Phosphorus Uptake Rate of Soybean Roots as Influenced by Plant Age, Root Trimming, and Solution P Concentration 1

An understanding of the phosphorus, P, uptake characteristics of plant roots is important for developing practices that improve P fertilizer efficiency. Since little is known about the nutrient uptake characteristics of soybean [ Glycine max (L.) Merr.] roots, the objective of this research was to measure the P influx kinetics of intact soybean roots. Experiments were conducted with soybeans ranging in age from 18 to 74 days. Rate of P depletion from solution vs. P concentration in solution was used to determine maximum P influx, I max ; Michaelis constant, K m ; efflux, E; and the lowest level to which the plants can reduce P concentration in solution, C min . The effect of increasing plant P requirement per meter of root length on P influx kinetics was studied by removing half of the roots 2 days before influx measurement. Average I max for P decreased with increasing plant age, however the rate of decrease was less than previously observed with corn ( Zea mays L.). Trimming roots increased I max of 18 to 35‐day‐old plants so that P uptake per plant on trimmed plants was similar to that for untrimmed plants. Both K m and C min values increased with plant age. Root trimming had little influence on K m , and E was not influenced by plant age or root trimming. Phosphorus influx kinetics of soybean roots as affected by plant age and root trimming were different from those observed for corn so that P fertilization practices that give the highest efficiency for use of applied phosphate may differ for the two crops.