Factors Affecting Solubility and Fixation of Phosphorus in Soils

The factors affecting solubility and fixation of phosphorus in soils include the following.

Soil pH

Phosphate retention or fixation in acid soils is due to the reaction of orthophosphate ions, H2PO-4, which is favored in acid media having high contents of iron, aluminum, manganese, and possibly silicate clays.

The phosphate ions react with soluble iron, manganese, and aluminum ions (Fe2+, A13+) to form insoluble phosphates. This fixation or chemical immobilization of phosphate ions by A1, Fe, and their hydrous oxides becomes more pronounced at pH below 5.5.

The more acid the soil the greater the amount of phosphate fixed. Aluminum phosphate is generally more soluble than iron phosphate.

These phosphate compounds when poured into the soil are either;

(i) Precipitated as small particles or crystals, or

(ii) Adsorbed on the surfaces of iron and aluminum oxides or of clay particles.

When the soil is alkaline, at a pH above 7.0, phosphate fixation occurs in different ways. Most of the phosphate ions are probably precipitated as calcium phosphates and magnesium phosphates.

If the soil contains free calcium carbonate such as black clays around the Zaria area or Biu, Plateau in Northern Nigeria, then the phosphate may be precipitated as dicalcium phosphate CaHPO₄ or apatite.

Therefore, the intermediate pH range of between 5.5 – 7.0 represents the soil reaction in which phosphate is most readily available to plants.

Mineralogy

The type and amount of clay in the soil constitute an important factor in the fixation of applied or mineralized phosphorus. As pointed out above, the 1:1 lattice clays such as kaolinite retain or fix more phosphate ions than an equal amount of 2:1 lattice clays such as montmorillonites.

The quantity of clay the soil contains is also important since the more heavily textured soils are likely to fix more phosphate than loams or sand. Banded placement minimizes contact between soil and fertilizer phosphorus before being taken up by roots.

The Presence of Hydrous Oxides in Aluminum and Iron

Apart from the reaction of phosphate ions with soluble Fe, A1, and Mn, H₂PO^–₄ also forms complexes with the insoluble hydrous oxides of these elements such as gibbsite (A1₂O₃. 3H₂O) and goethite (Fe₂O₃. 3H₂O).

These oxides are more prevalent in humid tropical soils where large amounts of H₂ PO^–₄ are usually fixed.

Presence of Organic Matter

Decomposition of organic matter librates phosphorus which participates in the equilibrium reactions between free and adsorbed P ions.

The presence of organic matter may also reduce P fixation by forming coatings around sesquioxides and hydrous oxides of aluminum and iron thus preventing them from complexing with phosphate ions.

Read Also: General Importance of Soil Organic Matter

Carbon Dioxides, CO₂

The decomposition of organic matter by soil microbes is associated with an increase in CO₂ production. The CO₂ forms weak carbonic acid with water and increases the solubility of P.

Redox Potential

In flooded soils (rice soils), redox potential could drop below 0.2V (anaerobic condition). Under these low oxygen or anaerobic conditions, the low soluble ferric ion (Fe³⁺) is reduced to the very active and highly soluble ferrous form (Fe²⁺).

This affects the Fe-containing skin of occluded P and can thus result in P release. Under reducing conditions, surface adsorbed P is also released.

Root Exudates and Organic Materials

In the rhizosphere soil, organic materials usually enhance the proliferation of soil microorganisms which in turn increase the mineralization of organic matter and the release of inorganic phosphorus.

Phosphorus status of the Soil

The degree of phosphorus saturation of the soil or the amount previously fixed by the soil also affects the subsequent fixation of added phosphorus fertilizer. Repeated application of high doses of phosphorus fertilizer could satisfy the P-fixation capacity of P-fixing soils.

Further application of phosphorus will then go directly into the solution where plants can take up almost all the applied phosphorus.

Phosphorus Availability Index

The availability index refers to the amount of nutrient that is available to the crops or the amount of nutrient uptake. It is an indication of how much of the nutrient is available to the crop. The objective is to find suitable extractants that could give suitable indices of the readily available (soluble) phosphorus that correlate with plant uptake.

Those extractants in use include water or carbonated water, dilute organic and inorganic acid, dilute alkaline, highly buffered, and salt solutions.

The surface P is usually measured since the amount released by organic matter will eventually go to the adsorbed surface – P. A combination of surface P and organic P gives a better P availability index especially in Western Nigeria if the soil has just been recovered from fallow.

In conclusion, the total phosphorus content of tropical soils is generally low with a range between 24-1087 ugg^-1. However, only a small percentage of these levels are present in available forms.

This low availability is more pronounced in tropical red soils which contain hydrous oxides and kaolinitic clays which usually lock up native phosphorus and also fix added fertilizer phosphorus.

In acid soils (pH<5.5), inorganic phosphate is fixed by iron and aluminum ions while phosphate is precipitated as calcium phosphates in calcareous soils. Maximum phosphorus availability is usually achieved at pH 6.5 for mineral soils and about 5.5 for organic soils and oxisols.

Liming practices that increase soil pH from 5.5 to 6.5 improve organic matter mineralization, control the amounts of soluble iron and aluminum and consequently increase the availability of phosphorus to plants.

Read Also: Soil Reaction: Meaning, Acidity, and Liming