Tuesday, November 14, 2006

Yield, Potassium Removal, and Post-Harvest Soil-Test Values as Affected by Potassium Fertilization for Corn and Soybean.

Matthew W. Clover1, Antonio P. Mallarino1, and Pedro Barbagelata2. (1) Iowa State Univ, Dept of Agronomy, Ames, IA 50011, (2) Instituo Nacional de Tecnologia Agropecuaria, Estacion Experimental de Parana, Parana, Argentina

Soil-test K and estimates of K removal with harvest are tools used to determine K fertilization rates for corn-soybean rotations and other crops.  Incorrect soil-test calibrations and estimates of K removal can have significant economic impacts.  A study based on 20 two-year field trials was conducted from 2003 to 2006 in Iowa to evaluate the effect of K fertilization on corn and soybean grain yield, plant and grain K concentration, and post-harvest soil K measured with ammonium acetate (AA), Mehlich 3 (M3), and sodium-tetraphenylboron (STB) procedures.  Five K treatments (0, 34, 67, 134, and 202 kg K2O ha-1) were applied for the first crop and in the second year the plots were subdivided to apply either no K or 134 kg K2O ha-1.  A grain yield response was observed in most years and locations when initial AA or M3 K was at levels at which K fertilization is recommended in Iowa (<171 mg K kg-1).  Plant and grain K concentrations were increased by increasing K rates at most sites, but sometimes K rate effects on post-harvest AA and M3 K were much less than expected.  Measurement of post-harvest non-exchangeable soil K with STB partly explained the results.  Lower than expected K fertilization effects on STK build-up often observed in the Midwest could be explained by too low estimates of K removal and retention in non-exchangeable forms.