Phosphorus Fractions in the Soil

The different phosphorus fractions in the soil are dynamic, this means that, through different processes, they can be transformed from one into another depending on their concentration. The rate at which the transformation of one phosphorus fraction into another occurs depends on the nature of the process, so different processes may occur at different rates.

Organic phosphorus

The organic phosphorus in the soil consists of humus and the microorganisms living in the soil. Humus is the organic matter that is partially degraded and typically has a dark brown color.

Soluble phosphorus

Soluble phosphorus is the phosphorus that is dissolved in the water present in the pores of the soil, mainly as phosphate ions. This phosphorus fraction is particularly important because it is the only one that can be directly taken up and used by photosynthetic organisms, and is typically the least abundant.

Adsorbed phosphorus

The adsorbed phosphorus is the soluble phosphorus that is attached to the surface of certain rocks, such as clays. The phosphate ions attached to the surface of the rock are then immobilized, making them not directly available for living organisms.