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 bg mixing sand (57%) and silt (28%), separated from one soil, with clay (15%) separa ted from two soils, One clay type (Urrbrae) was composed mostly of ill ite and kaolinite, 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 val ues of exchangeable sodium percentage (ESP) by equilibration with solu tions hating sodium adsorption ratios (SAR) between 0 and 30. Pea (Pis um sativum L.) straw was added at 50 g kg(-1) soil, and the samples we re incubated moist. After 67 d, cumulative mineralization of C was 16% greater in the soils with Urrbrae clay than in those with Claremont c lay. Sodicity had a slight negative effect on mineralization, but its interaction with clay type was not significant, Differences in the min eralization of C with time and between treatments were reflected in th e chemical nature of the remaining C, as determined by C-13 nuclear ma gnetic resonance (NMR) spectroscopy. The greater the loss of C, the lo wer the proportion of O-alkyl C relative to other types of C, An impli cation of the findings is that, following a large addition of organic matter to a sodic soil, retention of organic C would not be improved b y a prior reduction in ESP, irrespective of clay mineralogy.