ABSTRACT

Leaching is often an important aspect of nutrient cycling in agriculture (Brady and Weil, 2008). It occurs when mobile nutrients in the soil solution are displaced by percolating water to an area outside the rooting zone where plants cannot utilize them. Nutrients adsorbed to small mobile particles or colloids can also be leached to deeper soil horizons through facilitated transport. For nutrients dissolved in the soil solution, a migration of anions must be accompanied by an equivalent migration of cations for the maintenance of electro-neutrality. As such, the loss of highly mobile nitrate molecules after nitrogen (N) fertilization or organic matter mineralization must occur along with the loss of cations such as calcium (Ca), potassium (K), magnesium (Mg), etc.The amounts of plantessential nutrients lost from the rooting zone by leaching can be considerable: losses up to 80 per cent of applied N (Lehmann et al, 2004), 172 per cent of applied Ca (Omoti et al, 1983) and 136 per cent of applied Mg (Cahn et al, 1993) have been reported in the field. Values greater than 100 per cent indi-

cate that nutrients other than those added were also mobilized (e.g. by the process of desorption). Leaching, like most soil properties and processes, can be spatially and temporally highly variable.