Phosphorus Relationships in Flooded Rice Soils with Low Extractable Phosphorus

Phosphorus Relationships in Flooded Rice Soils with Low Extractable PhosphorusH. Shahandeh, L. R. Hossner,* and F. T. Turner

ABSTRACTRice (Oryza saliva L.) grain yields on some flooded soils are not

increased by P fertilizer despite their low soil test P status determinedon air-dry soils by soil test methods such as Bray, Olsen, or TexasA&M. Conventional soil test methods apparently do not accuratelyassess the capacity of these soils to supply P to rice grown underflooded conditions. To test the possibility of an Fe-P association,oxalate extractant (which would extract noncrystalline Fe and itsassociated P) was used to provide a more accurate measure of availableP in flooded soil. Phosphorus response experiments were conductedon 10 rice soils under greenhouse and field conditions and related tothe oxalate-extractable Fe and P, P uptake, P adsorption, and Pdesorption under aerobic and anaerobic conditions. Oxalate-extractable Fe significantly increased under anaerobic conditions inall soils including the soils that were nonresponsive to P and had lowlevels of extractable P using conventional extractants. Phosphorusadsorption and desorption data confirmed the importance of oxalateextractant in predicting P availability following anaerobiosis. Theinability of Bray 1, Olsen, and Texas A&M soil test methods toaccurately predict P availability in flooded rice soil appears to be dueto their failure to extract the active reductant-soluble P fraction. Themeasurement of P associated with poorly crystalline Fe by oxalateextractant is a useful soil test method for predicting the availabilityof P in flooded rice soils.

FLOODED RICE PRODUCTION on some soils is not in-creased by P application despite a low soil test ex-

tractable P by Texas A&M, Olsen, or Bray 1 methods(Folsom, 1975; El-khattari, 1973; Nambir et al., 1973;Willet and Higgins, 1978). Apparently the soil test meth-ods do not accurately reflect the available P status of ricesoils under flooded conditions. Factors not adequatelymeasured by soil test methods may determine P availabil-ity in rice soils. Verma and Tripathi (1982) concludedthat Fe was the most important variable contributing toP extracted by Bray 1 and Olsen soil test methods.

Iron-P has been indicated as the main source of avail-able P in rice soils (Ponnaperuma, 1972; Patrick andMahapatra 1968), and its availability increases on flood-ing (Goswami and Banerjee, 1978; Ponnamperuma,1965). Willet (1986, 1989) reasoned that the causes ofincreased P availability during flooding were the reduc-tive dissolution of ferric oxides and the liberation ofsorbed and occluded P; changes in soil pH that increasedthe solubility of Fe, Al, and Ca phosphates; and thedesorption of surface P. Direct measurements of theamount of Fe3+ reduced to Fe2+ during flooding, andof P sorption may be required to predict the net amountof P released during flooding. Little information is avail-able on the forms of Fe-P extracted by various soil testmethods used to predict the P requirements of soils usedfor rice production.H. Shahandeh and L.R. Hossner, Soil and Crop Sciences Dep., CollegeStation, XX 77843; and F.T. Turner, Texas A&M Research and ExtensionCenter, Beaumont, TX 77713. Received 2 June 1993. "Correspondingauthor.

Published in Soil Sci. Soc. Am. J. 58:1184-1189 (1994).

Several forms of Fe in soil are reported to correlatewith P sorption (Khalid et al., 1977; Willet and Higgins,1978). Soil P released under reduced conditions wasrelated to oxalate-extractable Fe (Khalid et al., 1977).Oxalate-extractable Fe is amorphous, poorly crystallineoxides and hydroxides of Fe (Schwartmann, 1973). Un-der flooded soil conditions poorly crystalline Fe oxidesproduce a more reactive surface for P sorption andconsequently more P could be released (Patrick andKhalid, 1974). It has been shown that poorly crystallineor oxalate-soluble Fe is released more readily than crys-talline Fe under reduced conditions (Munch and Ottow,1980). Poorly crystalline Fe correlated well with P sorp-tion (Sah and Mikklesen, 1989) and with P extractedwith oxalate (Wang et al., 1991). Therefore, employingan extractant that can more selectively dissolve poorlycrystalline Fe-P would best reveal the available P statusof rice soils and be more highly correlated with riceyield (Khalid etal., 1979).

Turner and Gilliam (1976a,b) illustrated that soil oxi-dation status had a major influence on P availability evenin calcareous rice soils where Ca phosphates predominateover Fe phosphates. They found that increased P avail-ability hi flooded or water-saturated soils occurred partlyas a result of decreased tortuosity causing a 10- to 100-fold increase in P diffusion through soil to roots as aresult of water saturation of the soils.

Using this information as a base, our objective wasto test the hypothesis that an extractant such as oxalatethat dissolves P associated with poorly crystalline Feoxides would be more selective than conventional extract-ants in assessing P availability in rice soils. The impor-tance of P adsorption and desorption on P availabilitywas also evaluated.

MATERIALS AND METHODSLaboratory Studies

Soil Tests for Extractable Phosphorus. Ten soil sampleswere collected from the top 15 cm of field research sites inthe rice-growing areas of the Gulf Coast of Texas (Table 1).Surface soil samples were collected from zero-P treatmentplots from four replications and composited. Soil samples wereair dried, crushed, and passed through a 2-mm sieve. Duplicateair-dried samples were extracted for P using the Texas A&M,Bray 1, and Olsen methods. Texas A&M P (NH^OAc-ethylene-diaminetetraacetic acid [EDTA]) (Texas Agricultural Exten-sion Service, 1980) was extracted by shaking 2 g of soil with20 mL of 1.43 M NtLOAc and 0.025 M EDTA for 30 min.Bray 1 P was extracted by shaking 1 g of soil with 7 mL of0.03 MNrL,F and 0.025 MHCl for 1 min (Olsen and Sommers,1982). Olsen P was extracted by shaking 5 g of soil with 20mL of 0.5 MNaHCOs (pH 8.5) for 30 min (Olsen and Sommers,1982). Samples were centrifuged and filtered and P in the soilextracts determined by the method of Murphy and Riley (1962).

Oxalate and Citrate-Dithionite-Extractable Iron. Dupli-cate soil samples were incubated for 14 d at 30 °C underaerobic or anaerobic conditions in a soil/0.01 M CaCl2 solutionratio of 1:10, as outlined by Khalid et al. (1977). Iron was

1184

SHAHANDEH ET AL.: PHOSPHORUS RELATIONSHIPS IN FLOODED RICE SOILS 1185

Table 1. Selected properties of soils used for rice production including extractable P using various extractants.

Extractable Pt

Soil series

BeaumontBeaumont (cut)DacostaEdnaGessnerLake Charles ILake Charles IIMoreyNadaWeston

Classification

Fine, montmorillonitic, thermic Entic PelludertFine, montmorillonitic, thermic Entic PelludertFine, montmorillonitic, hyperthermic Vertic OchraqualfFine, montmorillonitic, thermic Vertic AlbaqualfCoarse-loamy, siliceous, thermic Typic GlossaqualfFine, montmorillonitic, thermic Typic PelludertFine, montmorillonitic, thermic Typic PelludertFine-silty, mixed, thermic Typic ArgiaquollFine-loamy, silicous, hyperthermic Typic AlbaqualfCoarse-loamy, silicious, thermic Typic Ochraquult

PH1:1

5.545.366.516.376.486.337.105.786.575.80

OrganicC

• Eke"1e ™e

15.44.18.47.55.7

12.5114.613.06.6

18.6

Clay

48443619822413834038927378

185

TexasA&M

8.93.0

36.73.8

46.510.925.711.565.07.8

Olsen

— mg kg"1 —7.95.4

13.27.4

18.49.2

15.98.9

30.97.5

Bray1

7.53.7

32.53.0

33.38.2

10.99.2

61.75.9

t Extractable P in rice soils is given a low rating at 20.0 mg kg-' for Texas A&M (Texas Agriculture Extension Service, 1980), 8.6 mg kg-' for Bray 1 (Husinet al., 1987), and 13.0 mg kg-1 for Olsen (Nambir et al., 1973).

extracted by shaking the incubated soil suspension with acidicoxalate [0.2 M (NrL,)2C2O4 • H2O and 0.2 M H2C2O4] for 4 hin the dark (Sheldrick, 1984) in a soil/solution ratio of 1:40.Iron was analyzed by atomic absorption spectrometry afterfiltering the oxalate extract. Phosphorus in the oxalate extract-ant was determined by the phosphomolybdate blue method(John, 1970) after the residues were ashed at 500°C for 1 hand dissolved in 1 M HC1. Total free Fe oxide was determinedby the citrate-dithionite procedure of Holmgren (1967). Ironwas measured by atomic absorption spectrometry.

Phosphorous Sorption Study. Duplicate soil samples wereincubated under aerobic and anaerobic conditions for 14 d at30°C in a 1:10 soil/0.01 M CaCl2 suspension containing 0 to500 mg P kg"1 soil .as Ca(H2PO4)2. Anaerobic samples werepurged with N2 gas every 2 d and incubated in stoppered50-mL Nalgene centrifuge tubes in the dark. Both aerobic andanaerobic samples were shaken on a reciprocal shaker for0.5 h twice each day for 14 d. Aerobic samples were stopperedonly during shaking. After 14 d of equilibration, soil suspen-sions of aerobic and anaerobic samples were centrifuged andfiltered. The filtrate was analyzed for P by the Murphy andRiley method (1962). Phosphorus no longer in solution wasconsidered to have been sorbed by the soil (Roy and DeDatta,1985; Khalid et al., 1977).

Phosphorus Desorption Study. Phosphorus desorbed fromthe aerobic soils was measured by decanting the supernatantliquid of the P adsorption study and resuspending the soil in30 mL of 0.01 M CaCl2 solution (Roy and DeDatta, 1985).The samples were again equilibrated by shaking in a reciprocalshaker for 0.5 h twice a day for 3 d. The equilibrated sampleswere centrifuged and filtered. Phosphorus in the filtrate wasconsidered to be desorbed P.

Greenhouse StudiesTo determine the yield response to applied P by rice plants,

pot experiments were conducted with 10 soils collected fromcontrol plots of the field study sites (Table 1). The soils hada wide range of extractable P. Two kilograms of air-dried soil inpolyethylene-lined pots were fertilized with Ca(H2PO4)2 • H2O at0, 11, 22, and 44 mg P pot"'. Four P treatments were appliedin three replications in a randomized complete-block design.Nitrogen (50 mg kg"1 as urea) and K (12.5 mg kg"1 as K2SO4)were applied to each pot at planting. An additional 25 mg Nkg"1 was applied 14 d after flooding. Rice ('Lemont') wasplanted and the soil was brought to field capacity. Rice seedlingswere thinned to five plants per pot after germination and thento three plants when the soil was permanently flooded 14 dafter planting. The aboveground portion of the rice plants was

harvested 28 d after permanent flooding. Rice plants weredried at 65 °C and weighed to obtain dry matter yield.

Field StudiesThe rice yield response to P fertilizer was determined at

eight locations during the 1991 or 1992 growing season infields where soil samples had been collected for the greenhouseand laboratory studies (Table 1). Two of the field sites usedin the greenhouse study were abandoned because of variablerice stands or weather-related failure. Phosphorus was appliedpreplant as triple superphosphate (0-46-0) at 0, 18, 35, and 52kg P ha"1 in a randomized block design with four replications.Lemont rice was planted in April at 100 kg ha"1 rate to adepth of 1.2 cm with a grain drill at 15-cm row spacing. Plotsize was 0.9 by 5.4 m. Plots were fertilized with 150 mg Nkg"' as urea, applied in three 50-kg doses at planting, at thefour-leaf growth stage, and at the panicle initiation stage. Fieldswere flushed two to three times before permanent flooding inJune, and drained for harvest in August. At the panicle initiationstage, the entire aboveground plant was collected for P analysis.The plant samples were dried at 65 °C and digested in H2SO4and H2O2 containing Se metal and Li2SO4-H2O. The digestwas analyzed for P by the vanadomolybdophosphate yellowmethod (Jackson, 1970). Rice was harvested from a 0.6 by5.4 m area of each plot for grain yield determination.

RESULTS AND DISCUSSIONSelected properties of 10 soils used for rice production

along with corresponding extractable P measured by theTexas A&M, Olsen, and Bray 1 methods are presentedin Table 1. There was a wide variation among soils inthe amount of P extracted for each soil test method.Two of the soils (Beaumont [cut] and Edna) were verylow in extractable P, four were low (Beaumont, LakeCharles I, Morey, and Weston), and four were high tovery high (Dacosta, Gessner, Lake Charles II, and Nada)in P concentrations. The range of extractable P reflectsvariability in soil P and previous management practices.

Yield response to applied P of representative soils(Table 2) showed that two of the soils with very lowextractable P (Beaumont [cut] and Edna) responded toP fertilizer in both field and greenhouse studies. Foursoils with low extractable P (Beaumont, Lake CharlesI, Morey, and Weston) responded to P fertilizer only inthe greenhouse, and four soils with high extractable P

1186 SOIL SCI. SOC. AM. J., VOL. 58, JULY-AUGUST 1994

Table 2. Yield response of rice to applied P under field and greenhouse conditions.

OkgGrain yieldt Total dry matter yieldt

18kg 36kg 52kg Omg 11 mg 22 mg

t Field study using 0, 18, 36, and 52 kg P ha-'.$ Greenhouse study using 0, 11, 22, and 44 mg P pot~'.§ Means within the row for field or greenhouse experiment followed by the same letter are not significantly different (LSD[0.05]).

44 mg!,„ U_- l


5993a§3392a

—4852a5993a6408a6084a6483a

—4499a

5600a3914b

—5552b5600a6269a6117a6072a

—4635a

5769a3833b

—5743b5769a6500a6185a6334a

—4845a

541 la3997b

—5848b5411a6069a6311a6431a

—4881a

3.98a2.10a4.50aS.SOa7.25a3.47a7.80a4.92a6.04a4.0a

———————— g pot- ——————————————6.63b 8.81c4.50b4.62a9.40b6.56a6.70b7.64a5.40a6.16a6.80b

S.lOb4.75a

11.30c5.78a8.98c6.98a6.83b6.49a7.30b

lO.lOc9.70c4.92a

10.70bc6.91a

lO.Slc7.60a7.60b6.79a9.90c

(Dacosta, Gessner, Lake Charles II, and Nada) did notrespond to P fertilizer in the field or in the greenhouse.In the field, only two soils responded to P applicationand no significant response was obtained where P rateexceeded 18 kg ha"1.

In the field and greenhouse studies, two of the soils(Beaumont [cut] and Edna) had very low extractable Pand four of the soils had very high extractable P (Dacosta,Gessner, Lake Charles II, and Nada) and respondedto P application accordingly. The four soils with lowextractable P (Beaumont, Lake Charles I, Morey, andWeston) produced an increase in rice shoot yield drymatter at 42 d due to P fertilization in the greenhousebut no grain yield increase in the field (Table 2). Drymatter production after 6 wk of growth with a P applica-tion rate of 44 mg P kg"1 produced the highest yield.Phosphorus fertilizer addition to these soils resulted ina large increase in aboveground vegetation. The responseof rice to fertilizer P application in the pot experimentagrees with soil test P by the Texas A&M method(NHjOAc-EDTA) (Texas A&M Agriculture ExtensionService, 1980). According to the Texas A&M soil testmethod, soils with <20 mg extractable P kg"1 shouldrespond to P application. Soils having Texas A&M ex-tractable P >20 mg kg"1 did not respond to P applicationin the greenhouse study (Table 2). Yield response toP application in the greenhouse study is reported and

Table 3. Amount of P and Fe extracted by oxalate under aerobicand anaerobic soil conditions and citrate-dithionite-extractableFe in selected soils used for rice production._________

Oxalate- Oxalate- Citrate-extractable P extractable Fe dithionite-

——————————— ——————————— extractableAerobic Anaerobic Aerobic Anaerobic Fe


4039423675255246272303291287

3908922474249252239305284285

548623977542020541

1560209530375691803

,

73832798103628047312760309642347022892

11040887517774084155850429752530114793573

considered important because some of the critical levelsestablished for P are based on greenhouse studies.

The question is then why rice yield in the same soils(Beaumont, Lake Charles I, Morey, and Weston) underfield conditions did not increase with P application despitetheir low extractable P (Table 2). The lack of responseto P application under field compared with greenhouseconditions is partly due to an increased soil volumeavailable for root growth and because soil test methodsapparently failed to measure the P that becomes availableunder flooded conditions in the field. Additional factorsmust be considered when selecting a chemical test forP availability to rice.

Oxalate-Extractable Iron and PhosphorusThe four soils with low Texas A&M, Bray 1, and

Olsen-P that responded to P application in the greenhousebut did not respond to P application in field experiments(Beaumont, Lake Charles I, Morey, and Weston) hadabout the same or more oxalate-extractable P as thosesoils that did not respond to P application in the green-house or field experiments (Dacosta, Gessner, LakeCharles II, and Nada) (Table 3). Dacosta, Gessner, LakeCharles II, and Nada soils had high Texas A&M, Bray 1,and Olsen P. Oxalate-extractable P in two nonresponsivesoils in the field, Beaumont and Morey, was higher thanfor the highly fertilized Nada soil. Oxalate extractedmore P from nonresponsive soils than any other extract-ant (Table 2). Beaumont (cut) and Edna soils had <100mg kg"1 of oxalate-extractable P and responded to Papplication in the field. The amount of P extracted byoxalate under anaerobic conditions did not change, butoxalate-extractable Fe increased (Table 3).

Soil P availability under flooded conditions has beenrelated to ammonium oxalate extractable Fe (Khalid etal., 1977; Willet and Higgins, 1978). Data in Table 3show a considerable increase in extractable Fe fromnonresponsive soils under anaerobic conditions. For ex-ample in Beaumont, extractable soil Fe increased « 2000mg Fe kg"1 under anaerobic conditions. This is an indica-tion that extensive reduction of Fe3"1" to Fe2+ compoundshas occurred due to flooding. Willet (1989) reported thatchanges in extractable Fe during flooding could strongly


influence extractable P. Oxalate has been shown to extractpoorly crystalline Fe (Campbell and Schwertmann,1984). Poorly crystalline Fe oxides are the most reactiveFe oxides hi the soil because of their small size and highsurface area. Soils used in this experiment had a largeamount of poorly crystalline Fe, which constitutes alarge portion of free Fe oxides extracted by citrate-dithionite (Table 3). All of the soils contained at least30% of the Fe oxide as poorly crystalline when measuredunder anaerobic conditions. The Morey and Weston soilscontained >80% of the oxalate-extractable Fe as poorlycrystalline.

The correlation between oxalate-extractable Fe andP in four nonresponsive soils with low extractable P(Beaumont, Lake Charles I, Morey, and Weston) (Fig.1) is highly significant. This demonstrates that substantialamounts of P are associated with the oxalate-extractableFe fraction and that its release to plants would be expectedto be linearly related to the release of the noncrystallineFe fraction in the soil.

A graph of P sorbed vs. oxalate Fe was constructedfor the four nonresponsive soils with low extractable Pto investigate the influence of oxalate-extractable Fe onextractable P (Fig. 2). It has been found that extractableFe (Evans and Smillie, 1976) and clay content (Fox andKamprath, 1970) strongly influence P sorption underaerobic conditions, whereas oxalate-extractable Fe is themain soil property influencing soil P sorption underanaerobic conditions (Willet et al., 1978; Khalid et al.,1977). The high linear correlation between oxalate-extractable Fe and P sorbed in Fig. 2 confirms the abovefindings. The relationship indicates that at ==3000 mgoxalate-extractable Fe kg"1, 84% of the added P wassorbed under anaerobic condition. Ninety-five percentof the applied P was sorbed at 6000 mg oxalate-extractable Fe. Under aerobic conditions, less P wassorbed per unit of oxalate-extractable Fe. Large oxalateFe fractions under anaerobic conditions are an indicationthat Fe oxides in nonresponsive soils are poorly crystal-

OJ3

!tfl

600

500

400-

300-

200-

100

0

y = 379 + 0.014xR = 0.96

y = 266 + 0.034xI

R = 0.95

AerobicAnaerobic P Added = 500 mg P kg

2000 4000 6000 8000

Oxalate Extractable Fe (mg kg )

Fig. 2. Relationship between oxalate-extractable Fe and sorbed P forlow soil test P soils where rice did not respond to P application inthe field.

line with more reactive surfaces. Therefore, nonrespon-sive soils could be characterized as soils with high oxalateFe and P and a high tendency to sorb P. These characteris-tics make it possible for more P to be released underanaerobic conditions. Patrick and Khalid (1974) con-cluded that the greater surface area of gel-like reducedferrous compounds in anaerobic soils resulted in therelease of more soil P when solution P was low.

Iron uptake by the rice plants in the field study isplotted against oxalate-extractable P in Fig. 3. Therewas a high correlation between oxalate-extractable P andP uptake. This supports the earlier data that oxalateextracts a large amount of P associated with Fe.

Sorption and Desorption IsothermsPhosphorus release under aerobic and anaerobic condi-

tions was evaluated by constructing P sorption and de-

OJD

500

400"

300-

200'

100-

y = 202 + 0.036X1

R = 0.95

y = 188 + 0.027*2

R = 0.97

AerobicAnaerobic

2000 4000 6000 8000

Oxalate Extractable Fe (mg kg )

Fig. 1. Relationship between oxalate-extractable Fe and oxalate-extractable P for low soil test P soils where rice did not respondto P application in the field.

20

is-

le-Q.

y = - 1.52 +0.053X2

R = 0.77

100 200 300 400 500

Oxalate Extractable P (mg kg )

Fig. 3. Relationship between P uptake and oxalate-extractable P inthe field study.

1188 SOIL SCI. SOC. AM. J., VOL. 58, JULY-AUGUST 1994

DCJtBU

•ovao<n

10 100

P in Solution (mg L"1)

Fig. 4. Sorption isotherms for two soils on which rice did not respondto P application in the field and that have high (Beaumont) andlow (Gessner) oxalate-extractable Fe.

sorption isotherms for two of the nonresponsive soilswith high (Beaumont) and low (Gessner) oxalate-extractable Fe (Fig. 4 and 5). Phosphorus sorption anddesorption have been successfully used to determine Prequirements of rice soils (Khalid et al., 1979; Royand De Datta, 1985). Phosphorus supply to plant rootsdepends on the concentration of P ions in the soil solutionand on the capacity of the soil to maintain this concentra-tion. For example, Roy and DeDatta (1985) and Khalidet al. (1979) concluded that maintaining 0.12 to 0.20mg P L"1 in solution is required for a maximum growthof rice. Hossner et al. (1973) concluded that rice yieldswere >90% of maximum when the average soil solutionP concentration was >0.1 mg P L"1. Data in Fig. 4show that solution P increased substantially in the Beau-mont soil after reduction; however, solution P was below0.10 mg P L"1. This implies that after a 2-wk periodof incubation, Beaumont soil without applied P was notable to attain the reported critical P value in solution.Rice grown on this soil responded to P application in thegreenhouse but not in the field. Solution P for Gessner, ahighly fertilized soil, was X).2 mg P L"1. Rice grownon this soil did not respond to P application in thegreenhouse or in the field. The amount of P sorbedvaried with soil type. The sorption of P by Beaumontsoil was influenced by its degree of reduction. Littledifference in sorption was observed in Gessner soil afterflooding (Fig. 4). Willet and Higgins (1978) concludedthat P sorptivity of soils was increased after flooding.

The P desorption status of soils can be used to estimatethe P supplying power of the soil. The uptake of P byplants is governed by the ability of a soil to supply Pto plant roots and by the desorption characteristics ofthe soil (Roy and DeDatta, 1985). The shape of thedesorption curves of Beaumont and Gessner soils (Fig.5) follows the sorption pattern, indicating that the concen-tration of P in solution is a function of P adsorbed. Theamount of P desorbed from the Beaumont soil was greaterunder anaerobic conditions and P desorption increased

S 400-]Bf)

"8̂•D

.1 10 100

P Desorbed (mg L"1)

Fig. 5. Desorption isotherms for two soils on which rice did notrespond to P application in the field and that have high (Beaumont)and low (Gessner) oxalate-extractable Fe.

with quantity of P applied. There was no difference inthe amount of P desorbed from the Gessner soil underaerobic and anaerobic soil conditions. The amount of Pdesorbed was a function of extractable Fe. Beaumontsoil has a high P sorption capacity and it has been reportedthat more P will be desorbed from soils with a high Psorption capacity (Roy and DeDatta, 1985). Willet andHiggins (1978) concluded that rice grown in soils withlow extractable P and high P sorption characteristicsobtains some of its P needs from P sorbed under floodedconditions. Similarly, rice grown on Beaumont soil didnot respond to P application. The Beaumont soil hasvery low levels of extractable P and a high P sorptioncapacity. Applied P that had been sorbed by the soil wasapparently available to plants after flooding.

CONCLUSIONSRice grain yield data, extractable P, and adsorption

and desorption data confirm the importance of poorlycrystalline Fe as a primary soil property affecting Prelease under flooded soil conditions. Oxalate-extractableFe was significantly increased under anaerobic condi-tions. The greater the amount of oxalate Fe, the greaterthe active surface area available for P sorption and subse-quent P release. Oxalate-extractable P retained by poorlycrystalline Fe oxides under aerobic conditions could havea significant effect on P availability to rice followingsoil flooding. The poor performance of conventional soilP extractants (Bray 1, Olsen, and Texas A&M) can berelated to their inability to selectively extract P from thenoncrystalline Fe oxide fraction. An extractant such asoxalate, which dissolves P associated with noncrystallineFe, is more appropriate for predicting P availability insoils used for rice production.


Documents

Phosphorus Relationships in Flooded Rice Soils with Low Extractable Phosphorus