Fig. 1. Relationship between relative soil organic matter (SOM) decay rates and (a) Fe oxides and (b) kaolinite concentrations in the 20- to 53-, 2- to 20-, and <2-µm granulometric fractions of a sandy clay loam Acrisol. Mineral concentrations in soil granulometric fractions were analyzed in composite samples resulting from the mixture of three field replicates. Mean values showed <8% difference among cropping systems.