KARAN VERMA Ph.D Agronomy COA,CSKHPKVKARAN VERMA Ph.D Agronomy COA,CSKHPKV

The reaction by which phosphate fixation takes place in soil have been recognised into 3-groups.

1) Adsorption2) Isomorphous replacement3) Double decomposition

>ADSORPTION: Adsorption reaction may be classified into 2-typesa) Physical: phosphate held on the soil solid surface.b) Chemical: retain phosphate penetrates more or less

uniformly into the soil phases.

>ISOMORPHOUS REPLACEMENT REACTION:

Phosphate is fixed by the hydroxyl (OH- ) and

silicate ions through isomorphous replacement. Hydroxyl (OH- ) ions are attached to silicon and Al, and are liable either dissociate as: -(Si, Al)- OH -(Si, Al)+ + OH-

or eccepta proton (H+ ions) as follows……. -(Si, Al)- OH + -(Si, Al) – OH2

+

Another important mechanism for the phosphate fixation is by the replacement of silicate anions upto a certain amount of silicate released from the tetrahedron reaction of ions and hydroxide with the phosphate ions are perhaps most significant for phosphate fixation in soil.

>DOUBLE DECOMPOSITION REACTION Based on solubility product principles, a variety of reaction may be

regarded as significant in the fixation (precipitation) of soluble phosphates.However the formation of insoluble precipitates of phosphatic compound will largely depends upon the pH of system.

The fosphate fixation can be reduced with the increased pH with the exception increasing ph due to the presence of Ca(oH)2 or CaCo3 , the phosphate fixation may increases tempararily.

The avalability of Al3+ and Fe3+ for reaction with phosphate ions is regulated by the hydroxyl (OH-) ion cocentration and it is also found that phosphorus fixation can be decreased by increasing the pH of soil.

ORGANIC MATTER AND PHOSPHATE FIXATIONORGANIC MATTER AND PHOSPHATE FIXATION Organic matter also play important role in influencing the

phosphate fixation in the soil. It is evident that organic matter decreases the fixation of phosphate in soil as follows:

In adsorption reaction the organic matter containing various organic compounds, being dominately anionic in nature, compete with phosphate anion in polar adsorption phenomenon and their by decrease phosphate fixation in soil.

In double decomposition reactions involving Fe and Al. the acid produces during organic matter decomposition could decrease the pH and this increase fixation by solublising larger amount of Fe and Al.

PHOSPHATE FIXATION IN ACIDIC SOILS:PHOSPHATE FIXATION IN ACIDIC SOILS: Many acidic soils contain high amount of free Fe and of Fe and

Al hydrous oxide clays. The free Fe, Al and the sesquioxide clay react rapidly with phosphate, forming a series of not easily soluble hydroxy

phosphates. The amount of phosphorus fixed by this reaction usually exceeds

that fixed by phosphate retension. Generally clays with low sesquioxides ratio (SiO2 / R2O3) have p-fixing capacity.

PHOSPHATE FIXATION IN ALKALI SOILS:PHOSPHATE FIXATION IN ALKALI SOILS:

Many alkaline soils contain high amount of soluble and exchangable Ca2+ and frequent, CaCO3 , phosphate reacts with both the ionic and the carbonate form of Ca.

Phosphate fixation not be avoid entirely, but it may be reduced by addition of competing ions for fixing sites. organic anion from stable manure and silicates are reported to be very useful in reducing P-fixation.

FACTOR AFFECTING PHOSPHATE FIXATION:FACTOR AFFECTING PHOSPHATE FIXATION:

Nature and amount of soil componentsNature and amount of soil components: Adsorption and desorption reactions are affected by the type of surface contactsd by phosphorus in the soil solution.

pH: pH: soil pH has profound influence on the amount and manner in which soluble P-becomes fixed,

Other ions: Other ions: Both cations and anions affect fixation of phosphorus. Organic matter: Organic matter: The decrease in phosphate fixation due to

presence of sufficint organic matter in soils. Temprature: Temprature: Temprature affect the speed of chemical reaction.

High temp. are excepted to slightely increase the solubility of phosphstic compound like hydroxy and carbonate, octacalcium phosphate, varicite etc.

Overliming: Overliming: It increases the fixation of phosphprus by forming more insoluble Ca-P compound in soil

Boron fixationBoron fixation Molybedanum fixationMolybedanum fixation

MANGEMENT ASPECT OF P:MANGEMENT ASPECT OF P:

Most crops do not take more than 10-15% of the P added

through fertilizer during the year, the fertilizer is applied:

If possible, assure an optimum pH (6-7) Keep in mind that P is especially unavailable in cold soils. Apply P in bands in soil Use soil testing before planting each season, use appropriate

guidelines. Band-apply NH4

+ and P together--this usually increases P availability, particularly in alkaline soils.

Cation fixation in soils occurs when exchangable or water soluble cations are converted to a form that can not be readly extracted with a neutral solution of a salt. K+ and NH4+ ions have been primirily involved in such reaction.

Fixation of NH4+ takes place in the crystal lattices of different

clay minerals, montmorillanite, illite and vermicullite. Amorphous oxides of Fe and Al and allophane also adsorb NH4

+ in on unavailable form, leading ultimately to formation of + aronakite.

NH4+ NH+ NH+

K+ K+ K+

(solution) (exchangeable) (non exchangeable or fixed)

Animalmanures

and biosolids

Mineralfertilizers

Crop harvest

Runoff anderosion

Leaching

Soil solution potassium (K+)

Plant residues

Plantuptake

Mineralpotassium

Fixedpotassium

Exchangeable potassium

Input to soilComponent Loss from soil

The mechanism of NH4+ fixation is similar to that of K+ fixation ,

because of almost the some ionic sizes and polarizabilities. Addition of K+ to such minerals read to a strong K+ adsorption to

these position causing a of the mineral, to a unit distance of about 1nm against 1.4nm in weathered biotite and muscovite. This process is called K+ fixation.

The crystal chemistry of K+ fixation by soil collids is governed by the fact that the size of K+ ion is very close to that of the hole (hexagonal cavity).

Root uptake

Nitrate NO3-

Plant protein

Soil organic nitrogen

Biological fixation

Atmospheric fixationOut

gassing

Atmospheric Nitrogen4 000 000 000 Gt

© 2008 Paul Billiet ODWS

A lattice hole theory was proposed by “Page and Baver” (1940) that gave cosideration to the ionic size of the unhydrates ions, the expanding contracting nature of the montmorillonite type of minerals and geometric arrangement of the oxygen ions at the surface of the crystal unit.

Fixation by organic matter:Fixation by organic matter: Fertilizer containing ammonia when applied to the soil reacts with

soil organic matter to form compounds which resist decomposition and so it is said to be (fixed) by the organic matter. The fixation of added NH4

+ was linearly related to the %age of carbon in the organic matter.

Factor af fecting k+ and NH4+ fixation in soils:Factor af fecting k+ and NH4+ fixation in soils:

1) Nature and amount of clay.2) Soil reaction and pH.3) Cocentration of added NH4

+ and K+

4) Wetting and drying 5) Frrezing and thawing6) Lime7) Anions8) Moisture9) CEC

(NH 4+):

Total N content in soils depends on temprature, rainfall, and alltitude. It is observed that in black soils that N content increased as the rainfall increased.

Growing of legumes increases N content. The management of soil nitrogen consist of the maintenance of

adequate nitrogen supplies in the soil. The regulation of the soluble form of nitrogen.

(K +): Frequent light application rather than one heavy application of the

following reasons: 1) it is quickly subject to leaching from the soil solution, 2) it is subject to rapid fixation, 3) it is subject to luxury consumption by some crops.

Application of lime reduces leaching losses of potassium and promotes its utilization power in soils.

Contineous cropping without fertilization leads to the depletion of organic matter, available N, P, K, Fe, Mn, Zn and Cu, whereas contineous fertilization has beneficial effect on organic matter and N, P, and K, of the salts.

Precipitation - The formation of a solid from solution. The reverse of dissolution.

Dissolution - The process by which a substance dissolves. The reverse of precipitation.

Importance - (a) Selective precipitation is an important industrial purification process, especially when crystals are formed. (b) Scales that form on boilers and teeth are to be prevented, as are kidney stones. (c) Precipitation forms minerals - dissolution removes them.

Thank Thank youyou