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% clay (smectitic or illitic) and natural soil aggregates (from a smectitic clay soil [Pellustert] with 25 g kg−1 organic C and a pair of illitic loams [Rhodoxeralfs] with 15 and 29 g kg−1 organic C) were equilibrated 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 the prepared soils, which were then incubated. Changes in aggregation during incubation were similar irrespective of clay type. After 7 d incubation with no added straw at SAR 0 and 30, the amounts of spontaneously 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 (>250 µm) was 125 and 41 g kg−1 soil at SAR 0 and 30. After 67 d incubation with pea straw, spontaneously dispersible clay 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 soil at SAR 0 and 30. The effects of sodicity and organic matter on structural stability of the natural soil aggregates were similar to those in the prepared soils, but macroaggregation was less, and the smectitie 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 aggregation in sodic soils as in non-sodic soils