Soybean and Corn Rooting in Southwestern Minnesota: II. Root Distributions and Related Water Inflow
1975
Allmaras, R. R. | Nelson, W. W. | Voorhees, W. B.
Root length density (RLD) of corn (Zea mays L.) and two isolines (determinate and indeterminate) of Harosoy soybean (Glycine max. L. Merr.) in a Nicollet soil were measured from 1 July to 15 Aug. 1971. These measurements, along with water uptake sinks in these vertically developing root systems, were used to evaluate water relations of these three crops. Both soybean isolines had about 0.2 cm/cm³ RLD in the Ap layer with values < 0.1 below the Ap layer. This RLD distribution persisted with depth as the root system expanded vertically. Corn RLD were about 0.3 initially in the Ap layer, and as the root system expanded vertically, maximum RLD of about 0.3 was produced even to the 100-cm depth on 9 Aug. Vertical extension rates of corn after 1 July were 1.3 cm/day as compared with 1.7 for soybeans, but maximum rooting depths on 9 Aug. were 145 and 120 cm, respectively. Modal specific water inflows were 0.03 cm³/cm root per day—maximal values for soybeans were 0.2; maximal values for corn were about 0.05. Maximum soybean rooting depths coincided with maximum depth of water uptake sink, but corn rooting depths ranged 15 to 30 cm deeper than the water uptake sink. Specific water inflow generally increased as soil hydraulic conductivity increased and RLD decreased, which agrees with models for steady-state water flow into single roots. At low RLD, specific water inflows were either constant or increased as hydraulic conductivity decreased. This increased root resistance to water flow at the lower fringes of rooting may have resulted from low soil temperatures. Soybean root conductivities decreased with decreasing soil water potential. Corn root conductivities were largest at about −700 mbars and decreased as soil water potential decreased. Decreases of corn root conductivity resulted from potentials > −700 mbars when soil temperatures were low.
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