Is Air-Drying of Soil Samples an Appropriate Step in Determining Plant Available Potassium for Corn?.

Potassium (K) fertilizer recommendations are mainly based on ammonium acetate extraction  of air -dried soil samples which may lead to over- or under-estimation of plant available soil K levels. On-farm trials were conducted in North Dakota and Minnesota to determine corn response to six fertilizer-K rates and the variation of soil test-K between air-dried (K_Dry) and field moist (K_Moist) soil samples during the 2013 and 2014 growing season. Soil and tissue samples were collected at several corn growth stages and analyzed for plant-available soil K and plant K concentration. The differences between K_Dry and K_Moist decreased exponentially as soil K increased, but increased linearly with increasing soil moisture. Fertilizer K rate increased corn yield and plant K concentration only at Ada, which had the lowest initial available soil K values of the three sites in 2013. Soil sample drying influenced the plant available soil K-test value, generally producing higher K values compared to the moist soil K method. It is unclear based on these initial experiments which method might produce a more predictable K critical value to aid in directing K application for corn in this region.