Monitoring long-term changes in reserve potassium in some New Zealand soils using a modified sodium tetraphenyl-boron method

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. © 2003 Taylor & Francis Group, LLC.