DERIVATION OF CRITICAL NUTRIENT CONCENTRATIONS FOR GROWTH-RATE FROM DATA FROM FIELD EXPERIMENTS

Present methods for determining critical nutrient concentrations in plants and soils are unsatisfactory if concentrations change with time while the crop is growing. In such cases the critical concentration can only be applied in relation to growth rate at any given moment. For interpreting field experiments this introduces considerable difficulties: two possible approaches to these problems are suggested, one of which uses a simple simulation model. Results from a Brussels sprout [Brassica oleracea var gemmifera] nitrogen experiment are used to show how, using this approach, a single critical sap nitrate concentration (380 mg NO3 N.l-10 for 95% of potential growth rate) may serve to explain the results at all growth stages in three seasons.