Temporal Variation of Soil-Test and Nonexchangeable Potassium during the Non-Growing Season in Corn and Soybean.

Extensive research has been conducted to understand the relationships between K fertilization, K removal with harvest, and soil-test K but more research is needed to better understand K cycling and high short-term temporal soil-test K variation. The objective of this research was to study the temporal variation of soil-test K and nonexchangeable K from the time of corn (Zea mays L.) or soybean (Glycine max L. Merr.) grain harvest in the fall until planting of the next crop in spring as affected by the K rate, residue K recycling, and soil sample drying. Three-year field trials that included several broadcast K fertilizer rates were conducted at 35 Iowa sites with different soils and management histories. Soil samples (15-cm depth) and crop residue samples were taken at harvest (in early October), late fall before soils froze (early November), and in the spring (early April). Soil-test K (ammonium-acetate method) were analyzed using dry and field-moist handling procedures. Soil samples also were analyzed for nonexchangeable K by the Na tetraphenyl-boron method. Crop residue sampled each period was analyzed for total K concentration. The results showed that the time of sampling interacting with soil properties, rainfall, K recycling, and soil sample drying in the laboratory can greatly affect soil K pools and soil-test K interpretations for crop production.