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Change in Soil Test P Following Long Term P Fertilization Strategies

Start Date: 2011
Principal Investigator: Dan Kaiser
Organization: University of Minnesota, Department of Soil, Water, and Climate
Status: Completed

Background Info

According to current recommendations, response to applied potassium (K) is likely when soils test lower than 160 ppm. Simple strip trials can be used to correlate crop response to soil test values. Paired strips, with or without K fertilizer, can be used to determine relative crop yields, which can be correlated to soil test values. Three strip trials consisting of about 2 acres were planted each year, and soil samples will be collected from the 0–6-inch and 6–12-inch depths every 40 feet. Yield data will be collected at the same intervals and correlated with the soil data. The data will be compiled to determine the soil test level above which yield response to K is not expected. This information will be used to update the current recommendations.


Determine optimum phosphorus fertilizer application rates for corn and soybean over a long-term rotation.

Evaluate if timing of broadcast phosphorus fertilizer application impacts corn and soybean yield.

Determine if starter fertilizer for corn after broadcast P for soybean can maintain optimal yield of both crops.

Study the impacts of P application rate and P removal in grain on long term trends in soil test levels.

Evaluate critical soil test P levels (soil test where, on average, 95% of maximum yield is achieved).

Determine if sub-soil P concentration is being affected by removal of P in corn and soybean grain.

Key Findings

Medium testing P sites were selected to study the effect of timing of application and fertilizer rate in 2-year corn-soybean rotations. Corn and soybean yield was significantly increased by P over the years. Increase in soil test P varied by location and typically occurred more frequently within individual blocks within locations. It look less fertilizer P to increase soil P at Morris and the most to increase soil P at Saint Charles. However, greater rates of fertilizer could be removed at the previously mentioned sites in order to maintain soil test P.

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