Sorption of non-ionic organic compounds to soil is usually expressed as the carbon-normalized partition
coefficient (KOC), because it is assumed that the main factor that influences the amount sorbed is the
organic carbon content of the soil. However, KOC can vary by a factor of at least ten across a range of soils.
We investigated two potential causes of variation in diuron KOC – organic matter–mineral interactions
and organic matter chemistry – for a diverse set of 34 soils from Sri Lanka, representing a wide range
of soil types. Treatment with hydrofluoric acid (HF-treatment) was used to concentrate soil organic matter.
HF-treatment increased KOC for the majority of soils (average factor 2.4). We attribute this increase to
the blocking of organic matter sorption sites in the whole soils by minerals. There was no significant correlation
between KOC for the whole soils and KOC for the HF-treated soils, indicating that the importance
of organic matter–mineral interactions varied greatly amongst these soils. There was as much variation in
KOC across the HF-treated soils as there was across the whole soils, indicating that the nature of soil
organic matter is also an important contributor to KOC variability. Organic matter chemistry, determined
by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, was correlated with KOC for the HFtreated
soils. In particular, KOC increased with the aromatic C content (R = 0.64, p = 1 106
), and
decreased with O-alkyl C (R = 0.32, p = 0.03) and alkyl C (R = 0.41, p = 0.004) content.