DEFINITION OF SOIL ORGANIC MATTER
Phosphorus rank in importance with N and K as major plant nutrients. Phosphorus compounds in soil can be placed into the following three classes:
- organic compound of the soil humus,
- inorganic compounds in which the P is combined with Ca, Mg, Fe, Al and with clay minerals,
- organic and inorganic P compounds associated with the cells of living matter. Microorganisms are involved in transformations of phosphorus between organic and mineral forms.
From 15 to 80% of the phosphorus in soils occurs in organic forms, the exact amount being dependent upon the nature of the soil and its composition. The higher percentages are typical of peats and uncultivated forest soils. From the standpoint of plant nutrion, phosphorus is adsorbed by plants largely as the negatively charged primary and secondary orthophosphate ions (H2
) which are present in the soil solution. Small quantities of soluble organic P compounds are also present in water extracts of soil.
The phosphorus cycle
In a broad sense, the phosphorus cycle in soil involves the uptake of phosphorus by plants and its return ti the soil in plant and animal residues.
As can be seen from picture three general types of compounds make up the bulk of the organic phosphorus in plants, namely: phytin, phospholipids, and nucleic acids. Approximate recoveries of organic phosphorus in these forms are as follows:
|Metabolic phosphates tace|
When crop residues are returned to the soil, net immobilization of P will occur when the C/organic- P ratio is 300 or more; net mineralization will result when the ratio is 200 or less.