Conservation tillage and macropore factors that affect water movement and the fate of chemicals

A thorough understanding of how conservation tillage influences water quali ty is predicated on knowledge of how tillage affects water movement. This p aper summarizes the effects of conservation tillage on water movement and q uality mainly based on long-term experiments on Luvisols at the North Appal achian Experimental Watershed near Coshocton, OH, USA. 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 in crease with the adoption of conservation tillage and can contribute to a re duction in surface runoff. In some medium- and fine-textured soils most of the water that moves to the subsoil during the growing season (May-October) is probably transmitted by macropores. If a heavy, intense storm occurs sh ortly after surface application of an agricultural chemical to soils with w ell-developed macroporosity, the water transmitted to the subsoil by the ma cropores may contain significant amounts of applied chemical, up to a few p er cent, regardless of the affinity of the chemical for the soil. This amou nt can be reduced by an order of magnitude or more with the passage of time or if light rainstorms precede the first major leaching event. Because of movement into the soil matrix and sorption, solutes normally strongly adsor bed by the soil should only be subject to leaching in macropores in the fir st few storms after application. Even under extreme conditions, it is unlik ely that the amount of additional adsorbed solute transported to groundwate r will exceed a few per cent of the application when conservation tillage i s used instead of conventional tillage. In the case of non-adsorbed solutes , such as nitrate, movement into the soil matrix will not preclude further leaching. Therefore, when recharge occurs during the dormant season thoroug h Bushing of the soil, whether macropores are present or not, can move the remaining solutes to groundwater. Thus, the net effect of tillage treatment on leaching of non-adsorbed solutes should be minimal. Published by Elsevi er Science B.V.