PHOSPHORUS UPTAKE RATES OF COTTON ROOTS AT DIFFERENT GROWTH-STAGES FROM DIFFERENT SOIL LAYERS

A greenhouse experiment was conducted to determine P uptake at different growth stages by cotton plants from upper and lower 30 cm layers of 60 cm deep uniform soil columns. A radioisotope tracer technique was used to separate uptake from the two soil layers. Root lengths were measured in both layers at each growth stage. P fluxes were calculated for each layer. Root length increased significantly with time from square formation through peak flowering. At all growth stages, total P uptake from the upper soil layer was greater than that from the lower soil layer. P flux from either layer was greater during early growth stages than during later stages. A significant decrease in P flux occurred when plants reached the flowering stage. The flux from the upper layer was about twice that from the lower layer at square forming stage but the difference decreased as the plants matured. Both layers showed similar P fluxes at the first open boll stage. Our data indicate that roots located 30 to 60 cm below the surface are less effective per unit root length than those located at 0 to 30 cm depth when all roots are in the same soil environmental conditions. However, the effectiveness of P absorption from different soil depths under field conditions may depend on the environmental conditions that exist in each soil depth. © 1990 Kluwer Academic Publishers.