Forms of Soil Phosphorus | Soil Fertility | Fertilisers | Agronomy

In this article we will discuss about the organic and inorganic forms of soil phosphorus used for improving crop productivity.

Organic Soil Phosphorus:

Usual organic forms of soil phosphorus are esters of orthophosphoric acid and numerous mono and diesters. Five groups of organic phosphorus esters have been identified of which three esters are important.

i. Inositol phosphates (up to 20 per cent of organic P).

ii. Nucleic acids (0.6 to 2.4 per cent of organic P).

iii. Phospho lipids (0.6 to 0.9 per cent of organic P).

A C/N/P ratio of 100:10: 1 has been suggested for mineralisation of organic phosphorus. If the carbon/inorganic phosphorus ratio is 200: 1 or less, mineralisation of phosphorus will occur and if the ratio is 300: 1, immobilisation will occur. A concentration of about 0.2 per cent phosphorus is critical in the mineralisation of organic phosphorus substances.

Inorganic Soil Phosphorus:

Phosphorus dissolved in soil water reacts with soil constituents and converted into less soluble forms. Phosphorus, thus, removed from soil solution is said to be retained or fixed. Several mechanisms have been proposed to explain phosphorus retention. These include precipitation-dissolution reactions, sorption-desorption reactions and immobilisation-mineralisation reaction.

Removal of phosphorus from soil solution and its retention at soil surfaces is called sorption. Phosphorus held at the surface of a solid is known as adsorbed phosphorus. If the retained phosphorus penetrates more or less uniformly into the solid phase, it is considered to be adsorbed.

The less specific overall term sorption is often preferred due to problems in distinguishing between these two terms. The reverse reaction, desorption relates to the release of sorbed phosphorus into soil solution. The term fixation is used to collectively describe both sorption and precipitation reactions of phosphorus.

Phosphorus forms difficultly soluble compounds with Fe³⁺ and Al³⁺ at low pH, more soluble compounds with Ca²⁺ and Mg²⁺ at near neutral pH and difficultly soluble compounds with Ca²⁺ at higher pH values. Phosphorus retention is a problem in acid soils high in finely divided sesquioxides.

In the initial stages of acid weathering, phosphate becomes increasingly bound to Fe³⁺ and Al³⁺ ions released from silicate minerals by replacing OH^– from hydroxy minerals or oxygen from oxide minerals.