Prediction and Measurement of the Decline in the Critical-K, the Maximum-K and Total Cation Plant Concentrations during the Growth of Field Vegetable Crops

The declines in maximum and critical K concentrations in plant dry matter of 16 different vegetable species were predicted from emergence to maturity by assuming that both were proportional to the critical %N calculated in terms of plant dry matter per unit area using previously-described equations. Values of the proportionality constants were obtained from published measurements made, at commercial maturity, of crops grown in multi-level fertilizer experiments. Predictions were tested: ( a ) against measurements made at intervals during growth in experiments receiving a single level of fertilizer; ( b ) against measurements made of the highest K concentrations measured at the seedling stage in soil-K gradient experiments; and ( c ) against the results of a K-fertilizer-response experiment in which harvests were carried out at intervals during growth. Results were consistent with the predictions and, thus, with the view that both critical and maximum K concentrations are proportional to critical %N throughout growth. K concentrations in plant tissue water fluctuated widely during the growing season and values, averaged over the entire growing period for each of ten crops, varied by a factor of 3. These values were proportional to the mean cation concentrations (meq l −1 ) calculated by assuming that all the ions were in solution. Total cation concentration (meq 100 g −1 ) of dry matter was linearly related to critical N concentration. Evidence was also obtained that maximum K concentration, critical K concentration and total cation concentration (meq 100 g −1 ) during growth were linearly related to relative growth rate, provided that crop weights were greater than 2 t ha −1 dry matter. A hypothesis was developed to co-ordinate these findings.