Abstract
A previous study showed that there were large differences in the uptake of non‐exchangeable K (K nex ) by ryegrass grown in a glasshouse trial between 19 selected Pallic soils from both North and South islands. The present study evaluated a range of soil testing procedures for K for their ability to explain the variability between the 19 soils in both the uptake of total K from the soil and also the apparent uptake of K nex . A new, simple soil testing procedure, involving extraction of soil K with dilute nitric acid (HNO 3 ‐K), was found to be superior to a number of other published procedures, including estimates of exchangeable K (such as Quick Test K) and reserve K (involving multiple extractions with nitric acid), when correlated with dry matter yield and K uptake. An estimate of step K, calculated as the difference between HNO 3 ‐K and Quick Test K, proved to be better than reserve K in explaining variations in K nex uptake. The proposed HNO 3 extraction procedure is simple, cheap, and effective. Its validity should be tested under field conditions.
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