Potassium Recommendations for Corn in North Dakota: Influence of Clay Chemistry.

Corn has recently become a top-five crop in area within the state of North Dakota. Potassium recommendations for corn were originally borrowed from central Corn Belt states, and were little researched within the state until recently. About 30 K-rate experiments were conducted in corn over the past 4 years with the goal to redefine the K soil test critical level. However, the research resulted in unexpected yield increases at high soil test levels at some sites and no yield response at low K soil test levels at other sites. Clay mineralogy and potassium feldspar content of the soils was determined all soils in the experiments. The results indicate that highly smectitic clay soils require a higher critical K soil test level compared with soils having more illitic clay content. A State map of smectite to illite clay ratio has been developed, directing farmers and their consultants to the recommendation system they should use in order to better fertilize their corn crop with K fertilizer.