Soil Erodibility
Organic matter content ranked next to particle size distribution
as an indicator of erodibility. Both the rainfall energy needed to
start runoff and the final infiltration rates increased directly with organic
matter increases, while soil content of the runoff was inversely related
to organic matter content. Further analysis showed an important but
very complex interrelation between organic matter and clay. On silts,
silt loams, loams and sandy loams, the inverse relation of erodibility
to both aggregation index and organic matter level was strong, but it significantly
declined as the clay fraction increased. For a high clay soil with
4% organic matter, for instance, the inverse relation of erodibility to
aggregation index appears to hold only so long as the sand fraction exceeds
about 35%. For a 2% organic matter level this critical sand level
drops to about 10%. With clay content high and sand content appreciably
less than these percentages, the relationship reverses. This suggests
that aggregates comprised largely of clay particles are more susceptible
to erosion in aggregated forms.