Soybean and Corn Rooting in Southwestern Minnesota: I. Water-Uptake Sink

Sink terms in the transient soil water flow equation were estimated to provide accurate descriptions of field water uptake. Water uptakes by vertically developing root systems of three crops were described as a function of time during drying of a Nicollet soil which often has a perched water table at 1.3 to 2 m early in the growing season. During the period 1 July through 15 Aug., water uptake by corn (Zea mays L.) was compared with that by determinate and indeterminate isolines of ‘Harosoy’ soybean (Glycine max. L. Merr.). These accounts of water uptake were later compared with directly examined rooting patterns. Water depletion measurements (with neutron probe) underestimated transpiration beginning in late July when upward water flows at 140 cm were 0.4 to 0.6 of transpiration. Depletions by soybeans occurred at shallower depths than corn. The indeterminate isoline showed greater depletions than the determinate isoline at shallow depths. Water flows in the rooting zone were estimated from tensiometric measured hydraulic gradients and hydraulic conductivity, calculated to match in situ field estimates. Peak flows to the root system progressed with time from the shallow to deeper depths. Maximum flows at the 122- to 152-cm depth ranged from −0.6 for soybeans to −0.8 cm/day for corn. Soil water potentials at maximum flow were −180 to −600 mbars with lower potentials noted for soybeans. Sinks for soybean water uptake showed greater and earlier water extraction at shallower depths than corn. Sinks for water uptake by all three crops moved down with time until 30 July, when water extraction by soybeans became limited by depth of rooting. Pan evaporation increased significantly, and the corn root sink continued to move down. Integrals of the sink term early in the period overestimated by 20% the sum of depletion and upward movement into the rooting depth. Later in the test period sink term integrals underestimated the same sum by 25%.