Soil organic matter is thought to increase aggregate stability by lowering
the wettability and increasing the cohesion of aggregates. In southwest Fra
nce, thick humic loamy soils (Vermic Haplubrepts) have been intensively cro
pped for 40 yr, decreasing the soil organic pool and lowering the soil agre
gate stability. This study assessed (i) the contribution of organic matter
to aggregate stability by decreasing aggregate wettability and (ii) the spe
cific role of clay-associated organic matter. Soil samples with a C content
of 4 to 53 g kg(-1) were sampled and soil aggregate stability was measured
. Aggregate wettability was assessed by measuring water drop penetration ti
mes on individual 3- to 5-mm aggregates. The <2-mu m fractions were extract
ed without organic matter destruction and their wettability was determined
by measuring contact angles of water on clay deposits. Aggregate stability
against slaking was correlated to soil C content (r(2) = 0.71 for fast wett
ing). Water drop penetration time increased with C contents from 1 to 32 s
and was very heterogeneous among individual aggregates from a given soil. T
he contact angle of water on the clay fraction increased linearly with the
C content (r(2) = 0.86). This change in clay wettability could partly expla
in the higher water stability of soils rich in C.