Soybean Root Development and Soil Water Depletion1

Abstract

AbstractWe conducted a field study to investigate soybean (Glycine max L. Merr., ‘Williams’) root growth and water depletion patterns. Field water depletion and rooting data are needed to assist in developing and refining models that consider root‐soil‐water interactions.Irrigated and nonirrigated soybeans were grown on a deep, barrier‐free, Muir silt loam soil (fine‐silty, mixed, mesic, Pachic Haplustoll). Soil water content, determined using neutron moderation (15‐ to 150‐cm profile), was used to calculate water depletion rates. Soil water content and desorption curves (obtained for each 15‐cm profile depth increment) were used to estimate soil water potential. Available soil water was estimated using —15 bars soil water potential as the lower limit of availability. We collected soil volumes (centered on the crop row) 76 cm wide, 7.6 cm thick, and 180 cm deep (sectioned to 15‐cm increments) and washed roots free using 35‐mesh screen.During the first half of our study, maximum root and water depletion depths were nearly equal. Later, water depletion tended to be about 15 cm deeper than root growth (possibly as a result of upward water movement into the water‐depleted root zone). Our data indicate that, as soybean root weight increased, there was a drying effect so available soil water and soil water potential decreased. Depletion effectiveness (defined as cm3 of water/g of root/day) was greatest with lower root weights, higher soil water potentials, and greater available soil water. Depletion effectiveness increased with depth in the soil profile; probably as a result of the roots being younger, in wetter soil, and less crowded. Our data indicate that a small portion of the root system can be responsible for much water uptake.