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? Research in Iowa is showing that the current practice of using dried samples in soil test K procedures may sometimes provide inaccurate indications of whether or not crops will respond to additional K. This finding was used to alter soil test K recommendations in 2002. Ongoing small plot and on-farm research trials are showing that the new recommendations provide better predictability about whether or not crops will respond to additional K; however, much more research is needed into the possible causes of crop response variability to create further improvements in K recommendations. IA-09F