Estimating leaching losses from sub‐surface drained soils

Abstract. Leaching losses of solutes can be calculated if two variables, the amount of water passing through the soil and the concentration of solute in that water (a flux concentration), are known. Two simple approaches, soil extraction and suction cup sampling, were used to estimate the concentration of solutes in the water moving through a silt loam soil. The results were compared with actual concentrations measured in the drainage water from a sub‐surface (mole‐pipe) drained soil. Seasonal leaching losses were calculated as the sum of the products of estimated monthly drainage and the estimated average monthly solute concentration in the soil solution. These results were compared with the leaching losses measured in drainage water from the mole‐pipe system. For non‐reactive solutes such as bromide (an applied solute) and chloride (a resident solute), the suction cup data provided better estimates of the leaching losses than did the soil extraction data. The leaching losses calculated using volume‐averaged soil solution concentrations (obtained by soil extraction) overestimated the loss for the resident solute, but under‐estimated the loss for the surface‐applied solute. On the other hand, the data for non‐reactive solutes suggest that measurements on suction cup samples may be representative of the flux concentration of a solute during leaching. For nitrate, a biologically reactive solute, there was no clear pattern in the differences between the estimated and measured leaching losses. The flux‐averaged concentration in the drainage water was about midway between those measured in the suction cup samples and in the soil solution.