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Abstract
Changes in reserve‐K status were measured on a number of historical and current pasture trials using a modified sodium tetraphenyl‐boron incubation method (TBK incubation). Trials included four P, K, and S trials from North Otago, a long‐term (>40 years) P fertiliser trial from Canterbury (Winchmore), a long‐term (>20 years) nutrient depletion trial from Manawatu, and a 2‐year N × K trial from Southland. While only two trials showed a clear DM response to K, testing by both the original (Jackson's) test and the TBK‐incubation method showed changes in soil K status for all soils. The long‐term fertiliser trial showed that K reserves declined with normal annual superphosphate additions when no K fertilisers were applied. Failure of net changes in K reserve status to adequately account for K removal in nutrient budgets appeared to be best explained by significant plant uptake of K below the soil sampling zone (0–75 mm).
Prediction of a dry matter K response when exchangeable‐K levels are near or below critical levels i.e., at Quicktest K (QTK) values < 6–7, is governed by the buffering capacity of soil K reserve. This was linked in this study to measures of both readily available (short incubation; 1–20 h) and long‐term available (168‐h) K. Soils with a large long‐term K reserve can still exhibit a K response under high K demand, but this occurred in only one trial when QTK values were 3 or less, and when short‐term available K was also low. The biphasic nature of K release underlies the complications of predicting K response in many sedimentary soils.
The main means of classifying and predicting K response in New Zealand has historically been from Metson's soil class Kc values. This study showed that K reserve status for a significant number of New Zealand soils is not adequately represented by these values, and that better prediction of K response could be achieved using the new TBK‐incubation method after optimisation against field trial data. Implications for using this method to advance routine soil testing of K are discussed.