Evaluation of Phosphorus Management and Critical Soil Test Levels for Corn In the Northern Corn Belt.

Phosphorus management can be challenging in the northern Corn Belt due to soils with variable soil pH values. In Minnesota, the landscape is divided and in the eastern part of the state most soils are acid while in the west most soils are basic. Over the past ten years field studies have been established to determine how to sample variable landscapes and to better identify critical soil test values. In the past 15 years research has focused on using grid sampling to better identify areas within fields to fertilizer accordingly. Some of the earliest work studied small scale variability by sampling fields on a 20 m x 20 m grid. It was found that critical soil test levels were near 15 mg kg-1 according to the Bray-P1 test and the probabilities of response were 80% when soils tested 6-10 mg kg-1, 60 for 11-15, 25 for 16-20, and 7 when soils tested >20. A series of field studies were initiated in the spring of 2010 gridded in 15 m x 15 m areas and confirmed the earlier results. A follow up study was established to look at current recommendations for Minnesota over three cropping years comparing a more aggressive build and maintain strategy with the current sufficiency approach. A field was grid sampled at 0.25 ha and fertilizer was applied according to current recommendations. When soils tested Medium or Low in P either strategy produced positive economic returns. The combination of all studies indicate that 1) critical P values are at or near expected values based on past research; 2) probabilities of response are similar to neighboring states for similar soil P levels; and 3) targeting medium or lower soil test areas within fields will give the greatest returns to fertilizer.