Modeling Soil Flux by Manual Tillage as a Nonlinear Slope-Dependent Process

A review of seven studies addressing soil flux related to manual hoeing and weeding in Asia and Africa indicates that tillage erosion from hand-held implements on steep hillslopes can alter topography and influence the distribution of soil and nutrients within fields. Soil flux resulting from manual tillage increases with slope in a near-linear fashion until the slope angle approaches the angle of repose for displaced aggregates. On steeper slope gradients, the increase in soil flux is nonlinear because displaced clods roll, bounce, or slide relatively long distances downslope before coming to rest or breaking down. Because of this ravel-like transport process, soil flux is modeled most appropriately across a wide range of gradients as a nonlinear process. A modified nonlinear ravel model predicted soil fluxes in the reviewed studies better than a linear model, but it was not superior to a general exponential model. Additional field studies are needed to increase the basic understanding and predictability of soil erosion by manual tillage on steep slopes, as this activity is still practiced in many areas of the world.