Along with nitrogen (N) and phosphorus (P), potassium (K) is a primary macronutrient that is needed by crops in greater amounts. As a cation, K can be found in a readily available form on the soil complex (organic matter and soil clays). Soils with greater cation exchange capacity (CEC; e.g., higher in clay, organic matter) will hold greater amounts of K, and will be easier to manage for K fertility.
As K can be easier to manage than N or P, it is sometimes overlooked when fertilizer prices rise. We have observed some strong trends across the region related to K availability and corn yields. In 2018, we saw a significant positive relationship between corn yields and ear leaf K across Maryland and Delaware variety trials. While this trend was not evident in 2019, the site with the highest yields (Marydel, DE) also had the highest ear leaf tissue K.
As an exchangeable nutrient, K can have an antagonistic relationship with Mg and Ca, competing for space on soil exchange sites. This competition between Mg or Ca and K is why soils with greater CEC are easier to manage for all plant nutrients, since higher CEC soils can hold more nutrients. In addition, Mg and K may compete for uptake into the corn root. We have observed a negative relationship between Mg and K in the ear leaf tissue, where lower yields were also correlated to higher ear leaf Mg. These relationships may indicate that corn yields are reduced in soils with lower K or greater competition for uptake with Mg. Relationships between nutrients in soils and plant tissues are complex and rely on many factors. However, UD still recommends maintaining the optimum soil K levels. Also, make sure that excessive Mg applications do not suppress K uptake. For example, UD would still recommend 60 lbs K2O/ac at optimum soil test K levels (90-182 ppm; 50-100 FIV Mehlich 3) for corn with an expected yield goal of 200 bu/ac, while no Mg is recommended if Mehlich 3 soil test Mg concentrations are in the optimum range (>52 ppm; >50 FIV).