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 NO3, a saturated KC1 salt bridge and KN03 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 NO3 and K concentrations were measured with these electrodes, laterally and vertically, in the root zone, during active crop growth, from the N2 ½(PKNaMg), N2 PKNaMg, and N4PKNaMg 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 NO3 depletion was observed during the later stages of crop growth, at ‘pre-panicle emergence’ and at ‘grain filling’. Depletion decreased and the soil NO3 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 NO3.
Changes in NO3 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 NO3 and K concentrations, and also NO3 and K depletion, occurred consecutively. This indicates an alternating periodicity in the demands of the crop for NO3 and K respectively throughout growth.