Dynamics of K and NO₃ concentrations in the root zone of winter wheat at Broadbalk using specific-ion electrodes

Summary Procedures for measuring K + and NO - 3 activities in the root zones of field crops, using specific-ion electrodes, were standardized. For K, a 1·0 M-NaCl salt bridge and KC1 standards in water, for NO 3 , a saturated KC1 salt bridge and KN0 3 standards in water, and for both electrodes, a 1:0·5, soil: water ratio, and 30 sec equilibration time were found satisfactory. Recovery of added K in soil pastes by the K electrode and chemical analysis of the soil water extract compared well, but the recovery was about 8% only. The corresponding recovery of added N was about 87 and 95% respectively. Relative changes in the rates and magnitudes of NO 3 and K concentrations were measured with these electrodes, laterally and vertically, in the root zone, during active crop growth, from the N 2 ½(PKNaMg), N 2 PKNaMg, and N 4 PKNaMg treatments of the Broadbalk Winter Wheat Experiment. In all fertilizer treatments, at all times, the nutrient concentrations were most at 45 cm from the crop (in the uncropped area) and least within the cropped area. The differences between these extremes represent nutrient depletion by the crop, the ‘45 cm’ measurementsindicating changes in uncropped, but fertilized, areas. Soil nitrate depletion by the crop was much more at 12·5 cm and 20 cm depths than at 5 cm. Maximum NO 3 depletion was observed during the later stages of crop growth, at ‘pre-panicle emergence’ and at ‘grain filling’. Depletion decreased and the soil NO 3 level recovered partially as the crop reached maturity. Periodic changes in the K concentration at each site and the corresponding K depletions were much less. Periods of IC stress on the soil were few and less clearly demarcated. Soil K concentration started to recover at the ‘grain filling’ stage about a month earlier than with NO 3 . Changes in NO 3 and K concentrations seem to relate more to the amounts given of each nutrient, than to the N:K ratio in each fertilizer treatment. However, changes in NO 3 and K concentrations, and also NO3 and K depletion, occurred consecutively. This indicates an alternating periodicity in the demands of the crop for NO 3 and K respectively throughout growth.