From the Fields . . . Analyzing phosphorus levels in soil

From the Fields . . . Analyzing phosphorus levels in soil


By Jim Morrison

Extension Educator, Crop Systems

University of Illinois, Rockford Extension Center

Loss of phosphorus from agricultural lands into surface water is a growing environmental concern.

Factors that affect the movement of phosphorus from agricultural lands include the phosphorus soil test level, tillage, rainfall, slope, and amount, timing, and method of fertilizer or manure application. Many of these factors interact to either reduce or enhance phosphorus losses.

Runoff and erosion are the main reasons for this nutrient’s presence in surface water. However, there is limited, current research on phosphorus runoff from row crop fields associated with Midwestern soils and production practices. Dr. Robert Hoeft, University of Illinois Extension soil fertility specialist, recently shared results of on-going research comparing no-till and chisel plowing with phosphorus added as a commercial fertilizer and with hog manure that was either surface applied or injected into the soil.

Each practice was studied at four soil test phosphorus levels at the University of Illinois Agronomy Research Center near Monmouth.

Regarding phosphorus (P), it is important to remember two classifications. Soluble P is that portion which dissolves in water, and total P refers to the soluble portion plus P that’s attached to soil particles.

Research results to date indicate the following: Analysis of soluble P in surface runoff water showed that high soil test P levels affected both runoff P concentrations and contents with both being higher with higher soil test levels. Soluble P losses were greater from no-till plots than from chisel-plowed plots, but total P losses were greater from tilled than no-till plots.

Both higher soluble P runoff concentrations and higher runoff volumes of no-till plots resulted in significant differences in the total P loss between no-till and chisel plowed plots.

Surface–applied fertilizer substantially increased the soluble P losses compared to chisel plow-incorporated fertilizer. The method of manure application affected the P losses to a much greater extent than the manure application rates. Plots with surface–applied manure had the highest soluble P concentrations and content.

However, the P losses from plots with injected manure were similar to those from the control plots where neither manure nor fertilizer was added.

In summary, Hoeft said the research so far has demonstrated that fertilizer incorporation and manure injection have advantages compared with surface applications from both economical and environmental aspects.

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