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Leaching of a Soluble Chemical under Field Crop Conditions¹

ABSTRACT

The leaching pattern of surface-applied bromide is examined in an active rootzone of a sorghum-sudangrass [Sorghum sudanense (Piper) Stapf] crop in Southern Plains field plots. Data were collected for eight soils, comprising five soil orders, over three growing seasons. The position of the leaching front (solute center of mass) was better predicted generally by Burns' equation, which assumes chromatographic-type, sequential mixing of solute with soil water, than by Levin's equation, which assumes the leaching front moves with the velocity of water flowing above field capacity. With both equations effective rainfall was used, by correction for the soil water deficit caused by evapotranspiration above the leaching front. The leaching pattern in the soil profile was described quite well by a simplified version of the convective-dispersive solute transport equation, which assumed constant average values for the dispersion coefficient, D, a plant uptake factor, ß, and flow velocity, . Plant uptake of solute was assumed to be proportional to its concentration in soil water during passage through the root zone, with an effective average value of the proportionality constant applicable on a seasonal basis. The average value of D among soils was fairly uniform, 1.36 ± 0.56 cm² d^–1. Therefore, in practice a single value of D will probably suffice for many soils and conditions. Necessary inputs for obtaining can be obtained from plant growth and water balance components of CREAMS, or other appropriate models. In the case of ß, some additional information is needed regarding its relationship to specific growth factors.

¹ This work is a contribution of the Water Quality and Watershed Research Laboratory, Southern Region, Agricultural Research Service, P. O. Box 1430, Durant, OK 74702.

² Soil Scientists and Mathematician, respectively, USDA-ARS, P. O. Box 1430, Durant, OK 74702.

Received for publication March 3, 1983. Accepted for publication October 13, 1983.