Phosphorus Cycling in Agriculture// here is the normal content // ?>
Amy L. Shober, Department of Plant and Soil Sciences, University of Delaware
Based on original documents prepared by A.L. Shober for UF-IFAS
Introduction and Purpose
Crops often receive beneficial nutrients such as phosphorus (P) from manure and/or commercial fertilizer applications. However, the Delaware Nutrient Management Law limits the amount of P that can be applied to many agricultural soils in Delaware. These restrictions were instituted in response to elevated soil test P concentrations following historical poultry litter application at rates that exceeded crop P removal. Elevated soil test P levels, as well as improper use of manures/fertilizer containing P, can increase the risk of P transport from the field by rain or irrigation water to nearby water bodies, which can potentially lead to pollution of the water body. Understanding how P behaves in agricultural soils can help maximize crop productivity and minimize harmful losses of P to local waterbodies. This document helps agricultural producers understand how P interacts in the environment through the P cycle to guide maintenance and sustainability of agricultural crop production. Each agricultural crop differs in crop P removal rate. Therefore, fertilizer and manure applicators should consider the balance between P levels in the soil and removal rates per harvest and hydrologic conditions affecting the loss potential of P.
The Phosphorus Cycle
Sources of Phosphorus
We begin our discussion of the P cycle in a typical grain crop rotation (corn, wheat, double-crop beans) by considering the plant residue left on the field surface after harvest. Crop residues that are left on the soil surface or incorporated into the topsoil during tillage provide a source of organic matter to the soil. These crop residues contain forms of P that are not available for uptake by growing plants. However, soil microorganisms, including bacteria and fungi, change the complex, unavailable forms of P into an inorganic form that is plant-available. Inorganic P can then be dissolved in the water held in the soil (soil solution) and can be taken up by the roots of the next growing crop in the rotation. The cycle repeats when this crop is harvested (Figure 1). This description is a very simplified example of the P cycle; we will now go into more detail.
Decaying plant residues are not the only source of P in the soil. Manure and other organic residues (e.g., biosolids, composted manures, food waste residuals) contain phosphorus in plant available (soluble P), slowly available (phosphorus minerals and P attached to mineral surfaces), and plant unavailable forms. These materials are typically derived from plant residues or animal wastes. Like the crop residue in our earlier example, the forms of P in these materials that are not available to the plant must be converted to soluble P by soil microbes. Commercial P fertilizers are also available for use in crop production. Most of these materials are derived from P rich rocks that were deposited in ancient sea beds many years ago. This P rich material is mined from the earth and processed into more soluble P forms that plants can use. The ability of these P fertilizers to dissolve in soil water depends on the chemical form.
How Does Phosphorus Behave in the Soil?
Phosphorus must be dissolved in the soil solution to be taken up by plant roots. The dissolved forms of plant-available P in the soil solution are called orthophosphates (H2PO4- or HPO42-, depending on the soil pH; Figure 2). The amount of P dissolved in the soil solution at any particular time is usually very small.
Figure 2. Various forms of orthophosphate in soil solution at different pH levels. (Figure credit: FAO adapted from Boyd, 1982)
Once plant roots remove P from the soil solution, it is replenished by the residual P in the soil. We previously discussed how soil microbes transform organic forms of P in plant residues or organic soil amendments into plant-available P. This process is called mineralization, a