Submitted to: Soil & Tillage Research
Publication Type: Review Article
Publication Acceptance Date: July 2, 1999
Publication Date: N/A
An understanding of how conservation tillage influences water quality is predicated on knowledge of how tillage affects water movement. Conservation tillage can have a much larger effect on how water moves through the soil than it does on the total amount percolating to groundwater. Soil macroporosity and the proportion of rainfall moving through preferential flow paths often increase with the adoption of conservation tillage and can contribute to a reduction in runoff. In medium and fine-textured soils, most of the water that moves to the subsoil during the growing season probably is transmitted by macropores. If a heavy, intense storm occurs shortly after surface application of agricultural chemicals to soils with well-developed macroporosity, the water transmitted to the subsoil by the macropores may contain significant amounts of applied solute, up to a few percent, regardless of chemical affinity for the soil. This amount can be reduced up to an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Solutes normally strongly adsorbed by the soil should only be subject to leaching in macropores in the first few storms after application. Even under extreme conditions it is unlikely that the amount of additional adsorbed solute transported to groundwater due to conservation tillage will exceed a few percent of the application. In the case of non-adsorbed solutes, such as nitrate, thorough flushing of the soil during the dormant season can move solutes residing in the soil to groundwater. Thus, tillage treatment should have a minimal effect on leaching of non-adsorbed solutes.