Organically and conventionally farmed soil losses due to
water erosion between 1948 and 1985. The organically farmed Naff
silt loam has lost about 5 cm of topsoil, while the conventionally
farmed Naff silt loam has lost about 21 cm of topsoil leaving a
16 cm difference in topsoil thickness between the two soils in 1985.
The A2 horizon for Naff soils is characteristically only slightly
lower in organic matter than the Al horizon. So its mixing with
the thinning Al by ploughing, plus the addition of organic matter
from crop residues, helps maintain a fairly normal Al horizon
until the A2 horizon is completely depleted. Topsoil losses were
extrapolated from both field measurements of water erosion and
topsoil thickness (Table 2 ). The effects of tillage erosion on topsoil
thickness were not included.