Forms of Phosphorus in Soil

How Phosphorous Is Used in the soil

• Most of the phosphorous used by living organisms is combined into organic compounds. When plant material is returned to the soil, organic phosphate is released slowly in the form of inorganic phosphates or is incorporated into more stable materials that become part of the soil. Phosphorous is the least mobile of all of the major plant nutrients, which also include nitrogen and potassium. It is not toxic unless present in extremely high levels.

Organic

• Organic phosphorous is part of all living organisms such as plants and animals. It is also present in microbial tissues and plant residues. It is the main form of phosphorous found in most animal manure. Organic phosphates in the soil are formed primarily from plant or animal tissue and are contributed through biological processes such as animal waste and through dead and decaying organic material.

Soluble

• Also called available inorganic phosphorus, soluble phosphorus is contributed to the soil through inorganic materials such as minerals. This form of phosphorous is the form taken up by plants and the form most likely to be lost through runoff and leaching. Soluble phosphorous often makes up the smallest amount of phosphorous content in soil. Fertilizers and manures contain soluble phosphorous, but over time it transforms into a less soluble, or less available, form.

Bound

• Bound or attached phosphorous is unavailable inorganic phosphorous. Large amounts of the soil's phosphorous content is bound or attached to compounds that are created when the negatively charged ions of dissolved phosphorous become attached to cations, or positively charged atoms such as those in aluminum, iron and calcium. Loosely bound phosphorous, which is called labile phosphorous, remains in balance with soluble phosphorous in the soil. This means that when the amount of soluble phosphorus is decreased, through plant removal for example, this labile phosphorus is transformed to the soluble form to sustain the equilibrium.

Posphorous Loss

• Loss of phosphorous in the soil occurs primarily due to runoff, erosion and harvesting of crops. This is because phosphorous tends to be most concentrated on the surface of the soil, so erosion or runoff can remove a significant amount of phosphorous from the soil. In agricultural soils, this lost phosphorous is replaced with manure or fertilizers.