From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1999, No. 1


High-yield cropping systems depend on productive soils. One of the most important resources for building productive soils is livestock manure. Many crop yields have been set on soils with a history of manure use. Manure contributes nutrients, builds soil organic matter, and improves soil tilth. If manure applications can be targeted to low-risk areas for erosion and runoff, these areas can become focal points for high-yield crop management.

Manure builds soil productivity. But it can also pollute air, water and the soil. Manure must be applied with careful management, and this has led to the development of increasingly sophisticated nutrient management plans.

Most nutrient management plans emphasize a balance between nutrient supply and crop removal. While a balance is ideal, it is not realistic for many situations. In regions where livestock production is concentrated, the farm land base is not sufficient. Traditionally, most manures have been applied at rates that supply more phosphorus than the crop removes. This has not harmed the crop, as soil phosphorus levels several times higher than optimal do not reduce yields. Also, soil chemical processes gradually make applied phosphorus less and less available. Continued additions of fresh soluble phosphorus contribute to the health of the crop.

The buildup of soil phosphorus is a risk for the environment in hydrologically active areas—that is, parts of the landscape where surface runoff or erosion are likely. The risk of phosphorus loss to surface water depends on both source and transport factors. Areas at risk are where high soil phosphorus or high application rates coincide with zones of active surface runoff or erosion.

An environmental phosphorus index ranks vulnerability to phosphorus loss. The index incorporates transport factors affecting runoff and erosion—such as slope gradient, slope length, and distance to watercourse—and source factors including soil test phosphorus and rate and method of application of manure and fertilizer phosphorus. The phosphorus index often identifies a critical area comprising 2 to 15 percent of a field from which 90 percent or more of the phosphorus loss occurs.

The phosphorus index targets the low-risk areas for manure application to build soil productivity. The soils in these areas safely absorb the applied phosphorus and benefit from the other constituents of the manure. These are the areas where efforts need to be targeted at improving crop performance by supplying optimal combinations of manure and fertilizer nutrients to raise potential crop yields.

The phosphorus index is not a finished product. The index is an approximation of risk rather than a model of process, but its current form can effectively direct limits on manure applications and nutrient budgets to hydrologically active zones. However, more scientific work is needed, both to validate its accuracy and to improve the estimation of its component source and transport factors.

Soil tests that identify sorption capacity should be an essential component of an environmental phosphorus index. Some soils can adsorb up to 17,000 pounds of P2O5 per acre. Some calcareous soils have a phosphorus retention ability with no practical limit. On the other hand, some soils do become saturated with phosphorus to the point where both surface runoff and subsurface drainage water carry off excessive amounts.

The phosphorus index is site-specific. Appropriate application of the phosphorus index may demand the most intensive site-specificity that modern precision agriculture technologies can provide.

Working with an environmental phosphorus index, you can use manures and fertilizers to build soil productivity, resulting in high-yield cropping systems compatible with water quality.


For more information, contact Dr. Tom Bruulsema, Eastern Canada and Northeast U.S. Director, PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone: (519) 821-5519; E-mail:
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