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Effects of Initial Water Content on Macropore/Matrix Flow and Transport of Surface-Applied Chemicals

USDA-Agricultural Research Service, North Appalachian Experimental Watershed, P.O. Box 488, Coshocton, OH 43812-0488.

^* Corresponding author.

ABSTRACT

Pesticides and fertilizers are often broadcast on no-till fields in the spring when soil water content can be quite variable. Soil water content may influence the contribution of macropores and matrix porosity to water movement and chemical transport in subsequent rainfalls. Therefore, we surface-applied SrBr₂ 6H₂O, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), and alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide] on nine, 30 by 30 by 30 cm, undisturbed soil blocks obtained from a no-till corn (Zea mays L.) field and maintained at three initial moisture levels 1 h before a 30-mm, 0.5-h simulated rain. To distinguish applied water from resident water and assess interaction of the rainwater with the soil matrix, RbCl was added to the simulated rain as a tracer. Sequential percolate samples were collected from the base of the blocks in 10-mL increments using a 64-cell grid lysimeter. Flow-weighted concentrations of Cl^– and Rb⁺, respectively, were 75 and 836% higher in percolate from dry ( = 0.11 kg kg^–1) than from wet blocks ( = 0.21 kg kg^–1), indicating that displacement of resident water and interaction of rainwater with the matrix increased with initial soil water content. As a result, percolate concentrations of the reactive, surface-applied, constituents (Sr²⁺, atrazine, alachlor) decreased with increasing soil water content. High block to block variability precluded detection of significant differences in percolate volume and total chemical transport among moisture levels. The relative contribution of macropores to chemical transport and water movement appears to be greatest when the soil is dry and decreases as the soil becomes wetter.

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