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Leaching of Herbicides Through Undisturbed Cores of a Structured Clay Soil

White, R. E., Dyson, J. S., Gersti, Z., Yaron, B.
Soil Science Society of America journal 1986 v.50 no.2 pp. 277-283
bromacil, chlorides, clay, clay soils, equations, leaching, napropamide, preferential flow, soil aggregates, soil water, solutes, water flow
The leaching of the herbicides bromacil and napropamide through large undisturbed cores of a structured Evesham clay (Aquic Eutrochrept) was studied under continuous and discontinuous watering regimes and at different initial moisture contents. A greater proportion of both herbicides was leached from initially dry cores (θ = 0.24) than from prewet cores (θ = 0.35). This was consistent with differences in the amount of herbicide retained in the soil, most of which was retained in the top 0.05 m. Continuous leaching resulted in greater herbicide breakthrough than did the discontinuous treatment, presumably because some herbicide diffused from the conducting channels into the soil aggregates during the quiescent period, where it was less susceptible to subsequent leaching. At a continuous input rate of 12 mm h⁻¹ to initially dry soil, 85% of the applied napropamide (Kd = 17.7 L kg⁻¹) and almost 100% of bromacil (Kd = 1.73 L kg⁻¹) were leached out of the soil by one pore volume of water. Chloride was used as a tracer for water flow through the soil. Chloride breakthrough curves were analyzed using a transfer function equation, which yielded the median travel time of a solute molecule in the soil. It is suggested that preferential flow of water down cracks and channels between soil aggregates led to short travel times and apparently low volumes of soil water participating in solute transport (θₛₜ). The θₛₜ values were inversely correlated with the percentages of the applied herbicides that were leached.