This article throws light upon the three reactions by which phosphate fixation takes place in soils. The reactions are: 1. Adsorption 2. Isomorphous Replacement 3. Double Decomposition.
Reaction # 1. Adsorption:
Adsorption reactions may be classified into two types â€“ (a) physicalâ€”phosphate held on the soil solid surface and (b) chemical adsorptionâ€”retained phosphate penetrates more or less uniformly into the solid phases. Both types may be characterized by the Freundlich adsorption isotherm or by the Langmuir adsorption equation.
So adsorption isotherms give an idea of the adsorption reactions. At low concentrations of phosphate in solution; the adsorptions isotherm fits well to both types of equation. But at high concentrations and on contact for a longer period, other reactions take place.
m = ACB (Freundlich equation)
where, m = amount of phosphorus adsorbed per unit weight of soil,
C = concentration of phosphorus in soil solution
A and B = constants (vary from soil to soil)
m = ABC/1+BC (Langmuir equation)
where, m = amount of phosphorus adsorbed per unit weight of soil
C = concentration of phosphorus in soil solution.
A = maximum adsorption of phosphorus.
B = constant (related to bonding energy)
However, adsorption of phosphate takes also on surfaces of constant charge such as the crystalline clay minerals, which interact with phosphorus mainly through the cations held tightly to their plate like surfaces and on surfaces of variable charge including the ferric (Fe3+) and aluminium oxides and organic matter, for which H+ and OH– ions determine the surface charge, and calcite (CaCO3), for which Ca2+ and CO32- are responsible for charge development.
Besides these, there are some clay minerals such as kaolinite and allophones (amorphous clay minerals) are intimately associated with a hydroxy aluminium gel, which have pH dependent charge on their crystal edges and surfaces. Organic matter having pH dependent charge reacts with phosphorus through its cations (held by coulombic forces).
The degree of adsorption has been found to be increased with the increase in temperature and hence the adsorption reaction is chemical. Therefore, it may be concluded that the adsorption reaction is involved in the fixation of phosphate, but that fixation is, obviously, “adsorption plus.”
A simple mechanism for the phosphate adsorption is given in Fig. 17.1.
Reaction # 2. Isomorphous Replacement Reactions:
Reactions involving isomorphous replacement of compounds of a crystal lattice may be regarded as “adsorption plus” reactions of three general types:
(a) Continuation of the adsorption reaction through inter-crystalline absorption.
(b) Transformation of adsorption reaction to one of isomorphous replacement of hydroxyl or silicate anions from the crystal lattice.
(c) Decomposition of the isomorphously transformed crystal lattice as the limits of permissible isomorphous replacement are exceeded, followed by recrystallization as a new mineral compound.
It is evident that phosphate is fixed by the hydroxyl (OH–) and silicate ions through isomorphous replacement. Hydroxyl (OH–) ions are attached to silicon and aluminium, and are liable to either dissociate as:
giving rise to positively charged clays, which take part in anion exchange.
Another important mechanism for the phosphate fixation is by the replacement of silicate anions up to a certain amount of silicate released from the tetrahedrons. Beyond that the replacement was described as resulting in an unstable phosphate compound because of an infraction of the electrostatic valency rule when two phosphorus tetrahedrons share a common oxygen ion.
When more edges and corners are present for each unit mass, a larger amount of stable isomorphous replacement of silicon (Si) by phosphorus would occur. Reactions of iron and aluminium hydroxides with the phosphate ions are perhaps most significant for phosphate fixation in soils.
Reaction # 3. Double Decomposition Reactions:
Based on the solubility product principles, a variety of reactions may be regarded as significant in the fixation (precipitation) of soluble phosphates. However, the formation of insoluble precipitates of phosphatic compounds will largely depend upon the pH of the system. A solubility diagram for phosphate compounds is being shown below (Fig. 17.2).
The phosphate 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 increase temporarily.
Broadly this reaction falls into two categories:
(a) Reactions involving Fe and Al and
(b) Other reactions involving Ca.
Let us consider Al3+ system (supplied by aluminosilicates and free sesquioxides) as follows:
Ks = [Al3+] [OH–]3, where Ks = solubility product constant.
From the solubility product of the gibbsite, it can be calculated that at pH 5.0 the Al3+ concentration will not exceed 1.9 Ã— 10-6 M. At pH 4.0 it increases to 1.9 Ã— 10-3 if the phosphate ions are applied to a soil system in the form of a soluble fertilizer not exceeding 200 lbs P2O5 per acre the resulting phosphate concentration will be 0.007 M or 7 Ã— 10-3 M.
Now assuming that the phosphate will form a variscite like compound Al(OH)2H2PO4, having a solubility product of 2.8 Ã— 10-29(Ks).
The phosphate (H2PO4–, 7 Ã— 10-3 M) exceeds the equilibrium concentration at pH 4.0 when Al3+ is maintained at 1.9 Ã— 10-3 M by an excess of solid phase Al(OH)3.
Ks = [Al3+] [OH–]2 [H2PO4–]
[H2PO4–] = Ks/[Al3+l[OH–]2
= 2.8 x 10-29/[1.9 Ã— 10-3] Ã— [10-10]2
= 2.8 Ã—10-29/1.9 Ã— 10-3 x 10-20
= 2.8/1.9Ã— 10-29/10-23= 1.5 Ã—10-6M
Thus the phosphate [H2PO4–] concentration would be reduced from 7 Ã— 10-3 M to 1.5 Ã— 10-6 M.
It is evident that the availability of Al3+ and Fe3+ for reaction with phosphate ions is regulated by the hydroxyl (OH–) ion concentration and it is also found that phosphorus fixation (precipitation) can be decreased by increasing the pH of the soil.
But in case of reactions with calcium compounds like Ca(OH)2 and CaCO3, other reactions may take place to precipitate the phosphorus as unavailable to plants. Based on equilibrium constants and solubility product data of various soil phosphorus reactions, a phosphate cycle is depicted in Fig. 17.3.
When di-calcium phosphate dihydrate is held in aqueous system at above pH 5.0 and subjected to repeated extraction, more phosphorus comes into soil solution than that of calcium and as a result residue becomes more basic and approaching towards the formation of hydroxy apatite (less soluble), In a similar way di-calcium phosphate changes into carbonate apatite (less soluble) in presence of CaCO3.
However, a simple scheme for phosphate fixation in soils is appended below (Fig. 17.4).