Manure and commercial fertilizers contain nutrients essential for plant growth. Nitrogen, phosphorus and potassium are the most critical of these nutrients. This publication outlines some basic information about phosphorus and its interaction in the northern Plains environment.
Phosphorus (P) is a naturally occurring element that exists in minerals, soil, living organisms and water. Plant growth and development require phosphorus, like nitrogen, in large amounts. Phosphorus is essential for early root development and hastens plant maturity.
The forms of phosphorus present in soil can include organic, soluble or “bound” forms. Understanding the relationship among these forms of phosphorus is necessary to understand plants’ utilization of phosphorus and the extent to which phosphorus can move within the environment. Phosphorus is the least mobile of the major plant nutrients.
- Organic phosphorus — a part of all living organisms, including microbial tissues and plant residue. It is the principal form of phosphorus in the manure of most animals. About two-thirds of the phosphorus in fresh manure is in the organic form.
- Soluble phosphorus — sometimes called available inorganic phosphorus. It can include small amounts of organic phosphorus, as well as orthophosphate, the form taken up by plants. It also is the form subject to loss by dissolution in runoff and, to a lesser extent, leaching.
The soluble form accounts for the smallest proportion of the total phosphorus in most soils. When fertilizer or manure (both containing mostly soluble phosphorus) is added to soil, the soil’s pool of soluble phosphorus increases. With time, soluble phosphorus is transformed slowly to less-soluble (less plant-available) forms.
- Attached or “bound” phosphorus — unavailable inorganic phosphorus. A large amount of the soil’s phosphorus is bound in compounds that are formed when the anionic (negatively charged) forms of dissolved phosphorus become attached to cations, such as iron, aluminum and calcium. Attached phosphorus includes “labile,” or loosely bound, and “fixed,” or tightly bound, phosphorus compounds.
Phosphorus loosely bound to the soil particles (labile phosphorus) remains in equilibrium with soluble phosphorus. Thus, when plant removal reduces the concentration of soluble phosphorus, labile phosphorus is converted to the soluble form to maintain the equilibrium.
Much of the phosphate that living organisms use becomes incorporated into organic compounds. When plant materials return to the soil, organic phosphate will be released slowly as available inorganic phosphate or incorporated into more stable organic materials and become part of the soil organic matter. The release of available inorganic phosphorus from organic sources is called mineralization, and microorganisms carry it out.