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Relative Importance of Soil Resistance and Plant Resistance in Root Water Absorption¹

ABSTRACT

Soil water potentials, leaf water potentials, and transpiration rates of sweet corn (Zea mays L.), growing in a greenhouse, and grain sorghum (Sorghum bicolor L.), growing in a field, were determined to evaluate the magnitude of the combined soil and plant resistances to water flow in the plant system. Using a theoretical analysis of water absorption by roots, soil resistance was estimated. Plant resistance was inferred by the difference between the measured combined resistances and the estimated soil resistance. A wide range of soil hydraulic conductivity values for the plant rooting media which included nutrient solutions, peat-vermiculite-sand mixture, and a sand and clay soil maintained at various water potentials provided variations in calculated soil resistances. Our results showed that when root density was not unusually low, plant resistance to water transport was much larger than soil resistance, until the threshold soil hydraulic conductivity reached about 10^-6 to 10^-7 cm/day. This conductivity usually occurred at about –1 and –8 bars for the sandy and clay soils, respectively. These findings emphasize the need to consider plant resistance in water-uptake calculations when using equations that evaluate water potential gradients along the water flow path.

¹ Contribution from the Southern Region, Georgia-South Carolina area and Oklahoma-Texas area, USDA-ARS, in cooperation with the South Carolina Agric. Exp. Stn. and with the Texas Agric. Exp. Stn. 16 Oct. 1975.

² Soil Scientists, Southern Region, USDA-ARS, Florence, SC 29501, and Temple, TX 76501, respectively.

Accepted for publication December 15, 1975.