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 bel...