradesh the index of P binding energy K (mlfl.1.g p xl ()3\ranged from 45 to 362 with an average value of 134 (Sharma et ai.,1982). A signifkant and positive correlation of the index with extractable AI and total Fe was found in a bivariate correlation analysis. However, multiple regression indicated that different forms of Fe, and amorphous Al contributed significantly La the variation in P binding energy. The relationship between P-binding energy and soil factor suggests a mechanism of P adsorption by two point attachment, mainly through the colloidal surfaces offered by different forms of Fe oxides. The P adsorption followed Langmuir relationship in selected soils, representing different agro-climutic region of Punjab (Dev, 1984). The P desorption was far less than P adsorption; desorption followed pseudo first order kinetic"s and release of adsorbedP, diffusion was the main limiting step. High bonding energy value was responsible for low supply of P from soil. The soils having high P adsorption maximum value require low percentage saturation of adsorption maximum and vice­versa The P adsorption in black soils of Andhra Pradesh followed Langmuir's adsorption equation but in red soils it deviated at higher P concentration i.e. 30 Ilg/ml: or more (Krishna Kumari et al .• 1985). The P ad.<;orption was in monolayer at low doses and two or more layers at higher doses. In laterite soils:e adsorption was under two distinct regions

. ofhyper-bolic type, one up to lO)lg/ml and other in the range of 10 to 40J..l.g/mlbfadded P. The P adsorption sites were of two types with differentP bonding strength. These studies clearly show that the adsorption reaction is certainly involved in P fixation by soils and clay minerals but it may not be the only mechanism to explain the phenomenon of P fixation.

Anion Exchange Theory This theory involves metathetical reaction and anion exchange. Metathesis is the

substitution of one ion for another.In a broad sense, it is a chemical reaction without the stigma of law of mass action and stoichiometry. It involves chemical forces and affinities by which anions in solution may become associated with the solid phase. The fixation of P by the ,exchange or substitution has been suggested by many workers (Dean, 1949). When a clay suspension and phosphate solution of a similar pH are mixed, an increase in pH accompanied by the I disappearance of phosphate ions from solution, indicates that phosphate ions replace hydroxyl ions from the solid phase. Quantitative estimations of hydroxyl ions released by phosphafe ions have also been reported. Other corroborative evidence highlighting this phenomenon is the decrease in P retention with increase in pH and consequently the release of P from soil after its treaUllent with sodium hydroxyl. The exchange of phosphate by hydroxyl ions can be accounted for in several ways. It can be postulated that it entails an exchange for the hydroxyl ions associated with the sulface of clay minerals. The fixation of phosphate ions by hydrated oxides of Fe and Al can also be ascribt:d to a simple substitution of phosphate ions for hydroxyl ions. It is not necessary

PHOSPHORUS FIXATION MECHAN1SM AND TRANSFORMA nONS IN SOILS 15

for Fe and Al to be in solution in order to form Fe and AI phosphates, The Fc and Al phosphates can be formed by chemical precipitation on the surfaces of soil colloids. Anion exchange is a reversible substitution of one anion for the other. Phosphate ions which are fixed by the soil can be displaced by hydroxyl, f1uoride, silicate, arsenate and posssibly by some other anions also,

Phosphate transformations in soils The studies on the transfonnations of applied P fertilizers in different types of soils

elucidate the mechanism and natureofPfixation, Khanna and Datta (1968) reported that in some Indian soils transformation of added Pinto saloid-P depended on P fixation capacity of the soil and water solubility of the fertilizer. The maximum increase was in AI-P which was up to 90%. The quantum of increase in Fe-P was smaller than AI-P, Whereas, there was very little increase in Ca-P, even though the fertilizer added was in this form, Most of the added P to soils from different agroclimatic regions was recovered within 24 hours in different inorganic fractions, in the decreasing order of AI-P, Fe-P, Ca­P (Debnath and Hajara, 1972). Reductant soluble Fe-P increased in red, laterite and hilly . soils with no significant change in this fraction in alluvial and saline soils. On ageing, quantity of Fe-P increased while that of AI-P decreased irrespective of the soil. Ca-P increased in saline soils and decreased in alluvial soils on ageing, Ageing caused increase in reductant soluble Fe-P in red and laterite soils and decreased in hilly soils, In laterite, alkali, recent alluvial ,mar and alluvial soils mOSLof the added soluble P was transformed into insoluble AI-P. Fe-P artdCa-Pwithin 5 days (Singh and Ram, 1976). The percentage P transfonned into AI-P decreased and Fe-P increased with the increase in time. The percentage transformation of added Pinto Ca-P remained constant in laterite soil, decreased in mar and alluvial soils and increased in recent alluvial and aIka1i soils with time.

In acid soils ofPalampur (Himachal Pradesh) added P was transfonned mainly into AI-P(47 to 73% atpH4,7; 35 to 56% atpH 6.6) and Fe-P (18 to 44% at pH 4.7; 17 to 34% at pH 6.6) (Khanna and Mahajan. 1971). In alkaline and calcareous soils. saloid-P and Fe-P were the most and least abundant fonns, respectively. Singh· and Bahaman (1976) also reported that in acid soils ofPalampur, applied P was converted into Fe-P and AI­P. After 20 days of incubation there was significant increase in Fe-P and AI-P while Ca-P decreased with the period of incubation. In acid hill soils of Himachal Pradesh KHl04 applied at the rate of 50, 100, 150 ppmP was converted into Al-P)Fe-P,Fe-Pand small amounts to Ca-P, after one day (Sharma et aIJ1980). AI-P increased up.to 7 days but conversion to Ca-P was slow and very little even after a long period. Transformation of addedPtoAl-PandFe-PfractionswasrelatedtopH,clay,orgarucmatter,sesquioxidesand P fixing capacity of sons. Acidic hill soils adsorb more P than alkaline soils of plains irrespective of P concentration in the equilibrium solution (Vig and Dev,1984). In acid soils, more than 90% of adsorbed P was in the form of Fe and AI·P while in alkaline soils about 40% was saloid bound.

The P added to alkaline alluvial paddy soil of Hisar (Haryana), under submerged condition, was transformed mainly to Ca-P and Fe-P fractions after 12 weeks (Singh, '1973). In sorlie soils, the major portion of added P Was converted into Ca-P>AI-P>Fe-P> saloid-P (Gupta et al., 1975). The saloid-P and Ca-P increased,whereas. Al-P had a

16 PHOSPHORUS FJXA nON IN INDIAN SOILS

tendency to decrease with an increase in exchangeable sodium percentage. Saloid-P and AI-P decrease.d while Fe-P and Ca-P increased with time. The. water sol uble P in calcareous soils of Hisar was initially transformed into saloid-P, AI-Pand Fe-P which changed into Ca-P with time (Tomar el at.) 1984). The adsorbed-P underwent significant Ir'dl1sformation with time under maize culture in soils of Rajasthan (Singh and Saxena, 1969). Al-P also changed significantly in the rhizosphere but not in non-rhizosphere, while Ca-P and Fe-P did not change under both the conditions. In rice-barley rotation on alkaline alluvial soils of Delhi, the application of water soluble P increased Ca-P followed by Al­p in the first year and Fe-P in the second year, indicating the conversion of Al-P into the Fe-P with time (Lal and Mahapatra, 1979). In alluvial soils of Delhi, application of P fertilizer to a rice-wheat cropping sequence for over 10 years resulted in the increase in soil P status, particularly in the Al-P and Fe-P fractions (Nad and Goswami, 1984). In rice­wheat cropping system on alluvial, black, red and laterite soils; P applied to rice increased AI-P and Fe-P in all soils, reductant soluble-P in alluvial, black and laterite soils and; Fe­P and Ca-P in laterite soils only. Fe-P was the dominant form of soil and fertili~r P under rice growing situations while after the wheat crop succeeding rice in the same soil, content of Fe-P was reduced (Goswami and Kamath, 1984).

Application of 50 ppm P from (NH4)zHP04 to calcareous alluvial soils of Uttar Pradesh increased Ca-P and AI-P in the initial stages but Fe-P predominated after 2 weeks and Ca­P after 60 days when tbe soil was kept at field capacity or under saturated conditions (Gupta et al.r 1972). The application of 200 ppm P in the slightly alkaline alluvial soil of Uttar Pradesh at field capacity was almost completely converted into saloid-P, AI-P, Fe-P and Ca-P after three days (Sriyastava andPathak,1972). In Dhankar soils of Uttar Pradesh the amount of reductant soluble-P and residual-P decreased and that of Ca-P and Fe-P increased with time (GuPta and Nayan.1972). AI-P decreased at field capacity but its amount slightly increased under waterlogged conditions. Application of ammonium sulphate decreased Ca-P under both moisture conditions. In red and black soils of Uttar Pradesh. 90% of added water soluble P disappeared from solution of waterlogged soils after 30 days (Misra et at.11970). 60.5% in black soil and 50.4 % in red soil of the retained Pwas accounted for in theinorganic fraction. ThePapplicaLionfromKlP7andKHl04 increased AI-P and Ca-P in the red and laterite soils and decreased adsorbed-P (Misra and Gupta, 1971). Fe-P decreased with KZP20 7 but not with the application ofKHzP04, In red soils of Mirzapur of Uttar Pradesh, Fe-P content decreased at field capacity and increased under waterlogged conditions (Gupta and Nayan, 1975). Ammonium sulphate application decreased Ca-P and increased loosely bound-P with time. The amount of Al­p decreased while reductant soluble-P and residual-P increased with time under both conditions.

Application of 50 ppm P to calcareous soils of Bihar at field capacity increased Ca~P and Fe-P but decreased other forms ofP after 4 days (Sahay et al.j 1969).' After 60 days Ca-P was further inceased while other fractions generally remained unchanged. The reverse was true under waterlogged condition except that the decrease in Ca-P was somewhat less when incubation period was increased from 4 to 60 days. In acid soils of Bihar application of 50 ppm P increased reductant soluble-P and Fe-P (Chattopadhayay and Kar, 1973). In rice culture at Jabalpur (Madhya Pradesh), the Fe-P fraction fonned the major portion of native inorganic soil P fr:actions (Thakur et al., 1975). Al-p, Fe·P and

I'HOSPHOIWS FIXATION :VlECHANISM AND TRANSFORMATIONS IN SOILS 17

Ca-P fractions increased with the addition of P fertilizers. There was no appreciable increase in the reducL:'lnt soluble-P fraction.

In acid soils (pH 4.4 to 6.2) of West Bengal application of 225 kg Plha increa'>cd the AI-P, Fc-P, reductant soluble Fc-P and to lesser degree occluded AI-PandFe-P(Kar and ChakaravarLi, 1969). Ca-P was unaffected except in the soils with pH 6.2. The amount of added P (KHl04 at the rate of 8.75 mg P/lOg soil) to acid soils (pH 5.3 to 6.0) sharply decreased and there was marked increase in AI-P fraction but not in saloid-P, Ca-P fraction increased only slightly, and reductant soluble Fe-P increased in the iron rich soil only (Mandai and Das, 1970). The water soluble P added to acidic soils of West Bengal was transformed to AI-P, Fe-P, Ca-P and red-P and in alkaline soils it was transformed to Ca-P, AI-P, Fe-P and red-P in the decreasing order (Debnath and Mandai, 1982a). On ageing, Fe-P increased consistently whereas, AI-P and Ca-P increased up to 15-30 days and decreased thereafter. Water soluble P added to latosolic low land rice soil of West Bengal was rapidly transformed into AI-P and Fe-P (Mandai and Chatterjee, 1972). Transformation intO'Ca-P was low in these soils. 10 to 30% of addedp was transformed into reductant-soluble Fe-P. Transformation into occluded Fe and AI-P was negligible in these soils. In black and red soils of Mysore treated with ammonium phosphate and superphosphate at levels of 200,400 and 600 ppm p.; the largest single form of P was Fe­P and AI-P was only found in the superphosphate treated soils (Rao el a/. r 1972). The P application (582 kg P/ha) for three years to red loam soil (pH 5.4 to 5.6) of Hcbbal (Bangalore) increased the total-P, AI-P, Fe-P, Ca-P, reductant soluble Fe-P and occluded AI-P and Fe-P in 'both surface and sub-surface soils (Krishanappa and Rao, 1973). Over 50% of applied P was converted into inorganic forms, predominantly Fe-P and reductant soluble Fe-P.

Das and Datta (1969a) reported that NHiI2P04 and KH2P04 dissolved considerable \ amounts of Ca and Mg from black soils. S travite, brushite and monetite were the dominant reaction products formed from NH4~P04; new berryite and brushite were formed from the reaction of KH2P04;. Ca(H2P04)2HP produced an ionic environment dominated by Ca and Mg, which led to the formaton of brushite and monetite. Das and Datta (l969b) reported that appreciable amounts of ammonium taranakite and potassium taranakite were formed as reaction products when monoammonium and monopotassium phosphates respectively were applied to acid soils' of Tripura containing appreciable quantities of soluble aluminium. In alluvial calcareous soils the interaction of monoammonium phosphate, resulted in the formation of brushite, stravite and new berryite,while only brushite and monetite were formed from that of monopotassium phosphate. Monocalcium phosphate interaction with both the ~oils resulted in the dissolution of considerable amounts of Fe and Al which precipitated as amorphous Fe-AL-P. in addition to the formation of brushite and monetite as reaction products. The amorphous compounds dominated particularly in the acid soils. Mandal (1975) discussed the soil and fertilizer P reaction products such as strengite, vivianite, variscite, triCalcium phosphate, tnonetite, brushite, hydroxyapatite formed under varying soil conditions and their P supplying capacity to plants. He stressed that more work should be done for the identification and characterization of these products to understand the mechanism ofP fixation and the factors affecting P fixation.

These studies indicate that most of the applied Pis fixed due to adsorption on the soil

18 PHOSPHORUS FIXA nON IN INDIAN SOILS

particles. However, precipitation and anion exchange also play an important role in P fixation under certain situations. The nature of P fixation depends upon the soil type, P concentration, time for reaction and nature of the crops grown. In general, P is mainly fixed as Al·P and Fc·P in acidic red and laterite soils and Ca·P and Al·P in alkaline calcareous and sodic soils. The growing of rice crop favours formation of Fe·P in soils but crops like wheat and barley decrease Fe·P in soils.

3. FACTORS AFFECTING PHOSPHORUS FIXATION IN SOILS

Phosphorus fixation in soils is dependent on several factors which are operating at varying degrees in a particular soil system. To have a clear picture of the phenomenon, it is essential to understand the role of different factors affecting it.

Clay minerals The amount and nature of clay minerals of soil such a<; kaolinire, halloysilc,

montmorillonite, vermicultite and illite are the most potent factors determining the P fixing capacity. The P fixing capacity oflho clay minerals is mainly due to Lhe replacement of OH ions from the clay minerals surface, especially around the crystal edges and P reaction with soluble Al originating from the exchange sites and from lattice dissociation _ of clay minerals to form insoluhle P comilounds. Therallid P fixation by thcclay minerals is attributed to its reaction with readily available Fe and Al and slow fixation from the reaction ofP with Fe and Al released through decomposition of the minerals. H-kaolinitc had quite an appreciable P fixing capacity which increased with increase in concentration ofP (Chatterjee and Datta, 1951). H-Montmorillonites also fixed Pwhich increased with increase in P concenLration. H-kaolinite at low pH had greater P fixing capacity than H­montmorillonite, the relative difference diminished with increasing pH values but wa<; reversed between pH 7 to 9. The P fixation increased with decrease in particle size of the clay minerals to increase specific surface area of kaolinite and greater accumulation of free oxides or increased OR ions on the edges and of exchangeable Al on the surface of montmorillonite.

The amountofP fixation by vermiculite varied from 32.0 t04 7.1 % of 100 mlofmono­calcium phosphate solution of different concentration varying from 0.02 to 0.002 Madded to 5g of the powdered and oven-dried clay mineral (Mitra and Prakash, 1955a). The P fixation by montmorillonite, kaolinite, ,halloysitc, natrolite, vermiculite, muscovite and biolite was influenced by the concentration of P in solution, type of the mineral and the nature of exchangeable cations (Mitra and Prakash, 1955b). TheP fixation decreased gradually with the fall in P concentration. Montmorillonite and vermiculite fixed large amount ofP whereas, biotite, kaolinite, halloysite, natrolite and muscovite fixed only moderate amounts ofP atpH U.8. The decreasing order in which minerals fixed P was montmorillonite, vermiculite,biotite,natrolitc, kaolinitc, halloysite, muscovite. -Exchangeable Ca ions play an important part in P fixation by these minerals. This was the main reason that montmorillonite and vermiculite which had high exchangeable Ca possessed high P fixing capacity. The exchangeable Ca fixed P at the surface of the particle as Ca-P or that the exchangeable Ca ions act as bridge between the ,.phosphate ions and the clay surface. The chemical absorption of P by vermiculite, montmorillonite, biotite and halloysite from diluteP solution may probably be due to exchange WiLh OH ions in the crys­tallaltice or by reaction with At and Fe ions and with surface films of the hydrated oxides oCthese clements, resulting from llie weathering of clay minerals (Mitra and Prakash, 1957) .

. The P fixation by kaolinite and halloysite was appreciable (Sinha, 1956). The P

20 PHOSPHORUS FIXATION IN INDIAN SOILS

adsorptiorl or anion ex.change by kaolinite was complicated by the presence of AI, either in the free state or present in the laterally broken down lattice of clay minerals when the latter were finely powdered. Removal of free AI by means of dilute acid and or treatment of phosphatised kaolinite with ammonium oxalate solution brought down the P fixation capacity on phosphatisation of kaolinite. The retention of P by minerals occurred as counter ioris especially on muscovite andphlogophite(_Srivastava, 1961). The Pretention capacity of different minerals decreased in the following order: muscovite, phlogophite, montmorillonite, gibbsite, pyrophylite, kaolinite.

The fixation of P by montmorillonite ~Uld kaolinite increased with the decrease in pH (Dc, 1960). Montmorillonite fixed more P than kaolinite. Fix.ation of P was accompanied by the release of OH ions and it was affected by the nature of cations. The P fixation was more in the presence ofNH4, H, Na and ea than K cations. The P fixation in hydrogen derivative of Indian montmorillonite might be due to (a) the association of phosphate with minerals by physical forces, and (b) the removal of phosphate from the solution due to the formation of relatively insoluble AI-P (De, 1961a). In alkaline medium , !D0st of Al would tend to form AI(OH)3 where process (a) would be the more prominent, whereas, in other instances AIP04 would be formed. This was obviOUSly the reason that explains why the P fixation diminished with increasing alkalinity of the medium. The adsorption ofP from H3P04 , NH4Hl04. (NH4)zHP04 • (NH4)104 by NH4, Kand Li derivatives of Indian montmorillonite did not fit into Freundlich's adsorption isotherm and the curves were typically S-shaped (De, 1961b). The adsorption ofP by different montmorillonite derivatives decreased in the following order: NH4, Na, K, Li. The P adsorption increased as the pH of the P solution increased. During the process of P adsorption some silica was also released which was partly responsible for P fixation. During the preparation of montmorillonite derivatives by the treatment of respective salt solutions different amounts of AI, Caand Mg were released which influenced P adsorption. The P adsorption by Indian muscovite was mainly due to the precipitation of insoluble AI· P in which the ratio of Al:Pps was about 4.0 (De and Tewari, 1961). The application of ammonium chloride and ammonium nitrate increased P adsorption whereas, ammonium sulphate depressed the process. Sulphate ions appeared to reduce the associations of phosphate ions with AI.

SesquIDxides The sesquioxides present in the free and hydrated state are considered the main

causes of P fixation in acid soils, The Fe and AI containing soil minerals, including clay minerals are the sources of Fe and At The formation of insoluble P compound is governed by the solubility product principle. Under certain conditions i.e. in highly acidic soils when Fe and Al are present in the free state, Fe and Al phosphates are precipitated. Whereas, under other conditions hydrated oxide of Fe and Al adsorb the phosphate and later on form some defmite compounds. In many respects a distinction between reactions in which phosphate ions are precipitated from solution by Fe and AI ions and the reactions in which phosphates are removed from solution by hydrated oxide is arbitrary, since the final product of both the reactions would be identical. In a general manner, the fixation of P in add soils by Fe and Al can be visualized as follows (ij'emwall, 1957).

FACTORS AFFECl1NG PHOSPHORUS FIXATION IN SOILS 21

(M(H20). A ,.3 to + 1 +A + H2PO~ ( (M(H 0) A H PO) .' y • ? ,,2 • t,j.

M,03' M(OH)" Clay Minerals. and Exchange Sites Precipitated or Adsorbed

The symbol M stands for the cations of fie or Ai and A stands for oxide or hydroxide. The P fixation in red soils increased with the increase in sesquioxide in Hel extract

of soils (Raychaudhuri and Mukerjee, 1941). The P fixing power of acidic soils wa." considerably reduced by the chemical de-activation of Fe and Al by means of 8-hydroxy quinoline (Ghani, 1943b), This indicates that P fixation in acid soils was largely owing to the formation of Fe and Al phosphates by chemical interaction between the soluble phosphate and hydrated oxides of Fe and AI. Ghani and Islam (1943a, 1946) obtained direct and definite evidence in favour of chemical precipitation theory of P fixation in acid soils by the actual determination of Fe and Al phosphate farmed after the addition of soluble phosphates. More than 90% of the fixed P was recovered as Fe and Al phosphates, showing that chemical precipitation of soluble phosphates into Fe and Al phosphates account largely for the fixation of P in acid soils. No P was reverted La apatite form.

Mukerjee (1943a) reported that there were factors in addition to sesquioxides for P fixation in red soils. The P fixation may be owing to the following mechanisms.

(a) Some of the phosphates are converted into Fe and AI phosphates reacting with free Fe and AI.

(b) Some of the phosphates reacted on by the divalent cations and are transfonned into ea and occasionally Mg phosphate.

(c) Some of the phosphates are sorbed (absorbed or adsorbed) by the soils either by exchange or by simple physical processes.

The P fixation in the acid soils of Punjab was because of free sesquioxides whereas in calcareous soils it was owing to calcium carbonate (Singh and Dass, 1945).The P fixation capacity of hydrogen montmorillonite and coarse fraction of hydrogen kaolinite was mainly owing to scsquioxides (Chatterjee and Datta, 1951). In iaterite.podzol soils of Australia high P fixing capac.ity was mostly due to reactive sesquiQxides and the removal or complexing of reactive sesquioxides greatly reduced the fixation by soils (Kanwar, 1956). From the molectdar ratio, he showed that aluminium oxide in soils retained P more actively than equivalent amounts of iron oxides. The presence of free alumina in kaolinite was mainly responsible for its ,high· P fixing capacity (Sinha, 1956). The removal of free alumina greatly reduced the P fixation capacity of this mineral. He considered that Al.present, either in a free state or in the laterally broken down lattices of kaolinite. was mainly responsible for P fixation. He found Al in the ammonium oxalate extract of the mineral and confirmed the formation of Al-P on clay mineral by electronic diffraction pictures. About 72% ofP fixing capacity of acidic soils of Himachal Pradesh was due'to free sesquiQxides (Kanwar and Grewal, 1960). The removal .of free sesquioxides decreased the P fixation capacity to ~. great.extent From the molecular ratio's ofPO/RPj' they showed that basic phosphates of Fe and Al were formed.

The fixation of applied P to acidic lowland rice soils of West Bengal was affected by their FepJ and active FCP3 concentration rather than by soil pH (MandaI and Das, 1970). The quantity of sesquioxide in major soil· series of Thanjavur district of Tamil Nadu influenced their P fixing capacity (Mosi et al.f 1975). The.content of free iron oxides

22 PHOSPHORUS FIXA nON IN INDIAN SOILS

WIL~ a dominant factor contributing towards the P fixing capacities of major soil groups ofIndia (Nad et al., 1975). Free scsquioxides together with clay explained 76-77 % of the variation in P fixation capacities of these soils. The P retention depends on Al content in acidic soils and on Fe content in alkaline soil of Mysore (Prakash and Bhaskar, 1969). In ~andy loam soils of laterite origin, high in Al and with pH from 5.2 to 6.2, the sorption of}l had a good agre.ement with true exchangeable AI. The high amounts of Fe and Al oxides present in acidic laterite soils were responsible for P fixation (prakash,1975). There was a positive relationship (r=O.819) between P fixing capacity of black, rcd and mixed yellow soil of Rajas than and their sesquioxides coittent (Dhawan eC al,) 1969). Biddappa aml Rao (1973) also noted a close positive correlation between P fixing capacity of typical coffee growing soils of south IniHa and thcirpe

2 03 (r=O.665), free iron oxides (r=0.863),

Alp) (r=0.721) and RP3(r=O.655). It is now well-established that hydrous oxides of Fe Md Al fix P under conditions

that normally exist in soils. Support tl) this conclusion is drawn from four types of evidences: (a) correlations have been established between the P fixation and the amount of Fe and AI in soils; (b) removal of Fe and Al oxides from soil colloids reduces P fixation; (c) addition of Fe and Al compounds to the soils increased P fixation capacity 'of soils; and (d) compounds formed' during P fixation have been identified by comparing the effect of pHon the P fixation wilh that of the known effcct of pH on the solubility of Fe and Al phosphates.

pH Fixation of P in soil is genemlly high allow and high pH. The bac;ic Fe and Ai

phosphate have a minimum solubility around pH 3.0 to 4.0. At higher pH some of the P is released and the P fixing capacity is somewhat reduced. Even at pH 5.5 much of the P is still chemically combined with Fc and Al. As the pH approaches 6.0 precipitation of P as Ca compounds begins; at pH 6.5, the fOnTIalion of insoluble Ca-P is a factor in rendering the P unavailable. Above pH 7.0 even more insoluble compounds such as apatites are formed.

The P fixation was much more in Berhampur soil (Bengal) with pH 4.6 than Dacca soil with pH 5.2 (Ghani and Islam,1946). The acidic soil ofPaJampur (Himachal Pradesh) with pH 5.0 fixed more P than alkaline and calcareous soils of Punjab with higher pH (Singh and Da'>s, 1945). The H-kaolinite fixed 309 m.e. to 22.9 m.e. and H-montmorillonite from 157.7 m.e. to 43.7 m.e. P per 100 g of dry matter as the pH was raised from 3.0 to 9.0 (Chatterjee and Datta, 1951). The decrease in P fixation with increase in pH was attributed to two mechanisms, viz. (i) P fixation was due to reversible exchange between phosphate ion in solution and hydroxyl ion of the crystal lattice. As the hydroxyl ion concentration in solution increa..<;ed, the above reaction gets inhibited, resulting in a progressive decrease in P fixation, (U) P fixation was owing to the precipitation of phosphate as Fe and Al phosphate. As the pH increased, the activity of Fe and Al was gradually reduced and consequently the amount of adsorbed P decreased. The P fixation capacity of acidic soils of Himachal Pradesh decreased with the increao;c in pH and there was a negative significant correlation between the pH of the soils and P fixing capacity (Kanwar and Grewal, 1960). In .calcareous and alkaline soils the P fixation capacity increased with the increase in pH. In acidic,.neutral and alkaline soils, the maximum P

FACfORS Af·T:EC"TING PHOSPHORUS FLXATJON IN SOILS

fixation was in alkaline soils anclleast in neutral soils (Banerjee and MandaI, 1965). The fixed P was converted into AI-P, Fe-P and Ca-P in these soils. Fe-P was the highest in acidic soils and CaMp in alkaline soils. AI-P was nearly the same in all the soils.

From these studies it can be inferred that pH of a soil is an important factor to determine behaviour of applied P and nature of native soil P. In general, P fixation is high ill acid and alkaline soils and the availability of applied and native soil P is maximum in nearly neutral soils with pH 6.0 to 8.0.

Exchangeable Cations The nature of the exchangeable cations present on the colloidal complex of the soils

plays an important role in the P fixation. There was an increase in P fixation capacity with the increase in exchangeable Ca, exchangeable bases, total calion exchange capaci ty and [he clay content (Patel and Viswailath, '1946). The P fixation by montmorillonite and vermiculite was high because of high exchangeable ea in these clay minerals (Mitra and Prakash, 1955b). The exchangeable Ca leads to P fixation by precipitation of calcium phosphate or the exchangeable ea ions act as bridge between the phosphate ions and the clay surface. It is suggested that a linkage of phosphate to the colloid through Ca ions on the exchange complex is possible. The mechanism can explain the increased cation exchange capacity as a result of phosphate adsorption.

The P fixing capacity of acidic soils decreased with the increase in the degree of base saturation capacity of soils (Kanwar and Grewal, 1960). There was a negative significant correlation between the degree of base saturation and the P fixing capacity of soils. About 30 % of the P fixation in acidic soils of Himachal Pradesh was due to exchangeable Ca and Mg. In alkaline and calcareous soils nearly 70 % of the P fixation capacity was because of exchangeable Ca, Mg and CaC03• The exchangeable calions affected P fixation in the following decreasing order: Ca, Mg, H, NH

4, K, Na and non­

exchangeable ions in the order: Fe, Ai, Mn (Pathak and Shukla, 1963), Ca in the exchangeable form wasmosteffectiveinPfixation. Dhawan et at. (1969)reponed that P fixing capacity of black, red and mixed yellow soils of Rajasthan was positively related with their exchangeable Caand Mg (r=O.755). The H-saturatcd black and red soils of Uttar Pradesh retained more P than Na and Ca saturated soils (Misra and Ojha, 1969a). The P fixation by untreated acidic brown hi1l soils of Himachal Pradesh depended on the saturating cations in the following decreasing order: Fe, Ca, Mg. K (Singh, 1973). The availability of adsorbed P was greatest from NH4Hl04 in the presence of Ca and lowest in the presence of Fe.

These studies indicate the fixation and availability of P in soils is linked with the nature of the cations on the exchange complex and also on non-eXChangeable cations. The exchangeable cations also determine thepHofsoils. In general Ca and Mg cations in neutral and alkaline soils and H cations in acidic and laterite soils are positively related to their P fixing capacities.

Calcium Carbonate The P fixation in calcareous soHs was through the chemical combinations (Harrison

and Dass, 1921). The first combination was di-calcium phosphate which took place rapidly and afterwards the di-calcium phosphate reacted with more calcium carbonate

24 PHOSPHORUS FIXATION IN INDIAN SOILS

forming tri-calcium phosphate. The P fixation in the alluvial soils of Punjab varied with the nature of ~le soil and the amo~nt of CaCO~ 'pre~nt (Singh ~d D~ssl 194~). The P fixation capacity of calcareous sOlIs of the Punjab Increased With the Increase In Cae03

content (Kanwar and Grewal, 1960). Nearly(70.2 % of the P fixation in these soils was attributed to CaC0

3 and exchangeable Ca and Mg. They considered that phosphate is held

on the surface of the CaC03

particle. The CaC03

was closely related (1'=0.999) with P fixing capacity of black, mixed yellow and red soils ofRajaslhan(Dhawan et al. 1969). Nearly 90% of the P fixing capacity was due to the CaC03, exchangeable Ca and Mg and sesquioxides. There was a significant positive correlation (r=0.697) between P fixing capacity of alluvial soils of Gorakhpur and Deoria district of Uttar Pradesh and their CaCO) content (Singh and Pathak, 1973b). There was a negative correlation between available P and CaC0

3 content of soils of Malwa and Nimar regions of Madhya Pradesh

(Jain et al.J 1970). Similar results in nonnal, saline and alkali soils of Western Rajasthan and; alluvial, Bundelkhand and Vindhyan soils of Uuar Pradesh are reported by Mehta e/

al. (1971) and Bhan and Tripathi (1973). There was a direct correlation of Ca·P with free CaC03 in wide range of Indian soils

containing 0.5 to 15.25 % CaC03

under waterlogged conditions (Banerjee and Ghosh, 1970). It is also indicated that soluble phosphates are fixed as tribasic calcium phosphate and that the final conversion to calcium carbonate phqsphate is a very slow process. These studies revealed that CaC0

3 in soils is pOSitively related with their P fixing capacities.

Salts The addition of a Ca salt such as gypsum which involves no change in pH would be

expected to decrease P solubility by the common ion effect in soils containing high amount of ea-p. However, other metal salts would be expected to increase the Ca-P by lowering the activities of the phosphate and Ca ions. In sodic alkali soils of Punjab gypsum application increased the amount of Ca-P but decreased that of Olsen P, solaid P and Fe­P (Bajwa et al., 1982). Thus application of neutral Ca salts may decrease P solubility and salts of other metals may improve P availability.

Phosphorus concentration It has been observed that greater the ratio ofP to soil, the greater the P fixation. The

relationship between P fixation and P concentration has been shown to comply the Freundlich adsorption isotherm. The precipitation process of P fixation also gives results which fit in the Freundlich adsorption isotherm. In acidic soils of Dacca and Berhampur, the amount of P fixed increased with the increase in the amount of P added though not in the same proportion (Ghani and Islam, 1946). The amount ofP fixed in soils from Palampur, Rawalpindi and LyaUpur increase up to certain limit of P concentration beyond which it again decreased (Singh and Dass, 1945). However, percentage P fixed decreased WiUl increasing concentration of P. The amount of P fixed per 100g dry H­montmorillonite increased from 13.0 to 127.5 m.e. as the concentration of the added phosphoric acid increased from 0.002 to O.llN (Chatterjee and Datta, 1951). Most of added P (87 to 218 kg P/ha) was adsorbed by soil from Hebbal tank (Mysore) but the second dose of added P substantially increased the P content of water overlying the soil (Muddanna and Mlilliah, 1971). A decrease in P adsorption in acid soil (PH 5.8) of

FACTORS AFfECTlNG PHOSPHORUS FIXA TI001IN SOILS 25

Baruipur (West Bengal) from 98.0 to 41 % was observed with the increase in added P from 25 to 1000 ppm P (Basu and Mukerjee, 1972). TheP fixation in lowland rice soil of Gujarat decreased from 45.6% to 31.0% with the increase in added P concentration from 25 to 150 ppm P (Patel, 1975). The P sorption increased gradually with the increase in the equilibrium P concentration in most of the coastal alluvium, laterite, red mixed; red and black Karnataka, and clear maximum was not obtained (Parkash, 1975). The P sorptioon in general followed Langmuir isotherm. He stressed the need for determining Lhe kinetics of the soil-P reactions at different concentration for a better understanding of the system. From these studies it can be inferred that total P fixation in soils increases with increase in P concentration but percentage P fixation decreases with increase in P concentration, resulting in more availability of applied P at higher rates of P application.

Period of reaction The rate at which the P fixation proceeds in soils is an important factor in assessing

the P availability to ,the growing plants. In red soils, P fixation proceeded very rapidly up to 30 minutes, the rate then gradually sloweq down becoming almost constant aftei two hours (Raychaudhuri and Mukerjee,1941). In soils of Dacca (pH 5.2) and Berhampur (PH 4.6), 45 and 85 % of the added P was fixed up to six hours. As the contact period was increased to six weeks, the P fixation rose to 70 and 95% respectively (Ghani and Islam, 1946). The amount ofP fixed by H-montmorillonite increased with the time (Chatterjee and Datta, 1951).

The soluble P applied to acidic, neutral and alkaline soils of the erstwhile Punjab changed to slowly soluble forms and maximum change occurred in one day (Kanwar and Grewal, 1958). The equilibrium was established between readily and slowly available forms of P in 28 days. The P fixation in black soils of Madhya Pradesh includes a rapid and slow reaction (Motiramani el al., 1964). Whereas the rapid one takes place almost instantaneously and the slow one continues for a long time. The release ofP can also occur along with fixation as a result of microbiological activity and mineralisation of organic P. In soils from Bihar most of the applied 1:),oluble P was fixed up in 24 hours but gradually increased up to 30 to 45 days depending upon the type of the soil and thereafter it remained constant (Gupta, 1965). Availability of applied P to blac,k, red and laterite soils decreased considerably after 14 days but increased at 42 days when,incubated at water holding capacity (Mustafa and Durair!J,j, 1968). The P adsorption by Hebbal soils (Mysore) was increased substantially .up to 30 days (MuddannaandMallaiah, 1971). In alluvial and laterite soils of Tamil Nadu availableP decreased during incubation (Raja and Bhavanisankaran, 1973). In acidic soil of Baruijpur (West Bengal) P sorption was gradual up to five days and it reached a steady stale after 18 days (Basuand Mukerjee, 1972). The P fixation increased with time up to 60 days in alluvial soils of Madhaya Pradesh (Dravid and Apte, 1975). In black soils maximum P fixation was recorded at 30 days and there was some release of fixed P after 60 days of incubation. In representative soils of Kamataka i.e. coastal alluvial, laterite, red, mixed red and black, and black soils, P fixation was in two stages (Prakash, 1975). The first stage was so fast that it LOok place within the time required to mix the soil sample with P solution. This P fixation was attributed mainly to be exchangeable Al and Ca ions. The second stage was a slow P fixation which was attributed to the non-c,"Xchangcable At, presumably dissociated from

26 PHOSPHORUS FIXATION iN INDIAN SOILS

the non-exchangeable sites. The ex.change sites were responsible for the fastP fixation and non-exchilllgeablc sites for slow P fiXation.

TIle reaction of P with soils and clays would never be complete as long as an appreciable concentration of P ~emains in solution. Some of !.he surface reactions are completed quickly, however, tl'ie rale of reaction decreases rapidly. Reorientation of absorbed ions, debydration of the product fonned and phosphalolysis of the clay may apparently continue indefinitely at a very low rate.

Phosphorus fertilizers The nature of the P fertilizers applied to different types of ~oils also governs the

extent ofP fixation. In acidic brown hill soils of Himachal Pradesh the adsorption of P from the solution decreased in the following order: NH4

Hl04• K1-~P04' NalilO4 (Singh, 1972). The availability of adsorbed -P was greatest from NH4H,_P04• The Ca and Na saturated soils of Uttar Pradesh retained P from different P fertilizers in the following decreasing order: KHl04, NB4H,_P04, Ca(Hl04)2. (Misra and Ojha, 1969a): H­saturated soil retained more P fram NH

4l\.P04 than from the other two sources. In black

and laterite soils of Dharwad (Karnataka), P availability was higher on the application of superphosphate than nitrophosphate or- fused Mg-phosphate (Gumaste and Patel, 1:970). In acidic soils of Baruipur (West Bengal), the P fertilizer availability was in the following decreasing order: calcium phosphate, potassium phosphate. aluminium phosphate, Ferric phosphate (Basu and Mukerjee, 1972). The P fixation was more from diammonium phosphate than di-calcium phosphate in red and black soils of Madhaya Pradesh. On an average 79% of P applied as diammonium phosphate was fixed whereas only 62% was fixed from di-calcium phosphate (Dravid and Apte. 1975). These studies show that P fixation and availability from different P sources is governed by the accompanying cations, their solubility and the natme of the soil.

Organic matter The effect of organic matter in reducing the P fixing capacity of soils is well-known

and is discussed under the methods for preventing P fixation in soils.

Mechanical composition of'soils The size of soil particles plays an important role in P fixation. The sand, silt and clay

fraction of micaceous alluvial soils ofKanpur (Uttar Pradesh) fixed 2.5. 13.1 and 587 ppm P j .respectively (pathak et al., 1950). The P retention capacity of acidic and alkaline soils of Mysore was directly correlated with the surface area of the SQil (Prakash and Bhaskar, 1969). TheP fIXation by representative soils of Uttar Ppldesh wasassociated mainly with clay and silt fractions and their cation exohange capacity (Awasthi and Pathak, 1971). Nearly 90 to 98 % ofthePsorption in three acidic soils ofMaharashtra was due to the soil clay (KeneandDeshpande. 1972). Tbe1clayfracuon was the dominant factor responsible for P fixation in major soil groups of· India (Nad et al ®1975). Clay and free iron oxides together explained 76-77 % and clay and organic c3rbon 69% variation in P fixation in these soils. Negative correlation between available P and clay content in alluvial soils of Uttar Pradesh also indicates that clay is the main seat of faxed P (Srivastava and Pathak. 1970). From these studies it is evident that P adsOrption increases .

FACTORS AFFECTING PHOSPHORUS FIXATION IN SOILS 27

with decreasing particle size. More P fixation would be expected in the finer fractions with their large specific surface and a large number of active ions at the lattice edges, but in laterite soils, unlike the common belief, the coarser fractions of the soil also contribute appreciably to the P fixation capacity of soils.

Moisture The P fixation and transformations in different soils are greatly influenced by the

moisture particularly in waterlogged rice soils. Tile available P increased with increase in soil moisture in black and alluvial soils of Mad hay a Pradesh. Minimum and maximum P fixation was at 60 and 10% soil moisture, respectively (Vyas and Motiramani, 1971). The alternate cycles of wetting and drying increased P retention in black and laterite soi Is. The increase was greater in the laterite than the black soil (Misra and Gupta,1971). In latosol, red, black and alluvial soils of Tamil Nadu, availableP decreased with increasing moisture level during incubation (Raja andBhavanisankaran, 1973). In calcareous soil of Bihar, under waterlogged condition considerable amount of added 50 ppm P WilS released at 4 days but fixation occurred at 60 days (Sahay et al., 1969). In laterite, alkali, recent alluvial, mar and alluvial soils of Uttar Pradesh, the rate at P transformation was higherin waterlogged soils than in soils at 50% field capacity, except for AI-P which was higher at 50% field capacity in mar and alluvial soils (Singh and Ram, 1976). The transformation of P added to red, laterite, alluvial, hilly and saline soils from different agroclimatic regions of West Bengal to different P fractions such as AI-P, Fe-P and Ca­p was enhanced by waterlogging (Debnam and Hajra, 1972). The available P as well as inorganic. P fraction increased with waterlogging of Alfisols of Himachal Pradesh. Maximum increase 0[70.7% was in Fe-P (Verma and Tripathi, 1982). The repeated cycle of alternate waterlogged moisture conditions lowered AI-P in acidic and neutral rice soil of West Bengal (Mandai and Khan, 1975). The continuous waterlogged conditions decreased Fe.-P content of neutral soil during the initial stages but there was increase in Fe-Pin both types of soils after 11 0 oays of incubation. The,saturated moisture condition brought about an overall decrease in the content of Ca-P in both types of soils. However, the moisture conditions priorto flooding of rice soil of West Bengal did not influence the transformation of applied Pas Al-P (MandaI and Khan, 1976). The transformation into Fe-P was lower under submerged treatment than the air dried and moist treatments. The later treatffient was found to cause less fixation of P as Ca-P. The transfonnation into reductant soluble Fe·P was very small. The moist treatment recorded higher dry matter yield and P uptake by the rice crop than the other moisture treatments. In low land rice growing on P deficient submerged vertisoils of Andhra Pradesh, available P increased 2.5 times in wet season as compared to dry season (Katyal and Venkatramayya, 1983). 'These studies show that increasing moisture levels favour P transfonnation into diffeoot P forms, particularly into Al·p and Fe-P and at the same time improve available P content of soils.

Temperature The rate ofP adsorption is tempemturedependent Under sterile conditionlPretention

increased .only slightly as the temperature is increased fonn 25°C to 3Sac. If the temperature is increased up to 1{)()DC, the reaction.proceeded much more- rapidly but the.-

28 PHOSPHORUS FIXATION IN INDIAN SOILS

total retenlion of P may not increase. Under unsterile condition the rate of Pretention depends upon the rate of mineralization of organic matter and P adsorption by micro­organisms. Prakash (1975) reported that coastal alluvium, laterite, red, mixed red and black and black soils of Karnataka fixed more P on heating up to 400°C and their ability to fix P decreased at higher temperatures. The increase in P fixation on heating was probably due to the release of exchangeable Ai from the entrapped soil sites.

4. PHOSPHORUS FIXA 'fION CAPACITY OF INDIAN SOILS

The relative ability of the soils, clay minerals and hydrous oxides to fix. P has been determined by the different methods. There is no general accepted method for the determination of P fixation capacity of soils. By and large, two approaches are generally used: (i) measuring the decrease· in concentration of aqueous phosphate solution equiliberated with soils; and (ii) by measuring the amount of P extractable from soil equiliberaled Wilh P compounds as compared wiLh untreated soils. In the first instance, no attempt is made to define the nature of combination between soil and sol uble phosphate. Since it is conceivable that a part of the P taken up from solution will be available to plants. It has been urged that this method does not give adequate information. Consequently, other methods have been devised which entail incorporating P to soil, allowing fixation to take place and subsequently extracting the soil using plants or reagents capable of recovering readily available P. The assumption is that the added P remaining in the soil is in a sparingly soluble fonn and thus relatively unavailable to plants. As the available P varies with the methods of determination of available P, the P fix.ation capacity will also vary with the method of extraction. For example, the effect of degree. of saturation of soils with Ca on the P fixation depends on the method for dctennining P fixation. A low degree of base saturation tends to give a soil a greater P fixing capacity rendering P in difficultly available forms.

Mukerjee (1943b) suggested a method f'Or the detennination of P fixing capacity by leaching the soil with standard P solution till the P retention capacity of soil is satisfied. Weighed quantity of powdered soil is placed on a funnel. A definite quantity of standard P solution is added. P is estimated in the leachate. When the amount ofP added and the amount found in leachate are the same, further addition is stopped. To detennine any soluble P present in the soil, it is washed with water and the amount of P coming into solution is estimated. From these estimates the amount of P fixed is calculated. The advantages of this method are that dilution of leachate is not required. The buffering effect due to soluble salts are overcome, as the solution salts will affect the P fixation in certain determinations but at the last detennination the pH of leachate will be the same as that of solution. The method gives maximum P fixing capacity of a soil which is an absolute amount and as such the empirical nature of the other methods is overcome. The disadvantages of this method are that a large number of estimations have to be made and secondly it takes longer time. Mukerjee (1943c) suggested that while studying the P fixation by the method of shaking, it is necessary to fix up an arbitrary amount at a value not less than 1000 ppm P, preferably 2500 ppm P, to differentiale between the soils with low and high P fixing capacity. He observed tbatthe study ofP fixing capacity by adding low amounts ofP (200 ppm P) was not suitable for those soils which have high P fixing capacities. Mosi et ai. (1975) proposed a modified Waugh and Fitt's method for major soil series of Thanjavur district (Tamil Nadu) because of wide variations in water holding capacity of soils. They proposed soil to solution ratio based on maximum water holding

30 PHOSPHORUS FV(ATION IN Jl\D1AN SOILS

capacity for determining P fixing capacity. It is difficult to make a comparison of the figures given by various workers for P

fixation in different types of soils and clay minerals, as the amount of P fixed differs with the method used. Mitra and Prakash (l955b) studied P fixation capacity of minerals at pH 11.8 by adding 100 ml of K

3P0

4 solution of 0.02 molar concentration to four grams of the

mineral and shaking for 48 hours. The P fixation capacity of montmorillonite, kaolinite, halloysite, natrolite, vermiculite, muscovite and biotite was 15.66, 2.13, 2.03, 2.33. 10.17,1.61 and 2.76 millimoles PPJIOOg of mineral respectively. The decreasing order in which thc minerals fixed P was montmorillonite, vermiculite, biotite, nattolite, kaolinite, halloysite, muscovite. Palel and Vishwanath (1946) studied P fixation capacity of Indian soils by adding 100 ml ofP solution containing 100 mg pps to 109 of soil. The P fixation capacity varied from 1.54 to 87.05% with a mean value of 33.8%. They classified the Indian soils inlo three categories on the basis of their P fixation capacity: (1) soils with Jow fixing capacity, i.e. less than 20% P fixation capacity were that of North em India from Punjab to Assam; (2) soils with medium P fixing capacity, i.e. 20 to 60% P fixation capacity were that of North-West Frontier Provinces, part of Sind, south eastern parts of Madhya Pradesh and South India including coatal regions in the east and west; and (3) soils with high P fixing capacity, i.e. more lh,m 60% P fixation capacity were the black soils of Madhya Pradesh, Bombay <lnd Deccan. They observed that P fixation capacity was maximum in soils of semi-humid and humid areas (laterite and red soils). On the basis of colour, black soils retained maximum amount of phosphates. P fixation capacity increased with the increase in clay content, total exchange capacity; exchangeable basis, exchangeable calcium and also with Hel soluble silica; sesquioxide ratio. The P fixing capacity of major soil groups varied from 14.0 to 72.4 % with a mean value of 40.7% (Nad et al.,1975). Amongst the various soil groups black, red, laterite, mixed red and black, red and yellow and coastal alluvial soils exhibited higher P fixation than alluvial, grey brown, desert and other soils. The mean % P fixing capacity of these soils was red and yellow soils 72.4, coastal alluvial soils 63.0, black soils 59.2, red soils 61.6, mixed red and black soils 46.5. sierozem soils 38.0, laterite soils 36.7, submountaneous soils 36.0, alluvial soils 30.0, chestnut brown soils 17.0, desert soils 14.5 and grey brown soils 14.0.

The P fixation capacity of erstwhile Punjab soils varied from 7.2 to 63.5% with a mean value of 21.66% of the added 400 ppm P (Singh and Dass, 1945). The highest P fixation was in acid soil ofPaJampur. The calcareous soils also fixed appreciable amount of added P i.e. 30.5%. The P retention capacity of soils of erstwhile Punjab varied from 3120 to 18500 ppm P with a mean value of 6736 ppm (Kanwar and Grewal, 1%0). The P fixation capacity of acidic soils varied from 4840 to 18500 ppm P Witil a mean value of 8090 ppm. The Pretention capacity of alkaline and calcareous soiJs varied from 3120 ppm to 7440 ppm P with a mean value of 4889. The P fixation capacity of acidic soil was nearly double to that of alkaline and calcareous soils. The P fixing capacity waS highest in black soils and mixed yellow and red soils of Rajasthan (Dhawan etal.,1%9). It was the lowest in undifferentiated aUuvial and grey brown soils.

The P fixing capacity of the micaceous alluvial soils of Kanpur on an average was 0.092 mg P/IOOg soil (Pathak et 41.,1950). In soils of sugarcane growing tract of Bihar and Uttar Pradesh 32 to 52 % of the applied P was fixed up within 24 hours and it increased graduall y up to 30 to 35 days (Gupta, 19(5). In soils from Bihar P {'matioo ranged between

PHOSPHOROUS FlXA110N CAPAC1TYOFINDlAN SOILS 31

25 to 90% of the applied P \ViLhin 120 days. The P fixation capacity of black soils was higher than red and laterite soils of Uttar Pradesh (lv1isra and Ojha, 1969a; Misra and Gupta, 1971). Under waterlogged conditions 90% of the added P was retained by black and re',d soils after 30 days (Misra et al.,1970). 54.S to 99.3% of the applied 200 ppm P was fixed after three days in slightly alkaline alluvial soils of Uttar Pradesh (Srivastava and Pathak, 1972). Acid soils of Bihar fixed 80 to 90% of soluble P applied at the rate of 50 ppm over a period of 30 days (Chattopadhayaya and Kat, 1973).

Omanwar et al. (1961) determinedPfixation capacity of soils from Nagpurdivision by adding 200 ml of KHl04 of 1000 ppm P to 20 g of soil and shaking at intervals for 20 days. The P retained by the soil was considered as fixed. They reported that P fixing capacity of soils was high and varied from 23 to 93% of added P. Soils of Chanda and Bhal1dara districts had lower P fixing capacity as compared wilh soils of Vcotmol, Amravati, Wardha, Nagpur. Buldana and Akola districts. Soils of YootolJloJ, Buldana, Akola and Nagpur districts had the highest P fixing capacity. The P fixation capacity increased with increase in clay, cation exchange capacity, exchangeable calcium, free calcium carbonate and HCl soluble sesquioxides content-of soils. The P fixation was more in the black than alluvial soils of MadhayaPradesh (Dravid and Apte.1975). Nearly 73% of the applied P i.e. 84 to 252 kg P pJha was fixed in black soils whereas 68% in alluvial soils. The lowland rice soils of Gujarat iixed 26.4 to 47.2% of applied 25 to 150 ppm P with an average value of 39.6% (Patel; 1975).

The soils from Dacca (pH 5.2) and Berhampur (pH 4.6) fixed 45 and 85% of added P at the start of incubation and it increased to 70 to 95 % respectively after 6 weeks of contact (Ghani and Islam, 1946). In acid soils (PH 4.4 to 6.2) of West Bengal 30 to 50% of added 225 kg P/ha was fixed (Kar and Chakravarti, 1969) and in acid soils of Baruipur (West Bengal) 66 to 99% of 25 to 100 ppm applied P with a mean value of 90% (Basu and Mukcrjee, 1972). lrl acidic lowland rice soils of West Bengal 86 to 100% of the applied 25 to 218 ppm P was fixed (MandaI and Khan, 1972),

The P fixing capacity of 40 a~idic soils of NUgiri varied from 1111 to 100% of.added P with a mean value of79.6% (Mathan and Durairaj, 1967b). The P fixation capacity was not related with pH, Ca. Mg, rainfall or elevation but gave negative correlation with Ca­P and occluded Fe-P. In alluvial soil (Aduthurai), P fixation increased from 8.4% to 68% with increase in P rate from 23 to 1120 kg P Plha (Raja,1967). In black soil (koilpatti) the P fixation increased from 61 % to 83% with an increase in P rate from 79 to 1120 kg PpJha. In red soil (Coimbatore), P fixing capacity increased from 68 to 97% with the increase in P dose from 23 to 45 kg P :/.0/ha, and again decreased to 25% at 1120 kg P p, /ha. In latosol (Nanjanad), P fixation increased. from 81% to 95% with the increase in Pdosefrom225to675kgpp/haand again decreased to 87% of1l20kgPp~(ha. The black soils had the highest P fixation capacity followed by the red and laterite soils of South India: (Mustafa and Durairaj, 19(8). Most of the applied P (87 to 218 kg Pjha) to the soil from Hebbal tank was fixed by the soil {Uter 30 days (Muddanna and Mallaiah, 1971). The typical coffee growing soils of South India fixed 60 to 700 ppm P with an averagevalueof3,:?7ppmP(BiddappaandRao, 1973). ThePfixation was high in latosol, medium in red and black soils and low in alluvial. soil in. Tamil Nadu (Raja and Bhavanisankaran, 19(3). It was the lowest in red soils when very high rates ofP were applied.The major soil series of Thanjavur district (Tamil Nadu) retained 48.9 to 98.2%

32 PHOSPHORUS FIXA nON IN INDIAN SOILS

of the applied P with a mea.n value of76.1 when available P was extracted with Bray PI method (Mosi et 01.,1975). The P retention capacities ranged from 51.8 to 84.9% with an average value of 72.8 when available P was extracted with Olsen's method. They suggested that soil moisture should be kept at water holding capacity for determining their P fixing capacity. The black soils of AndhraPradesh adsorbed more P than red and laterite soils (Krishna Kumari e/ ai., 1985). The above studies indicate that P fixing capacity of most of the lndiart soils is high but the nature of P fixalion depends on the method used to delennine it and the soil type. In general, acidic, black and laterite soils fix more P than slightly alkaline alluvial soils.

5. AVAILABILITY OF FIXED SOIL PHOSPHORUS AND METHODS TO DETERMINE A\rAILABLE SOIL PHOSPHORUS

The P retained by the soil is generally considered as fixed P, although a pan of it can be utilized, depending 01'1 plant species and is exrracted by various reagenl~ being used to estimate available soil P. The rate of release of P from fixed form to replenish the immediately available soil P is the most important factor in determining the P supplying capacity of the soil because the quantity of P present in soil solution is not suflicicnt to meet the crop requirements. The release of fixed soil P depends upon the nature of its fixation ~md the extracting power of the crop or reagent used LO determine availability of soil P. The P is generally fixed as Fe-P and AI-P in acidic soils and Ca-P in alkaline soils. The fixed P in these forms was hitherto considered unavailable to the plants but recent studies conducted by a number of workers indicate that Ca, Al and Fe bound P is being utilized by the crops.

In representative Salls of Uttar Pradesh, available P determined by Olsen and Bray PI methods was highly correlated with Al-P exuacted with ammonium flouride (Srivastava andPathak, 1968). TheavailablePextracted with Bray P, and Truog's methods was highly correlated with Fe-P. In another study the available P determined by Olsen's method wac; positively correiated with absorbed P, AI-P and Fe-P (Srivastava and Pathak, 1971b). The available P determined by Bray PI method was positively correlated with absorbed P and AI-P. Available P determined by Bray-Pz method was positively correlated with absorbed P, AI-P and CaMP in linear correlations and with AI-P and Ca-P in mUltiple regression. In slightly alkaline soils of Uttar Pradesh, the saloid bound P had highly significant positive correlation with available P (Srivastava and Pathak, 1972). The available P was also significantly and positively correlated wilh Fe-P. The Fe-P and AI· P in alluvial soil of Delhi were good sources to tomato plants (Datta and Khera, 1969). Al­p and CaMp werc closely related to P contcnt of lettuce and wheaL, 'A' values and available P determined by Olsen and Bray PI methods (Patil and Datta, 1971). The P uptake by plants was highly correlated with Fe-P in soils from Varanasi (Gupta and Singh, 1969). In alluvial soils of Delhi, Ca-P and saloid-P in the surface soil and Ca-P and Fe­P in the subsoil· were the important forms conrributing to the grain yield of wheat (Ramamoorthy and Bisen, 1971). Ca-P and Fe-P in the surfaces soils and Ca-P and AI· P in the subsoil were important P forms for rice, The energy required for the absorption of P by wheat from Fe-P and saloid-P was less than that from Ca-P, In case of rice the energy required for P absorption was less from Fe-P than that from Ca~P. Rice had a greater capacity than wheat for extracting P from Fe-P. AI-P was utilized by both rice and wheat while rice utilized Fe·P better than wheat (Goswami and Kamath, 1984). In well drained sandy loam alluvial soils of Delhi, saloid-P, Ca-P and Fe-P in the surface soil and Ca-

. P in the subsoil contributed positively and significantly to the available P extracted by I Olsen's method (Bisen and Ramamoorthy, 1971). The available P determined by Bray PI method was positively and significantly related to Fe-P and AI-P both in the surface and

34 PHOSPHORUS FIXA nON IN INDIAN SOILS

subsoil. In medium black soils ofCoimbatore, Fe-P l_\nd Ca-P contributed positively and significantly to available P extracted by Olsen's meLhod, while Bray PI was positively and significantly related to Fe-P in surface soil and AI-Pin the subsoil (Bisen and Ramamoorthy, 1973). AI-Pexplained most of the variations in availableP, followed byCa-P and adsorbed P in surface soil of Uttar Pradesh (Misra and Verma, 1979). In alkaline alluvial soils with high Cacontent, water solubleP, fixed as Ca-P and Fe-P was associated with yield of barley and rice, respectively (La! and Mahapatra, 1979). The Al-P was the main source of Olsen or Bray extractable P in alluvial and black soils under both cropped (wheal-rice) and uncropped conditions, while in red soils, Fe-P was the main source under cropped conditions but AI-P under uncropped conditions. The available P determined by Olsen, BraY-P

J and water soluble methods had significamly positive correlations with Al-P and

Fe-P but not with Ca~P in paddy soils of Bihar (Duha and Joshi,1973). After prolonged Waterlogging Al and Fe-P was not correlated with available P determined by Bray PI method but available P determined by Olsen's method was still correlated. In calcareous soils of Haryana, Fe-P contributed significantly to available P, followed by AI-P and saloid-P. The contribution by Ca-P was negligible (Tomaret al.,1984).

The extraction with NaHC03 released more Fe-P retained in the red soil and extraction with HzS04 released more Ca-P retained in the black soils (Misra and Ojha, i969b). The release of retained P by citric acid was comparatively less in both.the soils. The saloid-P and AI-P showed significant correlation with available P i.e. r =0.924 and r = 0.807 respectively, in selected soils of Uttar Pradesh (Bhan and Shankar, 1973a). The saloid-P and Ca-P fractions in Bundelkhand soils (Uttar Pradesh) were significantly correlated with P uptake by paddy in alkaline calcareous soils and the correlation co­efficients were 0.801 and 0.556, respectively (Bhan and Shankar, 1973b). The available soil P determined by Olsen's method had highly significant correlation with saloid-P in alluvial and Bhat soils of Uttar Pradesh (Singh and Pathak, 1972, 1973a). Other fonus of P such as Fe-P, Ca-P, reductant soluble Fe-P and Al-P also had significant positive correlations with available P. .

The available P detenuined by Olsen' s method had positive correlation with AI-P and Fe-P in normal alkaline saline soils of Western Rajasthan and with saloid P, Ca-P and AI-P in sodic alkali soils of Punjab (Mehta et al., 1971; Bajwa et al.,1982). In well­developed, moderately developed and young soils of Mewar, Bray and Morgan's extractafitsrecoveredmostly Al-P(Deo and Ruhal,1970).IncalcareoussoilsofRajasthan, AI-P and adsorbed P was significantly correlated with available P extracted by Olsen and Bray's methods (Deo and Ruhal, 1972). Test plants utilized significant amounts of AL­p (r= 0.81), adsorbed P (r = 0.77) and Ca-P (r = 0.77). The maize plants grown on wide range of soils from Rajasthan significantly affected the adsorbed and AI-P fractions but had little effect on Ca-P and Fe-P (Singh and Saxena, 1969). There were significant positive correlations between P uptake by maize and the soil levels of adsorbed and Al­p and organic P in soils of Rajasthan (Singh and Saxena, 1970). The AI-P, Ca·P and Fe­P fractions in Rajasthan soils were the available source ofP to various crops (Choudhari et a/., 1974). AI-P had significant correlations with P uptake oy maize, jowar, cowpeas and urd; Ca-P had significant correlations with P uptake by jowar, cowpeas and urd; Fe-P had significant correlation with P uptake by jowar and urd. Olsen's P, Morgan's P ingredient elution exhibited a significant correlation with AI-P, indicating that AI-P was available

METHODS TO DE'mRMINE SOIL PHOSPHORUS 35

to the crops; Ca-P was also correlated wiLh Morgan's p, The saloid-P and AI-P had significant correlation Wilh Olsen and Morgan's available P in most of the textural classes soils of Rajasthan (Sacheti and Sa.xena,1974). Ca-P showed significant correlation in fairly large situations willI Olsen's and Morgan's available P. Fe-P was correlated significantly in a few cases only. In sandy loam soils, all the four inorganic P fractions were significantly correlated with available Pby both the metlJods, Saloid P and AI-P were also significantly correlated with equilibrium P potential in sandy loam soils. In north-western Rajasthan flood plain soils Ai-P had significant correlation will) available P content of soils (Talati ef

al.,1975). The AI-P and Ca-P of rice soils of West Bengal had significant correlations with

available P determined by Truog and Bray's methods (Khan and Mandal,1973). In soils from West Bengal on the basis of activity, Al-P was more available than Fe-P and Fe­P more available thun Ca-P (Debnalh and Mandai, 1982a). The availability of transformed fertilizer-P was morc than that of respectivcforms of native P in soils of West Benga.1. The specific activity of Al-P increased up to 15-30 days and then decreased,whereas tlJal of Fe-P showed two peaks and Ca-P decreascd constantly on ageing (Dcbnath and MandaI, 1982b).

In acidic brown hill soil of Shimla district,. AI-P and Fe-P were important source of P to potato (Sharma el ai" 1979). In Alfisols of Himachal Pradesh, Fe-P was the most variable contributing to total variation in the available P determined by Olsen, Bray PI Bray P2 Truog, Peech and Morgan methods and Ca-P in the North Carolina metlJod (Verrmi and Tripatlli, 1982). AI-P was tlJe second important variable contributing to the variation in the availableP determined by Olsen, Bray PI. Bray Pzand North Carolina methods and Ca-P in Peech and Morgan methods. In some Himalayan acid soils of north-west India Olsen's extractant removed most of P from Al-P fraction, followed by Ca-P fraction (Sharma and Tripathi, 1984). Multiple regression analysis showed that about 65.7% variation in Olsen P was accounted for variation in Al-P fraction alone. Bray's procedure derived P mainly from Al-P. followed by Fe-I> and Ca-P in almost equal proportions and the extent of variability explained by these fractions was 50.5% . The most important source of P for Bray P2extractant was Ca-P and about 36.9% variation could be attributed to it. In acidic soils of Jorhat (Assam), the release of both native and applied P, increased with pH of soil (Nath and Borah, 1983). The total release of applied fixed P was greater than that of native fixed P. The release of P increased progressively during 45 days of incubation. In acidic hill soils of Himachal Pr,adesh, At-P was the most important inorganic P which contributed to P nutrition of lentil and it was also dominant P fixation source against Olsen's extractant (Sharma et ai., 1985). From these studies, it can be concluded that part of the fixed P can be utilized by the plants. However, the availability of fixed P may decrease with the lapse of time. Thus it is important to know the nature ofP fixation in different types of soils to ascertain their effect on P availability.

Besides the nature of P fixation, the soil moisture conditions greatly influence the availability ofP from fixed forms. The P availability increased in red, black and alluvial soils with increasing soil moisiure up to 200% field capacity (Prabhakar et al., 1974). The effect of moisture level was more pronounced in red sandy loam soil than black clay loam or alluvial paddy soils. In general, lowland rice showed much lower response to P than the upland crops (patrick and Mahapatra, 1968). Transformation of native soil P under

36 PHOSPHORUS FlXA 110N IN INDIAN SOlLS

submerged condition lead to considerable incease in its availability, as for'the mechanism of P release in flooded soils, the conversion from reductant soluble fraction to ferrous phosphate was considered to be of great importance. Silt and clay particles under waterlogged condition became coated with Fe(OH)3 which on reduction to Fe(OH)/eleasc occluded phosphate. The reduction by bacteria, displacement ofP from Fe-P and AI-P, anion exchange llrc other possible mechanisms of P release in flooded soils. The waterlogging decrcase<.: total reactive P in paddy soils of Bihar (Dhua and Joshi, 1973). Ca­P increased considerably after 7 days but decreased subsequently. There was a net increase; in Ca-P in alkaline soil. Sa\oid-P, AI-P and Fe-P I.lecrcased after prolonged waterlogging. The submergence of rice soil ofGujarat, increased the availability of native P (Patel, 1975). The average increase was 16.6, 26.7 and 33.4% after 7 , 14 and 28 days, respectively. The waterlogging of acid soils of Pal am pur increased the available P after 20 days (Singh amI Bahaman, 1976). A similar trend in available P was also observed under upland soil conditions. The waterlogging increased P availability for paddy crop. The waterlogging increased AI-P and Ca-P availability after Olle day and that of Fe-P gradually in loamy alluvial paddy soil of Baruipuf of West Bcngal (BaSH and Mukherjce, 1969}. Addition of ferrous phosphate to waterlogged system decreased P availability from both Ca-P and AI-P, Drying after walerlogging produced a significant decrease in P availability but waterlogging led to a net increase, The P availabilty in alluvial soil reached a maximum after 32 days of flooding (Ba<;u, 1974). The alternate submergence and drying oflaterite rice soil of Poona increased available P in clay loam, loam and sandy loam soils but not inclay soils, probably due to the presence of rclativ ely more native Fe in clay soil (Savant et al.~ 1970). 1be repealed cycles of alternate waterlogged-saturated moisture condition lowered available P in rice soil of West Bengal but continuous waterlogged condition was beneficial for the availability of soil native P in acid soils (Mandai and Khan, 1975). The moist condition resulted in higher dry matter yield and P up lake by the rice crop than the submerged and air-dried soil conditions prior 10 flooding (Mandai and Khan, 1976). From these studies iLcan be ascertained that waterlogging increased available soil P either by reducing the P fixation or by increasing the availability of native and fertilizer P.

Methods for the determinatioll of available soil phosphorus

For the evalualion ofP fixation in soils, it is a pre-requisite to know t];;:; suitability of the melhods for extracting available P in different types of soils and for different crops. A number of methods have been proposed from time to time to assess the available P status of soil but none has proved to!>e ideal. The available soil P is usually determined by empirical methods using different extractants, applying principle of chemical potentials, detem1ining 'A' values from tracer studies.

The avaiIable P determined by the method of Olsen et al.(1954) was a better index of available P than Bray-P (Bray and Kurtz, 1945) in soils of Punjab with different fertility gradients (Singh and Brar. 1973). This method was also quite suilable for detennining available P in alluvial soils of lullundur (Punjab) with respect to potato (Grewal and Singh, I 976). The critical limits of available P for autumn and spring crops of potato were 10 and 13 ppm, respcctively. Dev (1984) reported that Olsen P can satisfactorily predict P deficiency in soils of Ludhiana district (Punjab) for rice (critical values being 4.5 ppm)

lvlETHODS TO DETERMINE SOIL PHOSPHORUS 37

and Bray-l for neutral to slightly alkaline soils of north-west Punjab for wheal and rieL (critical values being 8 and 6 ppm rcspeclively). In a study on acidic brown hill soils of Shimla region, Grewal and Sharma (1979) observed that Olscn's as weU as Bray P l

methods could predict the available soil P for the potato. The critical limits \VCfe 16 ppm P by Olsen's method and 35 ppm P\vith Bray PI method. The Oisen and Bray P methods were suitable for estimating available P in soil of Himachal Pradesh with pH 4.9 to 7.4 (Sharma el al.'fJ 1980). The available P was slightly associated with organic maller and clay content of soils.

The Olsen and Bray PI extractants were the most satisfactory for determining available P for lettuce and wheat in soils of Delhi (PatH and Datta, 1971). According to Ramamoorthy and Bisen (1971) Olsen's extractant was more suitable than Bray's PI method for wheal in alluvial soils of Delhi. For rice neither Olsen's nor Bray's extIacrant." were suitable. The Olsen's melhod and isotopically exchange P extracted similar forms of P from alluvial soils of Agra district (Tripathi el at., 1 Y70). The Olsen's mcthod was better than Bray's meUlOd for determining P needs of crops. The Olsen '$ method was more reliable Ulan BraY-Pl. Bray-Pz and Truog's method 'for determining available P in Bundelkhand soils of Uttar Pradesh (Bhan and Shanker,1973 b).The available P detennined by Olsen's mcthod was significiilllly correlated wim dry mattcr yield and P uptake by paddy.Srivastava and Jaffi (1974) compared Olsen's and Bray's methods of extracting available P with' A' values and per cent yield response using sugarcane as a test crop on eleven soils from Uttar Pradesh, Bihar,Haryana and Punjab. They reported that Olsen's and" A" values were significantly correlcated with per cent yield response. The Olsen's method was significantly superior Lo Bray's extractant. In slightly alkaline soils of Uttar Pradesh Olsen's method was suitable for available P estimation with respect to rice crop (pathak et aL ... 1975). . '

The Olsen's procedure was suitable for detennining available P in soils of Rajasthan with respect to maize. sorghum, cowpeas, urd, wheat and peas (Saxena, 1971). Morgan's procedure was also satisfactory for maize. sorghum and cowpeas. TheOlsen's method was superior to Bray's and Truog's methods for detemlining available P in non calcic brown soil group of Rajasthan with respect to wheat crop (Gattaniand Seth, 1973). In comparison with Olscns's method, Bray's and Tmog's methods could predict availability of soil P to wheat crop in only 75% and 60% cases, respectively. The critical limits for wheat with Olsen. Bray's and Troug's methods were: 10.5, 15.0 and 20.8 ppm P, respectively. The modified Olsen method i.c. 0.5 m NaHC0

3 at pH 8.2 was the best

for available P detennination for rice in black soils ofCoimbatore (Velayutham and Jain, 1971). The available P detennincd by OIesn's method in lowland rice soils (sandy loam to sandy clay loam) of Gujarat had significant correlation (r = 0.69) with P uptake by rice (patel, 1975). The Olsen's P was a better measure of available P than Bray, PI in major coconut growing acid soils of Kerala State (Deb et at., 1974). Both Bray p] and Olsen's methods were equally good in soils of Kalathur, Adanur, Ncdumbalam and Madukhur series of Thanjavur district of Tamilnadu (Mosi et al., 1975). Olsen' s extractant was found especially suitable for the soil of SHear and Avadakerkoil series while Bray Pl was found suitable for soils of Padugai series.

The mineralogy of the soil influenced the predictive value of the Olsen's test In

alluvial, red and black soils (Deb and Datta, 1971b). The predictive value could be

38 PHOSPHORUS FIXATION IN INDIAN§OILS

increased when soil pH and degree of P saLUration were also considered. Goswami el a/. (1971) compared yield data of wheat and rice with soil P lest (Olsen's method) and showe{\ thatthe criticalP values for rice and wheat varied with the soil. For rice the critical limits were 6.5 to 8.7 kg P/ha for red, 13 to 20 kg P/ha for mixed red and black and 29.3 to 30.0 kg P/ha for coastal alluvial soils. The limits for wheat were 7.0 to 8.7 kg P/ha for alluvial Or grey brown soils and 21.4 kg P/ha for black soils. The correlation between soil test data and yield was low but significant, the highest or' val ue was 0.7. Khera and Datta (1969) reported that methods using NaOH.N'lzCP4' HCI-NHl were suitable for measuring available P in soils of Delhi containing high amounts of Ca-P. NaHC03 extractant was comparatively less suitable .. Acid extractants were not suitable as these led to dissolution of high amounts of Ca-P which had highly significfmtcorrclation (r:: 0.95) with P fixation capacity of soils. Mild acidic solution proved most satisfactory and did not attack Ca-P in extraction of the soils.

Trllog (1930), Bray's and Olsen's methocls were suitable for available P dctcnnination in red, black and laterite soils when rice was the indicator crop (Rao and Rao, 1963). The Trnog' s method was the most efficient for all the soils. Bray's method was superior to Olsen's methbd on the red soils. The application of Neubauer's technique to representative acid soils of south India using ragi (Eleusine coracllnCl) seedlings gave results similar to those obtained by using Bray's Pzand Olsen's methods (Vijayachan­<Iran and Raj, 1973). Bray's extractant was most suitablefor dete~ning availableP from the acid soils of the Maluad tract of Mysore with respect to the wh¥t crop (Subramaniam, 1971). The gradient elution technique could correctly depict the available soil P in Rajasthan soils (Sacheti and Saxena, 1968 ). The gradient elution pattern can possibly be put to two uses. In first instance it will be possible to determine quantitatively the inorganic P components of soil individually. The second and more important is the estimation of most mobile or available P of soil. There were significant correlations between Morgan's P, Bray's P and uptake ofP by maize plants with the fITst and fifth peak of gradient elution curve (Singh and Saxena, 1968 ).

From these studies it can be concluded that Olsen's method is suitable for determining available P in most of the Indian soils. BraY-PI method is also promising for determining available P in many soils, particularly in acidic, hill, red and laterite soils. The critical levels of available P differ with the soils and crop species.

6. METHODS TO CHECK PHOSPHORUS FIXATION IN SOILS AND PRACTICAL SIGNIFICANCE OF PHOSPHORUS FIXATION IN SOILS

The recovery of the P fertilizers by most of the crops ranges between lOand20%. The fixation of P by the soils has been considered the important reason for the low recovery of applied P fertilizer. There is an urgent need to reduce the P fixation and increase its availability to plants. The available information on this aspect is reviewed and discussed.

Liming Liming the acid soils has been considered as one of the important methods to reduce

P fixation capacity of the soils and to increase the availability of native soil P. Ghani and Aleem (1942) incubated an acidic soil (pH 4.7), containing very low amount of available P, with calcium carbonate, calcium hydroxide, calcium sulphate and magnesium oxide, each at the rate of 2.5, 5.0 and 7.5 tonnes per acre. Transformation of soil P was studied by fractionating the sample at the interval of four, six, eight and ten weeks.

The available P regularly increased with time in all the treatments at all doses. The order of effectiveness in increasing P availability was in the following decreasing order MgO,Ca(OH\.CaCO).CaS04• The organic Pdecreased in all cases and the effectiveness of the salts in decomposing organic P was similar to the order of increasing P availability. The treatment did not produce any significant change in other fraction ofP, such as Fe· P, AI·P and apatites, indicating that the improved availability of soil P caused by liming was due to the decomposition of organic P and not because of chemical interaction of liming material with the Fe·P and AI·P. Ghani and Islam (1943b) reported thatP fixation can largely be counteracted by the application of lime to acid soils with P fertilizers. Lime rt:duces P fixation by controlling the rate offonnation of insoluble Fe·P and AI-P by increasing the pH of the soil. The application of lime lowered P fixation by Fe and Al but increased P fixation by Ca in acidic lowland rice soils of West Bengal (Mandal and Mandai, 1973). The application of Ca(OH\. KOH and Ca (NOJ)2 increased available P in acid soil of Bihar and West Bengal (Joshi and Dhua, 1972; K8Jj1974). Ca(OH)z was the most effective due to its Ca content and increase in soil pH with the addition of OR ions. In a red soil (pH 5.3). which was deficient in P, the addition of lime in the initial stages increased the release ofP which was taken up by the crop or leached into lower tayel:! and ultimately decreased available P in the soil P content (pal,1943). The addition oflime to acid soils along with P fertilizers or organic matter is needed to improve P availability. When an acid soil is limed, two processes operate in releasing the inorganic P to soluble form, viz. (i) hydrolysis of the Fe-P and AI-P leading to the foonation of Fe and Al hydroxides and increase in the equilibrium concentration ofP in solution, and (ii) increase in the rate of mineralisation of organic P. The increase in P solubility due to the acidification of calcareous or alkaline soil has also been reported. The opposite effect of decrease in P solubility on liming which is observed in many cases may be attributed to either (i) fonnation of solid phase Ca·P or low P solubility due to over liming or (ii)

40 PHOSPIIORUS FIXATI0~~ IN INDIAN SOILS

dccrca~c in the proportion of onho-phosphate prtscnt as HlO~, due to pH risc or both. MandaI (1964) studied the effect of lime on lIw transformation of inorganic P in

waterlogged rice soils. In the prescnce of lime o\,v'ing to hydroly~is of ferric and aillminium phosphate their amount (!el;fcased more in the former than the latter. The amount ofCa-P in soil jncreas(~d appreciably by liming. The effect of lime was to convert some of the ferric phosphate to calcium phosphate. It is felt that in acid soils having most of the inorganic P as ferric phosphate: the use of lime followed by the addition of organic maller will rcsult in an increase in the availability of soii P under a waterlogged condition. The liming of laterite soils increased the release of available P. l\'laLlian and Raj (1975) observed that lime produced an initial increase in U1C available P content of laterite soils ofNilgiri (Tamil Nadu). The liming improvclllhe efficiency of supcrph(1sphlltc in paddy soil of Maharashtra (Puranik ami Bapal, 11)75). Debnath :md i\landal (I %3) n:poncd lhal liming II ad no significant effect (In inorganic P fractions of acidic laterite and alluvial soils of West Bengal. however, it tlecmased the surface P and ~;lh.:ciric surfaces activity of AI·P and Fc-P but not or Ca-P. Liming increased !he rctcllI.ion of applied P from KHl04 in saloie! hound form but sllppressed its transformation to AI-P, Fc-P mHI reductant soluhle forms; the transformation to Ca-P rL~maincl! una!ll~ctell in the alluvial soils but increased slightly in laterite soil. De el al. (1974> observed thar calciulIl sail application significantly increased the availability ofP in highly alkaline soil by stimulating microbial activity. The order of effectiveness was: calcium nitrate> calcium sulphate> calcium citrate> calcium acetate> calcium sulphide> calcium oxide> calcium carbonate. The above results Lhus stress the need of liming to acid soils to bring the pH between 6.0 to 6.5 to increase the availabililY of native P and decrease the fixation of added soluble P.

Organic matter The role of organic malter in releasing Fe and Al bound P from acid soils has been

vigorously studied. The aliphatic and aromatic hydroxy acids, humates and lignin components of organic matter can prevent or reduce the chemical combination ofP with Fe and Ai. The decomposition of organic maller by HP2 or through microbial activity decreases the P fixation. P fixation capacity of kaolinite and montmorillonite depends upon the nature of their surface coatings; iron oxide coating increased whereas humic acid coating decreased the P fixation. The presence of large amount ofhurnic acid in soil decreases P fixation. A large Ca/Fe ratio which increases the efficiency of P fertilizer can be obtained either by keeping the adsorptipn complex saturated with ea in a slightly acid medium or by blocking the P fi?\.ing sites of Fe componnds with humic acid.

There was increased P fixation by alluvial soils after the destruction of humus with the use of HP2 (paUlak el al.,1950). The presence of humus decreased the P fixation partially by saturating the secondary valencies of the mineral lattice and partially by cementing the soil particles together. The addition of crop residues and green manuring or mixing of superphosphate with organic manure before its application increased the availability ofPfertiiizers (Dun, 1961). The phosphate ions are adsorbed by the organic colloids because organic colloids have many times higher adsorption capacity than inorganic soil colloids. The phosphate ion adsorbed by organic colloids are easily available to plants. The superphosphate treated manures added to the soil undergo rapid microbial decomposition and make their P readily available to crops.The addition of 10

\1FJIlODS TO (,!lECK PIIOSI'IIORUS FIXATION IN SOILS 41

Vila of farmyard manure along WiLh 90 kg P to Imerite soil of Nilgiri which had large amount of free scsqllioxitles ami of high P fixing capacity, incrc:l,c(] significantly lhl: yield of barley (RajagopaJ and Idnani, 19(3). The deactivation IJf free sesquioxides through chelatioJl and blockillg Lhe P fixation sites of clay by organic matter increased P availability.

Dawl and Srivastllva (1963) discussed the role of organic matter in reducing the intensity of P fixation by sesquioxides in acid soils. There were strong interaction betwecn organic matter, pH and s~~squioxides. The sesquioxides were highly correlated with P bonding encrgy. The organic matter reduced the intensity with which P is bonded to scsquioxides. The P bonding energy of two soils having" identical contcnt of free sesquiox ides is detcrmined by their organic maLLer content. In lite Iateri tic soils, poor in organic malter, till:: P bonding energy was high as compared with hilly soils which had th..: same content of se~qllioxilles but higher organic matter content. The high P fixing power of laterites is not only because of high content of sesqllioxides but also clue to low content of organic nlatter. In calcareous ancl neutral soils of Indore district, the P availability was positively correlated with their organic matter content (Iyer and Apte, 19{)7). This could be clue LO the reason thm organic matter lIlay decrease P fixation and incrcase P availability by producing CO2 which form carbonic acid with water amI decomposes certain primary soil minerals.

The application of fannyard manure and compost increased saloid-P and AI-P in soils and decreased Ca-P (Srivastava el al., 1969). The P fertilizer applied in combination with organic manures also increased saloid-? and AI-P and enhanced availability of soil and fertilizer P to paddy crop. The application of organic matter at Ig/lOOg increased the available P from native and fertilizer P added to black and alluvial soils of .Madhya \ Pradesh (Vyas and Motiramani, 1971), The application of organic matter as farmyard manure increased available P in alluvial soils (Basu, 1974). The organic manures like compost or finely powdered dry matter of Gliricidia maculata increased available P in red, black and alluvial soils (Prabhakar et al., 1974). The addition of organic matter to latosolic lowland rice soils of West Bengal increased availability of added P during initial periods of incubation but it lowered the amount of AI-P in these soils. It lowered the transfonnation of added P during initial periods of incubation (MandaI and Chatterjee, 1972). It lowered the transformation of added inorganic P into slow and reductant-soluble Fe-P in some soils. Organic matter did not show any significant influence on the transformation into Ca·P. The application of organic matter maintained higher amounts of P in soluble form than control in acid lowland rice soil of Kalyani (Mandai and MandaI, 1973). Application of organic matter lowered significantly the fixation ofP as AI-P and Fe-P.

The application of P digested compost to the soils with high P fixing capacity is suggested because a fraction of P fertilizer a<;lded to compost is transformed temporarily into forms which are not easily fixed in the soil and thus remains more available to plantS (Vinnani, 1966). The application ofP as superdigested compost was better utilized by rice because it increased P availability to plants in soils with high P fixing capacity (Virmani, 1967). The composted P was better than mineral P sources on equal P basis in acid soils, possessing high P fixing capacity (Singh and Subbiah, 1969). The availability of dicalcium phosphate was increased by cofupesting but the composting did not increase

42 PIIOSPHORUS l'lXA110N iN INDIAN SOILS

the availability of supcrphosphal(!. monocalcium phosphate and monoammonium phosphate in black, ~dluvial and ~:ak:arcous soils (Singh. 1970).lncubatiof] of P fertilizer., Wilh cattle dung prior to its application increased available and saloid-P in calcareous soil of Haryana and also improved the rate of mineralization of organic-P (Tomar el aL.. 1984). In wheat-grccngram cropping system coming of superphosphate with slurry increased the fertilizer P utilizal.ion from 14.7 to 19.0% by wheat in sandy loam soil of Delhi (Goswami and Kamath, 1984). These studies show that application of organic manures to soil or treating P fertilizers with organic manures increase P availability to plants especially in soils with high P fixing capacity.

Soil Amendments and Complexing Agents The addition ofS-hydroxy quinoline to acid soils greatly reduced theP fixingcapaciLy

by complexing the active SCS1luioxidcs (Ghani, 1943b). The availability ofP in soil from Karjat (district Koiaba, Maharashtra) of pH 6.8 was adversely affected when sulphur was applied along with superphosphate (Gokhalc el al., 1954). The organic amendments, such as molasses, mO}lIIa flower, and green manuring did not produce any effect when added wilh superphosphate. On the other hand the addition of sulphur with either rock phos!Jha~c or bone-meal increased the P availability. The organic amendments had little effect. Mohua flower depressed the rate of release of P when applied with bone-meal.

-Datta et ai. (1962) studied the effect of E.D.T.A. on the availability of soil and fertilizer Pta wheat and berseem, Application ofE.D.T.A. increased the dry matter yield and uptake ofP by wheat and rice in some soils. The "application of E,D.T.A. was not useful in any soil for berseem. The uptake of fertilizer P by wheat and berseem was not significantly affected. In laboratory study, the soil pH although remained unaffected, yet the available P tended to increase in most soils on the appli~ation of E.D.T.A. In their opinion this was owing to the formation of complexes between Fe, Al and Ca cations and E.D.T.A. Datta and Srivastava (1962) studied the factors affecting the availability of adsorbedP and suggested thai if corrections for bonding energy and saturation percentage are made, adsorbed P may show up highly significant correlation with plant uptake and response. Rajagopal and Idnani (1963) found that E.D.T.A. at 560 kg/ha plus 225 kg PPsfha . from superphosphate gave a significant increase in the yield of barley in Nilgiri soils which had large amount of free sesquioxides and high P fixing capacity. They suggested !.he chelation of active sesquioxides resulted in lessening the fixation of applied p, The application of starch to waterlogged rice soils increased the available P (Mandal, J964). The large amount of CO~ formed due to the decomposition of starch converted some of the insoluble tri-calcium phosphate to more soluble mono-, and dicalcium phosphate. Ghoshal and Chakravarti (1967) treated six acid soils with oxine, a!uminon,N-benzoyl, phenylhydroxylamine and humic acid as chelating agents, in the presence of 0.02 M KCl solution and after equilibrium the solutions were al)a1ysed for P. Appreciable release of P from soils was observed in most cases in the decreasing order: aluminon, oxine, N-benzoyl. phenylbydroxylamine, humic acid. Misra and Ojha (1968) studied the role of complexams. in decreasing P retention capacity of black and red soils of eastern districtS of U ttai' Pradesh. They found that with the addition of complexant there was a, marked decrease in P retention by soils. The decrease in P retention capacity wa~ proporti'onal to the amount of complexant added, the only exception was oxalate

METHODS TO CHECK PHOSPHORUS FIXATION IN SOILS 43

in which the efficiency decreased at higher concemration of complcxant. 8-hydroxy quinolint~, fcrrocyanide, citrate and aluminon had been found to be quite efficient in both types of soils, whereas E.D.T.A. was least efficient. The efficiency of complcxant to inhibit P rctenljon waS greater in black soils than red soils and was directly related to Pretention capacities. The P fixation capacity of red and black soils decreased when glucose, oxalate and ammonium sulphate were added with the P under waterlogged conditions (Misra el 01.,1970). Somani and Saxena (1970) reported the release of native soil P under active microbiological activity in Rajasthan soils. Glucose addition iilcreased the release of P. Soils rich in total P, organic P and with low organic carbon/organic P ratio provided better release. The citrate and to less degree E.D.T.A. salts greatly increase the mobility ofP in black, red and laterite soil. The P mObility increased with increasing concentration of the complexams (Misra and Gupta. 1974).

Raja (1967) applied amendments like farmyard manure, green leaf and lime along with P fertilizer in alluvial, black, red and lalosol soils in Tamil Nadu and found that application of superphosphate along with farmyard manure increased the yield of crops significantly. The incubation studies carried out with the above soils and the amendments including farmyard manure, reinformed compost, ordinary compost, green leaves, ferrous sulphate, lime, sodium sulphate. sodium citrate and E.D.T.A. showed that application of 560 kg green leaf per hectare increased the available P in alluvial soil significantly over control. Application of 5 tonnes of reinformed compost also increased the available P of soil significantly over control but to a less extent than green leaf. Combination of P with green leaf, ordinary and reinformed compost resulted in signifi. cantly higher ::1vailable P than rest of the treatments, indicating thereby a lower fixation of applied P in these combinations. In red soils, compost application significantly increased the available P of the soils. In black soils, application of amendment did not significantly increase the available P of the soil over control. In latosols, application of amendment did not increase the available P of soil apparently; although there was slight increase in case of farmyard manure treatment. Goswami and Krunath (1984) reported that the application of sodium silicate at 150 kglha increased the utilization of P from superphosphate from 11.0 to 20.5% by wheat in loamy soil of Delhi. The application of magnesium chloride also improved the P utilization from diammonium phosphate in wheat-rice cropping system. The green manuring in combination with ammonium phos­phate application was the best for paddy crop in rice soil of Andhra Pradesh (Kareem and Sastry,1966). The use of amendments like green leaf and compost increased P availability of alluvial and red soil of Tamil Nadu (Raja and Bhavanisankaran, 1973).

Many anions have been found to decrease the activity of AI, Fe and Ca in soils by chelation, forming stable 'complexes or by fonning insoluble precipitation products thus helping the P release. Organic anions, particularly. the hydroxy acids, such as citrate, tartarate, oxalate. acetate, succinate and inorganic anions, such as hydroxyl. fluoride, arsenate, ferrocyanide have been fopnd to reduce P fixation and assist in the release of

. native soil P. The fixation of added phosphate is prevented in a varying degree by 8· hydroxy quinoline, humic acid, tannin,E.D. T.A., cueferon aluminon, alizarin. fulvic acids, m-beuzyl and phenyl-hydroxylamine. ThePaddedalone to soil, reacts with Fe, Al and Ca, which are supposed to be the most active P fixers. However, when solubleP is added in combination with complexant to soil,there is a com,Wtition between P anions and the

44 PHOSPHORUS FIXATION IN INDIAN SOILS

complexant to react with Fe, Ai and Ca. The complexants have a greater affinity ,md form stable complexes wilh Fe, Al amI Ca in soils. This results in reduction of P relention capacity of soils. Of thccomplcxanls used. some have a high chclating power for Fe, viz. ferrocyanidc, other for AI, viz. aluminon, yet there are other like 8-hYdroxy quinoline and citrate which have cheiating power for both Fe and Al and finally there are those like E.D.T.A. and oxalate, which chelate Fe, Ai and Ca simultaneously. Consequently, when one Of the other complexant is used along with the soluble P, the soil exhibits reduced P retention capacity. However, the reduction in P reu.,;.ntion capacity can be expected more if the added complexant chclates with the very soil constituent with which added P combines.

AJumillon chelatcs with Al alone and the decrease in retention capacity is owing to an inhibition of the formation of AI-P in the soil. 8-hydroxy quinoline and fcrrocyanide, form chelates with both Fe and Al and the reduction in P retention capacity is due to the less formation of Al and Fc-P in soil. The retained P in acid soils exists mostly in Al-P and Fe-P, the highest efficiency of8-hydroxy quinoline and ferrocyanide in decreasing P retention capacity can directly be relatt<d to the chelation of Al and Fe and a subsequent decrease of Al and Fe-P combinations. The mechanism can be expressed by the following simple equation:

/OH

(AI. Fe)~ OH + complexant

'" H_,PO.

The complexant which can chelate with more than two constituents of soils simultaneously, generally exhibits a poor effect in decreasing P retention capacity. The best example is E.D.T.A. The reason for low efficiency of this complexant may be because of its multifarious activities or due to its small concentration.

It can be concluded that the complexant which can chelate AI and Fe simultaneously. produce better results in decreasing retention of added P in acid soils than those which can fOrm complexes with Ai or Fe singly. The presence of complexant in the soil or their introduction along with applied P may help in keeping more ofP in water soluble state, thereby increased availability of P to the plants. However, a high concentration of a complexant needs to be present. For practical application, the use of complexant has limited value because of their high cost as compared with cheaply available soil amendments like lime, orgfUlic manures and composts and green manures.

Suitability of Pbosphatic Fertilizer To decrease the fixation of applied P and increase the P availability to the plants.

application of different fertilizers in different types of soils has ~een suggested. Basu and Kibe(1943) conducted long-term (10 years) Pfertilizertrialson black cotton soils. low in available P. They found that P manuring can play an important role in enhancing the fertility status of these soils. For the purpose of choice of these fertilizers under perennial irrigation, the following descending order was obtained: superphosphate, baSic slag, bone-meal. Sreenivasan (1943) reported that dilute acid extra~tants remove more P from the acid soils treated with bone-meal than from those treated· with potassium dihydrogen phosphate. Ghani and Islam (1946) reported that in acidic soils of Dacca (PH

l\!ETHODS TO CHECK PHOSPlIOIWS FIXATION IN SOILS 45

5.2') and !3crhampur (pH 4.6) bone-meal was superior to nicifos as with its application [heI' rix~ltinll was millimum. Kanwarand Grewal (1958) studied the behaviour of different P fertiliz.ers in acid, neutral and alkaline soils of the erstwhile Punjab. They observed that lhc watcr soluble P fertilizers, like ammonium phosphate, mono-calcium phosphate and superphusphate were rapidly converted into insoluble phosphates in all the soils. The water-insoluble P fertilizers, like rock phosphate, bone-meal and U'i-calcium phosphates were slowly converted into more soluble forms in acid soils. They confirmed these observations by pot·culture experiments with wheat, mustard and maize. Suitability of booe-meal, tri-calcium phosphate, rock phosphate for acid soil and that of phosphoric acid and aluminium phosphate for calcareous soils wa~ indicated. Mono-calcium phosphar.e and sup~rphosphalc, in spite of their high retention by acid and alkaline soils had better utilization in both types of soils by wheat and maize than insoluble P fertilizers. The finely ground bone-mcal (0.5 mm) was effective as P source in acid soils and its residual effect was better than superphosphate (Kanwar and Kanwar, 1962). The effect of superphosphate in calcareous soils was uncertain and erratic, although better results were obtained when applied in conjunctions with organic manures (Das, 1944). Sodium meta and pyro-phosphatescontaining P03 and PP7 ions did notreact with calcium carbonatcan9 gave the best results in calcareous soils. The citrate soluble P fertilizers, such as di~cakium phosphate and nitrophosphatc, were as good as superphosphate in neutral to acidic soils (Tambaneand Ghosh, 1962). Superphosphate and ammonium phosphate were definitely superior to di-calcium phosphate in the soils of high pH and short season crop with restricted root system. Rock phosphate and bone-meal significantly increased the yield of barley in Nilgiri soils which contained a large amount offree sesquioxides and hada high P fixation capacity CRajagopal and Idnani, 1963). The superphosphate was the best fertilizer for soils from Pawarkheras, Indore and Chhindwara and di-caIcium phosphate for soil from GwaIior when applied at the rate of24.5 and 49 kg P per hectare (Motiranlani et al., 1964).

Rockphosphate was as good as superphosphate for paddy, wheat and maize in acid soil (Motsara and Datta, 1971). However, it was superior to supelphosphate for peas but inferior for potatoes. The residual effect of rockphosphate was significantly better than superphosphate on crop yield and available soil P. The citrate solubility of rockphosphate and the pH of soil were the most important factors governing the availability ofP from Laccadive phosphate earths and Mussoorie rock phosphate (S ingh and Dalta, 1973). The panicle size of rock phosphat Ii: had little effect on its solubility at low pH. The basic slag and rock phosphate were more effective than superphosphate for growing rice in acid lowland soils of Gayeshpur of West Bengal (Mandai and Khan, 1972). Bone--meal was slightly inferior to superphosphate. Mussoorie rockphosphate was less efficient than super­phosphate for guar and groundnut in slightly alkaline soil of Ludhiana-Punjab (Hunda! and Sekhon, 1976). The percentage efficiency of Mussoorie. Laccadive and Udaipur rockphosphates as compared with superphosphate was 78, 62 and 54, respectively for lucerne crop on soil with pH 6.8 (Singh el al.. 1976a). The availability coefficient ratio (ACR) was 0.60 for Udaipur, 0.75 for Mussoorieand 0.21 for Laccadive at 26 kg P/ha. In Laccadive P earths, the highest ACR of 0.97 was Observed at 78 kg P/ha while in other P sources, ACR tends to decrease with the increasing levels of P application. The effectiveness of rockphosphate on slightly alkaline soil was in the decreasing order:

46 PHOSPHORUS FIXATION IN INDIAN SOILS

Laccadivc. Mussoorie, Udaipur rockphosphale ; all rockphosphates were inferior to superphosphate (Singh et at., 1976b). The P availability was related to citrate solubility and increased as its application rate and incubation period was increased up to 75 days. Application of FYM had a little effect on rockphosphale solubility. The application of phosphobacterin reduced the availability ofP from Udaipur, Mussoorie and Laccadive rockphosphates up to 75 days and thereafter, increased in a soil of pH 7.5 (Singh et al., 1976c). Reduction in available P values with the use of phosphobacterin was attributed to microbial process.

Chatterjee and Dhar (1969) reported an increase in the uptake of P by maize from rockphosphaLe with its acidulation to 50% or higher in red soils. Misra and Panda (1969) also reported thatrockphosphaLe acidulated to 10, 50 and 100% with H3P04 gave good results in laterite soils ofpH4.0, 5.6 and 6.5.respecLively when maize and rice crops were grown. Panda and Misra (1970) observed that grinding and 10% acidulation of rockphosphate were effective in minimizing P fixation in laterite soil of pH 4'.0 whereas, 50% acidulation was required for soils of pH 5.6 to 6.5. For acidulation Hl04 was superior to HNOJ • Mussoorie rockphosphate of 80 to 100 mesh was quite effective to meet P needs of crops on acidic soils of Kerala (Nair and Aiyer. 1983). A mixture of rockphosphiie and low grade pyrites in suitable proportion would be quite effective as P source in alkaline soils of pH 7.65 (Rastogi et al., 1976). Rockphosphate improved P availability in acidic soils of Maharashtra (Zende. 1983). It was more effective than dicalcium phosphate but less effectiye than single superphosphate. The percentage recovery of rockphosphatc by crop was fairly good in acidic soils. The legumes like soybean, cowpeas and greengram could utilize 9.2, 7.0 and 5.2% of P application as rockphosphate in acidic laterite soil (Goswami and Kamath, 1984). The_release ofP from rockphosphate was apparently a slow process showing maximum availability at 30th day after application.,

On 'sandy loam soil of pH 5.5. basic slag was superior to superphosphate for peas but both were equally -effective for rice (Sinha et al .• 1968). Basic slag gave greater P uptake in both the crops but after liming its effectiveness was similar to that of superphosphate. On acid soil of Mysore, basic slag, rockphosphate and steamed bone-meal were at par in first year with superphosphate in rice, however, in the second year, basic slag gave significantly higher. yieJd (Sadasivaiah and Rao, 1970). Raw and steamed bone-m~ were usefulP ferilizersin tropical soils (Gupta, 1973). Suitability of calcinised bone~meal, rockphosphate and basic slag was indicated for acid soils and should be applied some time before sowing or planting and worked well into the soil so as to promote good rooting. The hydrolysis and solubility ofrockphosphate in carbonic acid revealed that this P source can also be utilized after mixing with organic matter. Prasad and Dixit (1976) concluded that P fertilzers containing partially water~soluble or no water-soluble P can be applied to acid soils, covering about 49 million hectares of area in India. P fertilizers like bone-meal, rOckphosphate, basic slag, dicalcium phosphate. thenna,l phosphate, nitrophosphate and ammoniated superphosphates were also good for acid soils.

The fused calcium and magnesium phosphates were superior to superphosphate for rice grown on laterite soils of pH 4.85 (patnaik.l1970). The suitability of (NH) HPO for laterite soil, CaC~(P04)z and NH4HzPO<4 for black cotton. soil. Ca-ph~:phate <4 in calcareous soils and 9A-metaphosphate in alluvial soils is reported (Datta et al., 1970}.

iVlETHODS TO CHECK PHOSPHORUS FIXATIO:-l IN SOILS 47

Ammoniated superphosphate was not really efficient. in any of these soils. Nitro­phosphate containing CaHPO'l was as effective as Ca(H2P04\ anci was superior to superphosphate in acid soil. The citrate soluble nitrophosphate was as efficient a~ (NH4)2HP04 in laterite soil, black soil and coastal alluvium for rice crop (Raychaudhuri, 1970). Dicalciul'll phosphate was more available than superphosphate in red, black and laterite soils (Mustafa and Durairaj. 1968). The citrate soluble P, especially by CaHP0

4•

added to calcareous or noncalcareous soils ofMaharashtra tended to maintain relatively high P availability, but this was not necessarily so in acid soils (Dcshpande and Zende, 1970). Field trials indicated that citrate solublePwas less effective than the water soluble form in increasing paddy yield. The water soluble P fertilizers were more efficient than citrate-soluble CaHP04 for dwarf wheat on sandy loam serozem soils whereas, response to rockphosphate was low (Singh et ai., 1970). The dicalcium phosphate was more available than diammonium phosphate in alluvial and black soils of Madhya Pradesh (Dravid and Apte, 1975). There were not so marked differences in P availability from Fe and Al phosphate applied to hilly forest, alluvial and laterite soils ranging in pH from 4.73 to 5.76 (Kar, 1973). The P availability increased with increasing soil pH. Venkateswarlu et al. (1970) concluded that at least 50% water soluble P is required for achieving comparable results with superphosphate and monocalcium phosphate with wheat crop on regur and chalka soils of Hyderabad.

The ammonium phosphate was the best P fertilizer for paddy in rice soils of Andhra Pradesh followed by superphosphate (Kareem and Sastry, 1966).Therockphosphate was the least effective. The superphosphate was superior than ammonium phosphate, bone­meal and rock phosphate for paddy crop in alluvial soils (Raja. 1967). In Ca and Na saturated Uttar Pradesh soils, the availability of P fertilizer was in the following decreasing order: NHlO",NH4I\P04,Ca(HzP04)2 (Misra and Ojha, 1979a).InH-saturated soil K~P04 and ea (Hl04)2 were more effective than NH4HzP04• The superphosphate was superior to nitrophosphate and fused magnesium phosphate in black and laterite red soils of Dharwar of Mysore (Gumaste and PatH, 1970). The NIV4.P04 and single superphosphate were equally effective for rice, potatoes, maize and sugarcane (Prasad et al., 1971). Superphosphate was better for grain legume. On neutral to alkaline soils, single or triple superphosphate . is superior to nitrophosphates but the latter is better on acid soils. The superphosphate was the best P fertilizer for soils of Chambal Command

, area of Rajasthan (Chauhan et al.}1972). The efficiency ofP fertilizers was in the following decreasing order: superphospfiate. diammonium phosphate, ammonium phosphate, dicalcium phosphate. The mobility of orthophosphate and pyrophosphates was. greater than that of metaphosphate in black, red and laterite soils (Misra and Gupta. 1974). The availability of P in paddy soils· was in the following decreasing order: Orthophosphoric acid, triple superphosphate, nitrophosphate, potassium hydrogen phosphate, single super­phosphate, sodium polyphosphate, dicalcium phosphate, ammonium phosphate, monocalcium phosphate, . iron phosphate, tricalcium phosphate, fish-meal. bone-meal (Puranik and Bapat, 1975). The water soluble fertilizers like superphosphate, diammo­nium phosphate were suitable for direct application to the potato crop with shallow root system on acidic brown soils of Shimla hills (Sharma et al., 19.76). Rock phosphate was of little use but can be used as supplemental to build up the available P status of the soils. The P fertilizer containing 50 to 75% water soluble P was. needed for rice in alkaline

48 PHOSPHORUS FIXATION IN INDIAN SOILS

alluvial soils, as their transformation to Fe~Pcontriblltcd significantly to available P(Lal and Mahapatra, 1979). Water insoluble P sources were inferior in such soils because of their reversion to insoluble Ca-P compounds.

In field practice, where water insoluble P fertilizer is applied to the soil, it is brought slowly into solution and is taken by plants as and when it is converted to water soluble form, although the dissolved P may also be got fixed by the soil adsorbing complex. The application of water soluble phosphate. on the other hand, would be followed by its fixation by the soil adsorbing complex. The application of the water soluble phosphate, on the othcr hand, would be followed by its fixation by the soil colloids and the plants may not be able to utilize the absorbed P. It has been observed that application of superphosphate to soil results in its almost immediate conversion into relalively insoluble forms but their availability is not completelY lost. These studies indicate tnat application of water insoluble P fertilizer like rockphosphale, bone-meal, dicalclum phosphate, basic slae and nitrophosphales can be of use to rice and peas (leguminous crops) in acid and laterite soils which are known for their high P fixing capacity. For ShOft duration crops like potato. the water soluble P fertilizer like superphosphate, monoammonium phosphate were suitable. In alkaline .md calcareous soils also water soluble fertilizcrs were more effective in spite of P fixation from their P sources.

Methods of Phosphatic Fertilizer Applications The method and lime of application of P fertilizers to soils is one of the important

factors affecting the fertilizer use efficiency. In pot experiments both with calcareous and non-calc~eous soils, the application of P fertlizer at a depth of 6.25-11.25cm gave better response than the surface application (Singh and Dass. 1945). In calcareous soils P fertilization increased the P concentration only in the surface layers where it was applied. The crop yields showed that application of superphosphate at the depth of 10-15 cm gave the maximum crop production (Das, 1945,1946). In the soil of Sabour (Bihar) which was loamy in texture with a pH of 7.3 to 7.5, very good increase in crop yield of wheat was obtained by applying P at the turning of sunnhemp (Green~manuring) (Mukerjee et al., 1955). An additional yield of303 to 321 kg per hectare was obtained w*,en P was applied to sunnhemp as compared with its application while sowing wheat. DeePJ,Iacement ofP fertilizer was beneficial to gram but had no effect on wheat crop. For the efficient use of the superphosphate it should' be applied in the granular form or it should be mixed with the organic manures before applying to the field (Dult, 1961). He considered the band placement of the fertilizer at the time of planting as the best method of application. The band placement and drilling behind the plough were superior to broadcast and plough sole application of P fertilizer for grain yield of dwarf wheat varieties (Singh and Gupta, 1969). The deep placement of P fertilizer was more effective in coconut (Muliyar and Wahid, 1973). The furrow placement of P fertilizer was significantly better for potato than broadcast and band placement in acidic brown hill soils of Shimlaregion (Verma and Grewal. 1978).

Raheja (1965) suggested that P fertilizer should be placed close to the seed so that seed roots may absorb the largest amount of P and the contac~ of fertilizer with the soil mass is reduced to the minimum. In soils which fix. P more readily than the others, the

METHODS TO CHECK. PHOSPHORUS FIXATION IN SOILS 49

application ofP fertilizers to green manure or responsive lcg~mecrop can be more effective than direct application to the soil before Ille main crop. The P fertilizers are less fixed in moist soils'than in dry soils and they should be applied at the germination stage. The optimum dose ofP a<; recommended for each crop or type of soil should be used. The use of super-digested compost for soils with high P fixing capacity was recommended (Virmani,1966). The incubation of superphosphate with organic matter increased itS efficiency by 35 % over an application of superphosphate and organic matter separately. The soaking of potato seed tubers before planting in 0.4% P solution from mono­ammonium phosphate for 3 hours was able to meellhe P requirement of the crop (Sharma et at., 1977). This technique can be effectively used to economize on P fertilizerin soils with high P fixation capacity. These studies indicate that placement of P fertilizer ncar the active rool zone of the plant is necessary for its better utilization by the plants. particularly in soil with high P fixing capacity.

Practical Significance of Phosphorus Fixation in Soils The subject of Pfixationin soilis not only important from theoretical consideration,

indicating the nature of its fixation in soils but it has its great practical implications. The fundamental question with which the common agriculturist is concerned is the determination of minimum amount of P fertilizers required to be added to a particular soil in order to maintain a concentration ofP in the soil solution which is sufficient to meet the P needs of the crop. The main interest of the farmer, therefore, centres round the determination of P fertilizers dose which can maintain sufficient available P in the soil solution throughout the crop growth period. The P fixation to a moderate degree may be considered a..~ blessing because it acts as a means of withholding the P against leaching. However, when P fixation is high, large amount ofP fertilizers is required for successful crop production.

The knowledge regarding the P status, the transformations of added phosphorus, the P fixation capacity. the factors responsible for P fixation, the availability of fixed P and methods to prevent P fixation in different soils as discussed can. be of great help in understanding the extent of the problem ofP fixation and device different ways and means to solve it. Various methods of practical utility to reduce P fixation in soil or increase the availability of fixed soil P are: lim ing or use of basic slag or other liming material to raise the pH of the acidic soil before the addition of a suitable dose of P fertilizer. Application of fannyard manure, compost or green manuring so as to provide the soil environment which is less conducive to P fixation. Furrow or band placement of fertilizers instead of broadcasting or spreading. Exploring possibilities of application of P through non­conventional methods of application such as soaking treatment of seeds and tubers etc. The use of granular fertilizers instead of powder or fine crystalline materials. Composting ofP fertilizers with organic wastes before application 10 the soil. Application of one heavy dose of p. to satisfy the P fixation capacity to a point so that its mobility of the nutrient becomes large enough to meet the P needs. of the crop. Using P fertilizers of low water solubility but of greater citrate solubility, particularly in soils of high acidity and high P fixation capacities. Making use of the inConnation about P fixation properties of the soils and adding such amounts and types ofP fertilizer which are most suitable under those conditions.

PHOSPHORUS FIXA nON IN INDIAN SOILS

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