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The Influence of Sorghum-Sudangrass Roots on Nutrient Leaching¹

An active root system can reduce leaching by taking up water and nutrients and transporting them to the aboveground portion of the plant, but little is known concerning the quantitative influence of roots on leaching. This study was conducted to determine the influence of sorghum-sudangrass (Sorghum bicolor [L.] Moench x S. sudanense (Piper) Stapf) roots on the movement of NO₃, K, Ca, and Mg in a simulated field soil profile of Dothan loamy sand (Plinthic Paleudults) in the Auburn rhizotron.

Two rates of N (250 and 500 kg/ha) and one rate of K (63 kg/ha), Ca (50 kg/ha), and Mg (25 kg/ha) were applied. Nutrient movement was monitored by analysis of soil solution samples taken at 48 to 72-hour intervals over a 132-day period from depths of 20, 40, 60, 80, 100, 120, 140, and 180 cm. Soil water status was determined by tensiometers and the neutron-scattering technique. Rainfall and supplemental irrigation maintained soil water tension generally less than 0.3 bar. Sorghum-sudangrass roots proliferated the soil profile to 160 cm and reached a maximum density of 3 cm root length per cm⁸ of soil in the Ap horizon, with lesser amounts in the A2 and B horizons. Roots took up NO₃ and did not allow any appreciable leaching below 80 cm at 250 kg of N/ha or below 100 cm at 500 kg of N/ha. Root uptake of K was sufficient to prevent any appreciable accumulation of K at any depth measured. Uptake of Ca and Mg was sufficient to virtually prevent leaching of these nutrients below 40 cm at 250 kg of N/ha or below 80 cm at 500 kg of N/ha. From the results of this study, it is evident that nutrient leaching through soil can be virtually eliminated by the natural mechanism of root uptake, thereby preventing any possible pollution of groundwater.

Key Words: Nonpoint source pollution • Fertilizer • N • K • Ca • Mg • Fertilizer use efficiency • Soil solution

² Soil scientist, USDA, Auburn Univ., Auburn. AL 36849.

Received for publication September 12, 1980.