Phosphorus is critical to all life forms because of the role it plays in many important biomolecules such as DNA (deoxyribonucleic acid), phospholipids, and ATP (adenosine triphosphate). The primary mineral form of phosphorus is rock phosphate, or apatite. The amount of total
phosphorus found in a surface soil can vary greatly, ranging from <100 μg P g-1 (200 kg ha-1) in a very sandy soils to >1,000 μg P g-1 (2000 kg ha-1) in soils derived from basic rocks.
A model of the phosphorus cycle shows the various compartments of phosphorus in the terrestrial environment. Phosphorus is affected by both biological and chemical reactions. This model divides the phosphorus cycle into a geochemical sub-cycle and a biological sub-cycle, with the solution phosphorus pool serving as the central point in the overall cycle.
Solution phosphorus is the source of orthophosphate for plant and soil
Biological phosphorus sub-cycle
In the biological phosphorus sub-cycle, orthophosphate can be taken up by plants or immobilized into microbial biomass. As plant residues and animal remains and wastes are returned to soil, the organic phosphorus may be: directly incorporated into stable humus mineralized to orthophosphate, or immobilized into the microbial biomass. Biomass phosphorus is subject to incorporation into humic substances and mineralization and immobilization reactions. The turnover or cycling of the biomass contributes significantly to the labile organic phosphorus pool. Crop removal and erosion are two mechanisms for loss of organic phosphorus.
Geochemical phosphorus sub-cycle
In the geochemical phosphorus sub-cycle, orthophosphate is solubilized from primary and secondary minerals by chemical and biochemical weathering processes. Dissolution of these compounds in the soil solution or solubilization through microbially produced organic acids releases orthophosphate to the soil solution for plant and microbial uptake. Orthophosphate can adsorb to aluminum and iron oxides (labile inorganic phosphorus) or precipitate as aluminum, iron, or calcium phosphates (secondary minerals).
Syed Shabbar Hussain Shah & Ali Turab
Department of Soil and Environmental Sciences
University of Agriculture, Faisalabad