Variability in Soil Test Potassium and Crop Yield

How great is the variability in potassium (K) soil test levels across fields and years? How does this variability impact crop responses to applied K? How well do currently-used soil test K methods predict the likelihood that crops will respond to K additions? These questions are being actively researched in Iowa, both at research stations and in farmer fields.

In 2006, eight long-term K trials revealed large yield reductions when no K was applied to soils testing less than 150 ppm; however, in central and northern Iowa on poorly drained soils, yield reductions without K were observed when soil test levels were less than 170 to 180 ppm.

A question that has sometimes been asked about corn response to K in these studies is how much of the response is to K and how much is to chloride (Cl)? In 2006, potassium chloride (KCl) and calcium chloride were compared as nutrient sources. In the long-term trials, it was found that most of the response to potassium chloride is from K; however, results also indicated a 5 to 8 bu/A response to just Cl, meaning that on soils where no KCl has been applied for a long time, Cl may be limiting and applications needed. IA-09F