A general theory of vertical solute movement in a soil is presented, which takes into account uptake of water and solute by roots, irrigation or rainfall, and solute application and adsorption by the soil. Irrigation, rainfall, and the surface application of fertilizers are arbitrary functions of time. The main limitation of the theory is the neglect of the variability of soil-water conductivity with position. The theory is illustrated by comparing predictions and experimental observations of solute leaching losses measured in a lysimeter. Solute transport in soils, and porous media in general, is of crucial importance in a variety of fields, from agriculture to chemical engineering. Increasing food demand, in particular, has resulted in greater use of fertilizer and pesticides. Also of concern in some irrigated areas with saline aquifers is the upward movement of salts through soil profiles to crop rooting depths, or even the surface. Accurate formulae are required to predict and control the movement of such fertilizer, salts and pesticides. This will lead to their efficient use, resulting in minimal chemical or salinity build-up in aquifers.
In profiles not subject to soil erosion processes, solutes are primarily carried by water movement. As water moves in the soil, some solute adsorption can take place and, during water uptake by plants, solutes may be partially excluded by the roots. Diffusion, and hydrodynamic dispersion taking place during water movement, tend to smooth out abrupt changes in solute concentrations and these effects are important for soil column experiments in the laboratory. However, the present theory is intended more for field use than for laboratory miscible displacement experiments. Jury (1982) states that, in the field, the "extent to which a pulse of solute is dispersed depends both on variations in water application rate and on variations in travel time within the soil". Thus, since the extent of solute spread caused by hydrodynamic dispersion is unimportant in field soils, it will be ignored in the following.
We can then recognize two extreme cases to be considered in the study of solute dispersion at field scale. The first is where solute dispersion is due primarily to spatial variability of soil transport properties, viz. travel time or hydraulic