Effect of upper-profile soil water potential on soybean root and shoot relationships

Abstract

Effects of water distribution in the soil profile on the dry matter yields of roots and shoots of soybeans ( Glycine max (L.) Merr.), and depth distribution of the root mass were studied in plants (cv. ‘Forrest’) grown in 90-cm deep containers (two 45-cm sections) for 50 days in the greenhouse. Containers were filled with equal weights of Dubbs silt loam surface soil which was uniformly wet at planting. Soil in the bottom section of each container was maintained at −0.1 to −0.2 bar SWP1 (soil water potential). Soil inthe top section was maintained at −0.1 to −0.2 bar (SWP1), −0.3 to −0.5 bar (SWP2), or −0.5 to −0.7 bar (SWP3) once each level was reached. After 50 days, roots and shoots were harvested. Dry weight of roots in the upper section did not differ significantly among treatments, but was significantly greater in the lower section of the SWP3 treatment. Shoot dry weight from the SWP1 treatment was significantly greater. Shoot-root weight ratio declined significantly with each decrease in SWP level of the top section, and this decline resulted from decreased shoot weights in combination with increased root weights. The amount of water added to maintain the uniform SWP level in the bottom section of all containers increased significantly as SWP level decreased in the top. Yields of dry matter per unit of water added to the soil were nearly identical for all treatments and indicated that the increased yield from the wettest soil treatment (SWP1) was only in proportion to the extra water added.