| Monitoring the nutrient dynamics in the nutrient solutions of hydroponically-grown sweetpotato plants gave the following results. (1) Monitoring the concentrations of critical individual chemical species over time in the hydroponic solution would allow for optimal nutrient management: (a) While the time variation in the Electrical Conductivity (EC) level of a hydroponic solution could suggest normal nutrient uptake, nitrate uptake inhibition, or increased nitrate uptake, the time variation in EC levels could not identify which specific nutrient species were being inhibited or increased in their uptake; (b) Even when the total nitrate assimilation per plant increased over time, the specific nitrate uptake over time actually decreased significantly and correlated well with the saturation of average growth rate, justifying the addition of nitrogen in the solution to achieve optimal growth during the plant's vegetative phase; (c) Doubled-N by ammonium resulted in the significant suppression of the uptake of nitrate and potassium as well as calcium, phosphorus, magnesium, iron and boron, among others; (d) Under doubled-N by nitrate, approximately twice as much nitrate was taken up from the solution relative to the control, indicating that the uptake of nitrate was nitrate-concentration-dependent; (e) Under doubled-N by nitrate, the uptake of potassium was unaffected; and (f) Doubled-K, designed to initiate sweetpotato rooting, significantly suppressed nitrate uptake as desired and kept the potassium uptake unaffected. (2) The fairly reasonable regularity of time variation of EC level and nutrient uptake would allow for mathematical modeling, useful for biomass prediction and stress diagnostics: (a) EC modeling over time by exponential fits resulted in reasonably acceptable r-squares under doubled-N by nitrate treatment and the control condition; (b) EC changes in the standard solution provided reasonable inverse correlation with the plant's average growth rate; (c) Exponential fitting of nitrate concentrations over time resulted in reasonable r-squares both for the doubled-N by nitrate treatment and the control condition; and (d) Under the doubled-K treatment, nitrate uptake was significantly suppressed, so that the resulting variation in nitrate concentration over time deviated significantly from that for the control condition or even that for the doubled-N by nitrate treatment, indicating physiological stress for the plants. |
