Characterisaton of Phosphorus Present in Soil

After reading this article you will learn about the inorganic and organic characterisation of phosphorus present in soil.

Inorganic Characterisaton of Soil Phosphorus:

Inorganic phosphates in soil may be usually classified on the basis of physical,mineralogical, chemical nature or their combinations. Various crystalline phosphates formed in soils from phosphate fertilization may not be always stable under normal soil conditions.

As the phosphate concentration in the soil solution decreases, because of diffusion of the soluble phosphate outward into the surrounding soil and reaction with the soil minerals, the crystalline phosphates formed upon fertilization tend to release soluble phosphate, at the same time either dissolving or undergoing transformation from a less soluble to more stable form.

However, a number of mechanisms are involved for the retention of phosphorus in soils which includes precipitation-dissolution reactions, sorption-desorption reactions and immobilization mineralisation reactions. Sorption means the removal of phosphorus from the soil solution and subsequent retention on soil surfaces.

When phosphorus is held at the surface of a solid, the process is known as adsorption. If retained phosphorus penetrates more or less uniformly into the solid phase, it is said be absorbed or chemisorbed or sorption. Desorption means the release or sorbed phosphorus into the soil solution. Fixation includes both sorption and precipitation reactions of phosphorus.

It has been mentioned earlier that there are mainly four groups of inorganic phosphorus dominantly present in soils namely, A1—P, Fe—P, Ca—P and reductant soluble or occluded Fe and A1—P.

Some of the most important soil-phosphate fertilizer reaction products of different solubility are formed due to application of soluble phosphatic fertilizers which are as follows:

Variscite –AlPO₄.2H₂O

Strengite – FePO₄.2H₂O

Fluorapatite – Ca₁₀(PO₄)₆.F₂

Hydroxyapatite – Ca₁₀(PO₄)₆(OH)₂

Carbonato apatite – Ca₃(PO₄)₆.CaCO₃

Octa calcium phosphate – Ca₈H₂(PO₄)₆.5H₂O

However such formation of reaction products usually depends on the nature of soils and other climatic conditions. Phosphorus forms difficultly soluble compounds with Fe³⁺ and Al³⁺ at low pH and more soluble compounds with Ca²⁺ and Mg²⁺ at near neutral pH and also difficultly soluble compounds with Ca²⁺ at high pH.

There is a wide range in solubility of these various phosphate compounds and their availability to crops is usually highest within a soil pH range of 6-7. It is evident that the phosphorus fixed is held with five times more bonding energy in acid soils than in calcareous soils.

Organic Characterisaton of Soil Phosphorus:

Organic phosphorus can be extracted from soil as organic form and not simply as inorganic orthophosphate.

Naturally occurring organic forms of phosphorus are esters of orthophosphoric acid and various mono- and di-esters have been characterised from soils and such esters of phosphorus have been identified as the following five classes: inositol phosphates, phospholipids, nucleic acids, nucleotides and sugar phosphates, of which first there are the dominant organic phosphorus present in the soil.

a. Inositol Phosphates:

Inositol is a homo cyclic sugar like compound, C₆H₁₂O₆, which can form a series of phosphate esters ranging from mono to hexaphosphates. It is the most dominant group of all organic P fractions and it is present in soils about 35% or more of the total organic P. Phytic acid (myo inositol hexa phosphoric acid) is the most common phosphate ester present in soils.

The inositol phosphates are released from organic materials in soil at a very slow rate, but they are rapidly stabilized and can accumulate in some particular soils to the extent that they account for more than half of the organic phosphorus and about one- fourth of the total phosphorus.

Inositol hexaphosphate forms various insoluble salts through reactions with Fe and Al under acidic soil conditions and with Ca under alkaline conditions. It also can form stable complexes with proteins and with some metal ions. It is resistant to enzyme attack.

It is also adsorbed by soil clay minerals. Myoinositol phosphates are sorbed, being lower with the decrease in the number of phosphate groups. Sorption of inositol penta and hexa phosphates occurs at the same sites like that of inorganic orthophosphate ions in soils.

b. Phospholipids:

Phospholipids are fatty in nature and are conventionally extracted by the use of fat solvents. Most common phospholipids are derivatives of glycerol. Phosphatidylcholine (lecithin) and phosphatidy lethanolamine are the dominant phospholipids found in soils. The phospholipids content in soils vary from 1-5% of the total organic phosphorus.

c. Nucleic Acids:

Nucleic acids are major components of the organic phosphorus in plants and micro-organisms. It can be determined with the absorption of ultraviolet light in the wave length range of 255-275 ml by extracts of soils high in organic phosphorus. Two distinct forms of nucleic acids—RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) occur in all living organisms.

Each consists essentially of a chain of sugar units either ribose or deoxyribose, attached by phosphate ester linkages. Nucleosides are units containing one molecule of sugar attached to one molecule of nitrogeneous base. Phosphate derivatives of nucleosides are called nucleotides.

Nucleic acids are possibly released into the soil more quickly as compared to inositol phosphates and also are broken down rapidly. The amount of nucleic acids present in the soil will vary and however, in most soils, it contains up to 2.4% of the total organic phosphorus.