These results demonstrate that the absence of agricultural chemicals,
especially herbicides, in organic farming systems can reduce soil erosion
for rowcrops due to the development of weeds in the furrows. However,
our results also show that a reduced crop yield associated with crop–
weed competition or herbivory outbalances the positive effects of
weeds, and can therefore produce higher erosion rates in organic farming
systems. Nevertheless, in both cases the difference in soil loss
between the farming systems were relatively small, and the effects
of weed coverage and crop yield were highly variable depending on
the timing of planting and harvest in relation to the occurrence of rainstorm
events, and the degree of soil disturbance. The simulated average
annual soil loss for both management systems exceeded, by far, any
tolerable soil loss rates. The OECD (2001) defined soil loss as tolerable
when it is less than 6.0 t ha−1 yr−1, and severe when it exceeds
33.0 t ha−1 yr−1. The average annual erosion rate for all four row
crops in this study was at least at the limit of severe erosion, and well
above in many cases. Our results also show that the maximum erosion
rates can be three to four times higher than the average values depending
on field topography.