Phosphorus Loss: Can We Predict It?

	Robert Hoeft Professor of Soil Fertility Department of Crop Sciences (217) 333-4424; rhoeft@illinois.edu
		Ines Ruffo Graduate Research Assistant Department of Crop Sciences (217) 333-4424; daverede@illinois.edu

In recent years, society has become concerned about the influence of excess phosphorus on water quality. Unlike nitrogen, phosphorus does not create a human health hazard in water supplies. However, it is a pollutant, contributing to excessive growth of algae and other aquatic plants. The ultimate decomposition of this excessive growth limits the use of surface waters for recreation, fisheries, industry, and drinking. In most water bodies, phosphorus is the element most limiting for plant growth.

Figure 1. Measuring phosphorus runoff.

Phosphorus contamination of surface waters comes from point source emissions (industry and waste water from sewage treatment systems) and non-point sources-primarily, agriculture. The primary mechanisms by which agriculture contributes phosphorus to surface water is through runoff and erosion.

Figure 2. Concentrations of soluble P in surface runoff from experimental plots as a function of manure applications rates and methods.

Figure 3. Concentrations of soluble P in surface runoff from experimental plots as a function of fertilizer application rates and methods.

Prior research has indicated that there are a number of factors, including soil test level, residue cover, manure and fertilizer application, slope, and proximity to water body that influence the concentration of phosphorus that reaches water bodies. Unfortunately, most of this research has been conducted on soils of the southern United States and/or on pasture soils. In an attempt to determine management practices that will minimize the potential for contamination from farm fields in Illinois, an experiment was established at Monmouth, IL in 1999 (Figure 1). While the results are still considered preliminary, some interesting observations can be made from the first year's results.

Injection of manure (Figure 2) or incorporation of fertilizer (Figure 3) with a chisel plow on soybean stubble minimized the concentration of soluble P. However, if either the fertilizer or manure were left on the soil surface, soluble P concentrations were excessive.

Figure 4. Concentrations of soluble P in surface runoff as affected by soil test P levels.

Increasing soil test levels from approximately 30 to 300 lb/acre resulted in an increase in soluble phosphorus concentration with both no-till and chisel plow systems. However, the increase was several magnitudes greater with no-till than with chisel plow (Figure 4). These results were not unexpected, as it is well known that phosphorus is relatively immobile and thus does not move down into the soil unless it is physically moved with a tillage operation. When it is not moved down from the soil surface, it will be susceptible to runoff in the drainage water.

Even though soluble phosphorus concentrations are well above the generally accepted level that causes eutrophication, the total amount of soluble P lost is of no economic significance to a farmer (compare Figure 5a and 5b). Note that the total soluble P loss is less than 1/4 pound per acre with a 3-inch-per-hour rainfall storm on no-till land testing near 300 lb P/acre.

Figure 5a. Concentrations of soluble P in surface runoff from no-till bean stubble at four soil test P levels.

Figure 5b. Total amount of soluble P lost in surface runoff from no-till bean> stubble at four soil test P levels.

Results obtained to date confirm that following certain management practices will minimize the potential for soluble P loss in runoff. These practices include:

Ensuring that excessively high phosphorus soil test levels are not maintained.
Minimizing erosion.
Matching nutrient applications to crop needs.
Incorporating or knife-injecting fertilizer or manure where possible without destroying crop residues required for soil conservation purposes.

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Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension