ORGANIC-MATTER, SODICITY, AND CLAY TYPE - INFLUENCE ON SOIL AGGREGATION

Organic matter influences aggregation in non-sodic soils, but less is known about its role in sodic soils. The purpose of this study was to examine the role of organic matter in aggregation and clay dispersion in soils with different levels of sodicity. Prepared soils with 15% cl ay (smectitic or illitic) and natural soil aggregates (from a smectiti c clay soil [Pellustert] with 25 g kg(-1) organic C and a pair of illi tic loams [Rhodoxeralfs] with 15 and 29 g kg(-1) organic C) were equil ibrated with solutions having sodium adsorption ratios (SAR) of 0, 5, 15, and 30. Pea (Pisum sativum L.) straw was added at 50 g kg(-1) to t he prepared soils, which were then incubated. Changes in aggregation d uring incubation were similar irrespective of day type. After 7 d incu bation with no added straw at SAR 0 and 30, the amounts of spontaneous ly dispersible clay were 5.9 and 23.7 g kg(-1) soil, and mechanically dispersible clay was 11.9 and 23.3 g kg(-1) soil. Macroaggregation (>2 50 mu m) was 125 and 41 g kg(-1) soil at SAR 0 and 30. After 67 d incu bation,vith pea straw, spontaneously dispersible day contents were 2.1 and 5.4 g kg(-1), mechanically dispersible clay contents were 9.0 and 17.7 g kg(-1) soil, and macroaggregation was 533 and 416 g kg(-1) soi l at SAR 0 and 30. The effects of sodicity and organic matter on struc tural stability of the natural soil aggregates were similar to those i n the prepared soils, but macroaggregation was less, and the smectitic clay soil was more sensitive to sodicity than the illitic loams. This work showed that organic matter has at least as great a role in aggre gation in sodic soils as in non sodic soils.