Phosphate Retention and Fixation in Soil

After reading this article you will learn about phosphate retention and fixation in soil.

Kaolinite has a strong affinity for phosphate anions. Phosphate anions can be attracted to soil constituent with the force strong enough to make them insoluble and difficulty available to plant roots.

Tisdale and Nelson (1975) have differentiated phosphate retention from phosphate fixation by the fact in the first case, the phosphate anion held by the soil clay can be removed by treatment with the soil clay with dilute acids e.g. 0.005 N sulphuric acid in contrast with the latter case when the phosphate ion held by the soil clay cannot be removed by the treatment of the soil clay with the same acid.

Phosphorus retention may be caused by anion exchange mechanism when the phosphate anion replaces the hydroxyl anion held by the aluminum ion of the clay mineral especially of 1: 1 type as illustrated in the under mentioned equation:

Phosphate fixation is caused by the reaction of the orthophosphate H₂PO₄ anions with:

(i) Soluble aluminum and ferric ions

(ii) Exchangeable aluminum ion

(iii) Protonated aluminum occurring at the broken edge of kaolinite

(iv) Hydrous oxides of iron and aluminum

(v) Exchangeable and soluble calcium ions and

(vi) Calcium carbonate.

The 1:1 clay i.e. kaolinite decomposes with the decrease in pH in acid soils to liberate aluminum ions in the soil solution. The aluminum ions thus liberated in the soil solution react with water.

Al⁺⁺⁺ + 3 H O.H  → Al(OH)₃ + 3 H⁺

The increase in soil acidity caused by the above-mentioned liberation of hydrogen ions in the soil solution leads to further decomposition of clay that further liberate more aluminum ions in the soil solution.

The aluminum hydroxide thus formed, reacts with the orthophosphate anions to form insoluble aluminum hydroxyl phosphate:

Acid soils also contain high amounts of hydrous oxides of iron and aluminum which react with the soluble orthophosphate anions and precipitate them as insoluble iron and aluminum hydroxyl phosphates.

The phosphate anion also replaces hydroxyl ion held by the aluminium ion of kaolinite

> Al – OH + H₂PO₄^– → Al – H₂PO₄ + OH^–

Hydroxyl ions attached with aluminium ions located at the broken edge of kaolinite gets protonated.

The orthophosphate anions react with these protonated hydroxyl anion:

> Al – OH₂⁺ + H₂PO₄^– → Al – H₂PO₄ + H₂O

The exchangeable calcium ions occurring in soil of alkaline reaction acts as bridge to link the orthophosphate anion with the clay as shown below.

The soluble calcium ion precipitate the orthophosphate anion as insoluble tricalcium phosphate.

The under-mentioned reaction takes place in presence of calcium carbonate.