Phosphate and Lime Potential of Soil

After reading this article you will learn about the phosphate and lime potential of soil.

Phosphate Potential:

The reactions involved in the soil phosphate equilibrium are dissolution and precipitation of sparingly soluble compounds by the common ion principle, solubility product principle, salt effect and by the adsorption, of phosphate on the surface of the soil particles.

Phosphorus exists as the following equilibrium conditions:

The phosphorus activity in soil solution is controlled by the most stable P compounds like Al-P, di-calcium phosphate and possibly octacalcium phosphate. Since these compounds are all metastable under ordinary soil conditions they slowly form more insoluble phosphates like hydroxy and Fluor apatite, strengite and variscite.

The phosphate potential is defined as ½p Ca + pH₂PO₄ with the hydroxide potential of the cations Ca, Al and Fe defined as pH— ½p Ca or pH— â…“ Al or Fe and one used in determing the nature of the solid phase and lime.

The term P-potential is introduced by Schofield (1955) to be used as an index for availability of soil phosphorus. The phosphate potential is defined as the amount of work that must be conducted to move reversibly and isothermally an infinitesimally small amount of a phosphate ion from a pool of phosphates at a specified location at atmospheric pressure to the point under consideration.

By measuring the pH and the total P concentration in 0.01 M CaCl₂ solution, the solubility product of phosphate ions can be expressed as:

K_sp = (Ca²⁺)^½(H₂PO₄)

By taking – log, this can be changed into,

– log K_sp = – ½log (Ca²⁺) + – log (H₂PO₄)

or pK_sp = ½ PCa + pH₂PO₄

where, p = log.

It is called the phosphate potential. It can be used to make indirect prediction as to the phosphate availability to the plants.

Low phosphate potential suggests high availability of phosphorus, while high phosphate potential suggests low phosphorus availability.

For the evaluation of the availability of phosphate to plants and their capacity to mineralize it in the soil, the most important is the concept of mobility of phosphates or their compounds in soils. The mobility of phosphate in the soil is its capacity to enter the soil solution from the solid phase of soil. Mobility is controlled by intensity and capacity factors.

Intensity factors which do not depend on mass are temperature, redox potential, solution P concentration, activity of compounds etc., whereas capacity or quantity factors which depend on mass, are the amount of heat in the system, amount of P substances etc. the intensity factors are the concentration of phosphate ions in the soil solution whereas the capacity factors are the content of solid phase phosphate compounds in soils which provide the level of intensity of phosphate in the soil solution.

As an intensity parameter, the change in Gibb’s free energy or chemical potential (µ) may be used; µ = µ⁰+ RT In a_i. This makes it possible to express the mobility as work which must be done to change the concentration (activity) of the component in the soil solution.

The value of a_H2PO4^– is used as an intensity index of mobility of phosphates. However, this value is not convenient, since it depends on the dilution and concentration of different cations in the soil solution.

Hence it has been suggested by Schofield that the chemical potential of mono calcium phosphates in the equilibrium liquid phase of soil is used to evaluate the possible entry of phosphate from the solid phase of soil into soil solution. This value is called as the “phosphate potential”.

If the phosphates of the soil solution are in equilibrium with the precipitates of mono-calcium phosphate [Ca(H₂PO₄)₂], then the activity of phosphate ions in the soil solution will decrease with an increase in the concentration of Ca²⁺.

In fact, phosphate potential is the negative logarithm of the activity of phosphates in the soil solution and is proportional to its negative thermodynamic potential.

The higher the phosphate potential value, the lower is the activity of phosphates, so in general, the lower will be the phosphate concentration, whereas in case of lime potential, the higher the lime potential (pH—1/2 PCa) the greater will be the activity of calcium hydroxide in the solution.

Lime Potential:

The negative logarithm of the ratio of the hydrogen ion activity to the square root of the sum of the activities of calcium and magnesium in the soil solution. It is generally written as pH –1/2p (Ca + Mg). It can also be written as 1/2 log [(Ca + Mg) (OH)₂ + pKw, where Kw is the ionic product of water. Lime potential is an expression of the sum of the activities of calcium and magnesium hydroxides in the soil solution.

The relation between the lime and phosphate potential for a saturated solution of di-calcium phosphate is:

2(pH— ½ pCa) – (pH + pH₂PO₄) = 0.66 at 25°C

and for octacalcium phosphate:

8(pH— ½ pCa) – 3 (pH + pH₂PO₄) = 11.7

Under conditions when the solid phase dissolves consistently, and for variscite:

3 (pH— â…“ pAl) – (pH + pH₂PO₄) = -2.48 at 25°C

and for strengite:

3 (pH— â…“ pFe) – (pH + pH₂PO₄) = -6.3 at 25°C.

But these values are only valid if all the Al or Fe in solution is present as the simple ion, that there is no gibbsite or aluminium hydroxide in variscite and the pH is sufficiently low to ensure the phosphates dissolve consistently.

The term “equilibrium phosphate potential” (EPP) is a better estimate of the chemical potential of the phosphate in the original soil than the Schofield’s phosphate potential. The EPP is determined in a phosphate solution of such a concentration that there was no loss or gain of P when shaken with the soil. The EPP is also considered a better index of the phosphorus available to plants.

Quantity/Intensity Relationship:

Intensity of supply and quantity indicate the total amount or quantity of soil P. The importance at which this function decreases as the phosphate is taken up by plants from the ‘pool’ of phosphate in the soil which is at the same chemical potential. The capacity or the phosphate buffering capacity of the soil to resist depletion is determined by the nature of the relationship between intensity (I) and quantity (Q) factors.

If Q and I are expressed as the weight of phosphate per unit volume of soil and soil solution respectively, the capacity, a dimensionless parameter (If Q and I are expressed in same units) depends on the bulk density of the soil affecting the volumetric soil moisture content.

Within a limited P concentration where Q/I relationship is linear, the capacity factor ‘C’ is constant.

The phosphate buffering capacity is defined as the reciprocal of the change in chemical potential of mono-calcium phosphate per unit change in the total labile P. The logarithm of P concentration is linearly related to the values of labile P.

The slopes of the lines relating the two quantities (∆Q/∆I) represent the phosphate buffering capacity of the soil either for precipitating P or for releasing P. For a steeper slope (smaller buffering capacity), the change in the intensity factor per unit change in the quantity of labile phosphate will be greater.

As the capacity of the soil depends on specific soil properties viz. nature and amount of clay, amount of CaCO₃, organic matter content and the amount of P present in the soil etc. Soils vary greatly in their capacity to maintain concentrations at equilibrium. The quantity factor depends on the volume of the soil tapped by the plant roots and hence physical properties of surface and subsurface soils.