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, and the end products are soluble the plant-available orthophosphates. Once in the soil solution, the orthophosphate form of P can be taken up by plant roots. The soil solution can also be replenished from several pools of inorganic (mineral) P in the soil. Solid P minerals in the soil can dissolve in the soil solution when concentrations of soluble P diminish. This process can be compared with adding sugar to a glass of iced tea. When solid sugar is added to tea, it will dissolve in the liquid. Phosphorus can be attached to soil particles such as clay or specific minerals that contain iron (Fe) or aluminum (Al). Phosphorus can detach from these soil particles, thereby supplying P to the soil solution via a process called desorption. Finally, solid rocks can be a source of P as they break down into soil over a long period of time by a process called weathering.
Just as soil solution P can be replenished when the concentration of P becomes low, P can be removed from the soil solution if the amount of P in the soil solution gets too high. Consider the iced tea example again. If too much sugar is added to the tea, some of it will not dissolve and will remain in solid form at the bottom of the glass. Similarly, when concentrations of P in the soil solution are too high, some of the dissolved P will form solid P minerals by a process called precipitation. Depending on soil pH, precipitation can result in the formation of solid calcium phosphate minerals (high soil pH) or aluminum and iron phosphate minerals (low soil pH). Alternatively, P can be removed from the soil solution and attach to soil particles like clays or iron (Fe) and aluminum (Al)-bearing minerals via a process called adsorption.
Phosphorus and Water Quality
Phosphorus can move from the soil to surface or groundwater as the field drains following heavy rain or excessive irrigation. When the rainfall or irrigation rate is higher than the soil’s ability to absorb water, the result is runoff. Runoff can transport soluble P, P attached to eroding soil particl