nd 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 particles, plant residues, and/or any recently applied commercial fertilizers into lakes, ponds, streams, rivers and bays. The risk for soluble P losses is increased when the concentration of P in the soil exceeds the natural soil P holding capacity. The sandy soils of Delaware have naturally have low P-holding capacity. In some cases, P can be carried downward, or leached, through the soil and may eventually reach groundwater.
Once transported into water bodies or groundwater, inorganic P can become a water pollutant. For example, P lost in runoff or leachate may contribute to eutrophication of Delaware’s surface water bodies. Eutrophication is the enrichment of water with nutrients that results in excessive aquatic plant (mostly algae) growth. With time, oxygen depletion of eutrophic waters can lead to fish kills and the accompanying foul smell. Eutrophic water bodies can often no longer be used for fishing, swimming, boating, or other recreational activities.
Can I Test My Soil for Phosphorus?
Phosphorus is one of the elements for which soil testing is useful. When testing is performed by a reputable agricultural laboratory, the results indicate the relative availability of soil P. Soil test results help producers and consultants properly manage application of manure. The soil test report from the University of Delaware Soil Testing Program, or other reputable regional laboratory, will include recommendations for P fertilizer rates for crops if results indicate that P fertilization is necessary.
Contact your county Extension office for more information about soil testing and taking a soil sample.
How Can You Help Protect Water Quality
As a grower in Delaware, you can help protect water quality by following best management practices (BMPs) when using fertilizers and manure. Fertilizer and manure applications are restricted to a three year crop-removal rate on “high P” soils (>150 FIV) to protect water quality. Alternatively, growers may opt to conduct a P Site Index to determine if other P management strategies (i.e.,