Chemistry and Behaviour of Phosphorus Present in Soil

After reading this article you will learn about the chemistry and behaviour of phosphorus present in soil.

Chemistry of Phosphorus:

1. Sorption Reactions:

The surfaces on which phosphate ions enter into sorption reactions of two types-surfaces of constant charge e.g. crystalline clay minerals and surfaces of variable charge including Fe³⁺ and Al—oxides and organic matter where H⁺ and OH^– ions determine the surface charge and calcite (CaCO₃) in which Ca²⁺ and CO ions involve the charge development.

Besides, some other clay minerals including amorphous such as allophane also involves in the phosphate sorption.

Hydrated Fe and Al oxides are the most important surfaces of variable charge in most soils excepting peats and highly calcareous soils. These oxides have surfaces of negatively charged OH groups which take up and dissociate protons (H⁺) and hence they are amphoteric having either negative, zero or positive charge depending on pH.

The pH at which there are equal numbers of positive and negative charges on the surface is known as point of zero charge (PZC). At pH levels below the PZC, phosphorus and other anions like SO₄^2- and H3SiO₄^– are attracted to the positively charged oxide surfaces.

2. Precipitation Reactions:

Precipitation reactions mainly govern by the solubility product principles which are controlled by the pH of the system.

When some common phosphatic fertilizers like super phosphate, mono ammonium phosphate, Di-ammonium phosphate, some poly phosphates etc. are applied to the soil, within a very short time the released soluble phosphorus converts into very less soluble forms rendering unavailable and with time passes the strong insoluble phosphate fertilizer reaction products will form depending on the nature and type of soil as well as soil reaction.

In acid soils mono-calcium phosphate produces a number of substances like di-calcium phosphate (dihydrate and anhydrate), CaFe₂ (HPO₄)₄. 8H₂O; CaAl H(PO₄)₂.6H₂O etc. whereas in calcareous soils, di-calcium phosphate (CaHPO₄) is the dominant initial reaction product and in presence of excess amounts of calcium carbonate (CaCO₃), octacalcium phosphate may also form.

Further, when di-ammonium phosphate is applied to soils, the following reaction products viz. Ca₄ (PO₄)₃.3H₂O; Ca₂ (NH₄)₂ (NPO₄)₂.2H₂O, CaHPO₄-2H₂O; CaNH₄PO₄.H₂O; CaxH₂ (PO₄)6-5H₂O etc. will form. Dicalcium phosphate dihydrate is one of the most dominant reaction products formed in high-calcium soils followed by octacalcium phosphate.

When polyphosphate fertilizers are applied to soils it undergoes precipitation and adsorption reactions. In addition the orthophosphate present initially plus which formed by the hydrolysis of polyphosphates react with the soil components similar to that happened in orthophosphate compounds.

Hydrolysis of polyphosphates results in a stepwise breakdown forming orthophosphates and different short chain polyphosphate fragments. Then such short chain polyphosphates undergo further hydrolysis. However, reactions of polyphosphates in soil and the nature of substances produced are dependent upon the rate of their reversion back to orthophosphates.

Slow rate of hydrolysis permits condensed phosphates to sequester or form soluble complexes with soil cations and hence reduce phosphate retention in soils. Two mechanisms namely chemical and biological are involved in the hydrolysis of polyphosphates. In soils, where both mechanisms can function, the rate of hydrolysis will be rapid.

Enzymatic activity is the most important factor which controls the rate of hydrolysis. Phosphatases associated with plant roots and rhizosphere organisms are believed to be responsible for biological hydrolysis of pyro-and polyphosphates. Various factors like, temperature, soil pH, moisture, organic carbon content etc. can affect the transformation of polyphosphates.

Behaviour of Phosphorus:

Both organic and inorganic forms of phosphorus undergo transformation in soils leading to either release or retention of phosphorus. It is evident that decomposition of organic phosphorus substances gives both active and inactive substances.

The active substances are primarily the portions of the residues that have not yet been transformed into microbial products, whereas the inactive forms of phosphorus behave similarly to the resistant forms of nitrogen in humic acid.

1. Organic Phosphorus:

During mineralisation of organic phosphorus substances, the release of inorganic phosphorus takes place in the soil solution and such released phosphorus reacts very quickly with various soil components forming insoluble complex phosphatic compounds and there by unavailable to the plants.

Mineralisation of organic phosphorus is of three types:

(i) Based on the lowering of organic phosphorus level in soils due to long term cultivation.

(ii) Based on the results of short laboratory investigations decreasing the level of organic phosphorus with simultaneous increase in the amount of inorganic phosphorus in the soil and

(iii) Based on monitoring levels of soil organic phosphorus in the presence and absence of plants considering seasonal variation.

Mineralisation of organic phosphorus is carried by phosphatase enzymes and these enzymes are broad group of enzymes which catalyze the hydrolysis of both esters and anhydrides of phosphoric acid. However, there are a wide range of micro-organisms that are capable of mineralising (dephosphorylating) organic phosphorus on soils through their phosphatases activities.

Phosphatase activity of a soil is due to the combined functioning of the soil micro-organisms and any free enzymes present. Mineralisation of organic phosphorus is not entirely similar to that of organic carbon and nitrogen mineralisation and the mineralisation of organic phosphorus increases with an increase in soil pH but organic carbon and nitrogen mineralisation did not.

Most of the organic soil phosphates are present as inositol phosphate esters and these are prone to adsorption resulting less available in soils having higher adsorption capacity. The ultimate process by which organic phosphates are rendered available is by cleavage of inorganic phosphate by means of a phosphatase reaction.

The principle of this reaction is hydrolysis which is shown below:

For carrying out the mineralisation of organic phosphatic substances in soils it is essential to have some idea about C: N: P ratios in the soil. A carbon: nitrogen: phosphorus (C: N: P) ratio of 100: 10: 1 for soil organic matter has been advocated, but its values ranges from 229: 10: 0.39 to 71: 10: 3.05—depending on nature and type of soils.

C: P inorganic ratio – Process Operates

200: 1 or less – Mineralisation

Above 200: 1 but – Neither net mineralisation nor

Less than 300: 1 – Net immobilisation

300: 1 and above Immobilisation

A concentration of about 0.2% phosphorus is critical in the mineralisation of organic phosphorus substances. If the system contains less than this, net immobilisation takes place, as both the plant and the native soil phosphorus are utilised by micro-organisms. The transformation of P takes place both in upland (aerobic) and low land submerged (anaerobic) soils.

2. Inorganic Phosphorus:

It is evident that most of the soluble inorganic phosphorus either released from the mineralisation of organic phosphorus or applied as soluble phosphatic fertilizers are rendered unavailable to the plants and hardly 20% of the applied phosphatic fertilizers are available to the plant.

The reasons for such recovery are the conversions of soluble form of phosphorus to a form which is very less soluble through reactions with various soil components involving different mechanisms.

Such mechanism for the removal of phosphorus from the solution phase in the soil is known as “retention or fixation”. However, the retention of phosphorus in the soil involves various mechanisms namely, sorption and precipitation reactions.