Sodicity and Clay Type: Influence on Decomposition of Added Organic Matter

Sodicity and clay type influence the decomposition of organic matter, and hence its accumulation in soil. In this study, the interaction of these two factors was examined. Two soils were prepared by mixing sand (57%) and silt (28%), separated from one soil, with clay (15%) separated from two soils. One clay type (Urrbrae) was composed mostly of illite and kaolimite, with a mean particle diameter of 1050 nm. The other (Claremont) was composed of smectite‐dominated materials, with a mean diameter of 104 nm. The prepared soils were brought to different values of exchangeable sodium percentage (ESP) by equilibration with solutions having sodium adsorption ratios (SAR) between 0 and 30. Pea ( Pisum sativum L.) straw was added at 50 g kg −1 soil, and the samples were incubated moist. After 67 d, cumulative mineralization of C was 16% greater in the soils with Urrbrae clay than in those with Claremont clay. Sodicity had a slight negative effect on mineralization, but its interaction with clay type was not significant. Differences in the mineralization of C with time and between treatments were reflected in the chemical nature of the remaining C, as determined by 13 C nuclear magnetic resonance (NMR) spectroscopy. The greater the loss of C, the lower the proportion of O‐alkyl C relative to other types of C. An implication of the findings is that, following a large addition of organic matter to a sodic soil, retention of organic C would not be improved by a prior reduction in ESP, irrespective of clay mineralogy.