USING THE WILLIAMS EQUATION TO EVALUATE NUTRIENT-UPTAKE RATE BY INTACT PLANTS

The Williams equation uses root growth and nutrient accumulation data to evaluate nutrient uptake by plants. Accuracy of this equation depen ds on basic assumptions of constant uptake rate with time and space. T he validity of the Williams equation and the assumptions underlying it were tested with sorghum [Sorghum bicolor (L.) Moench] plants grown i n solution culture and analyzed for N, P, K, Ca, and Mg uptake. Only N and Ca influx were independent of time, as required by the equation. Influx of P, K, and Mg decreased linearly during the entire growth per iod. Mean root radius, which is assumed constant, decreased linearly w ith time. Consequently, a cautious and selective use of the equation i s recommended.