Variability in Soil Test Potassium and Crop Yield

The primary objective of this project is to improve Iowa State University K fertilizer recommendations for maize and soybean. This research project is actually a compilation of several interrelated experiments being conducted to meet this objective. These projects include long-term K soil fertility trials, maize and soybean residue decomposition experiments, on-farm K fertility trials, examination of different soil sample preparation techniques, and studies that are examining the interaction of nitrogen (N) and K on maize nutrient composition and yield. In this summary, we highlight research on an alternate laboratory method for preparing soil samples prior to K determination. This research focused on the effects of drying soil samples on soil K extracted by a solution of ammonium acetate and then used field response data to correlate K tests based on oven-dried (45-50 35 to 40^oC) and field-moist samples for maize and soybean based on 162 single- and multi-year response trials conducted during six years.

Potassium concentrations extracted from dried samples were higher than those extracted from field moist samples (an average of 1.92 times higher). The ratio of dried:moist soil test K levels decreased exponentially with increasing K. Soil test K concentrations analyzed with field-moist preparation were better correlated with grain yield response than those associated with oven-drying. Critical concentration ranges for maize were 144 to 301 mg K/kg for dried samples and 49 to 84 mg/kg for field-moist samples. Field-moist samples predicted crop response to K fertilization better than the commonly used oven-dried samples. These new guidelines have now been published in the extension bulletin of the university and Soil Science Society of America Journal.