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On-Farm Assessment of Critical Soil Test P Values in Minnesota

Start Date: 2010
Principal Investigator: Daniel Kaiser
Organization: University of Minnesota, Department of Soil, Water, and Climate
Status: Project Merged with Evaluation of Critical Potassium Levels in MN Soils in 2011

Background Info

Fluctuating fertilizer prices can make the management of nutrients, especially phosphorus, challenging. Phosphorus (P) soil tests are used to estimate the potential availability and response to P. However, these tests only measure a small fraction of the total P in the soil, and can be affected by soil chemical properties (i.e., pH, organic matter, etc.). Across Minnesota, soils can differ in their relative potential to retain or fix phosphorus. In general, soils in the western part of the state tend to be high in calcium carbonate. This can be problematic since calcium can react with P, making it unavailable for crop uptake. Critical values should be established so producers know how much additional P is required to support crop production in their region. The critical level is the level where there is only a very small chance (less than 5%) the crop will respond to additional fertilizer.


Evaluate corn yield response to P and K based on initial soil test level on irrigated and rain-fed fields

Assess the potential impacts on soil P and K below 6″ on response to P or K fertilizer

Evaluate the potential of luxury uptake of P and K in corn grain

Compare soil test P and K values according to the Mehlich-3 test and commonly used tests for P or K

Determine if extracting K on field moist samples better correlates to yield than dried samples

Key Findings

The probability of response to phosphorus was greatest when soil tested low.

There was a high potential for a large increase in yield in ‘Low’-testing corn and soybean fields, and a moderate chance of a slight increase in yield in the ‘Medium’ class.

After two years of study, the critical soil test level for P was found to be similar to current guidelines.

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