Effects of aggregate structure and organic C on wettability of Ustolls

Soil wettability is especially important for rainfed agriculture in climates with a dry period during the growing season. The effect of aggregate structure and soil organic C content on wettability of soil aggregates was determined for grassland (grass) and tilled fields (tillage). Soil organic C, plastic limit, aggregate total porosity, and wettability at 100 mm (rapid wetting) and 300 mm (slow wetting) water tension were measured on soil at 0–0.2 m depth. Natural aggregates from tillage and grass were compared to soil pellets formed by remolding aggregates. At both tensions, wettability of grass aggregates was significantly greater than that of tillage aggregates (P ≤ 0.001). Pellets were significantly less wettable than natural aggregates at 300 mm tension and during the initial wetting at 100 mm tension, but became significantly more wettable with time at 100 mm tension. Cumulative water uptake during 60 min exceeded the initial total porosity of pellets and natural tillage aggregates, suggesting incipient failure (formation of microcracks) during fast wetting. Grass aggregates contained twice as much organic C as tillage aggregates (26 g kg−1 versus 13 g kg−1). Organic C was linearly and positively related to plastic limit, total porosity, and the wettability of natural aggregates at 300 mm tension. At 100 mm tension, organic C was negatively related to wettability of natural aggregates under grass, but unrelated to wettability under tillage. Aggregate wettability was positively related to organic carbon content, except when the arrangement of soil constituents reduced or prevented incipient failure and soil dispersion during rapid wetting resulted in cumulative water uptake (60 min) similar to initial aggregate total porosity. Organic C increased wettability of grass aggregates when compared to tillage aggregates and also stabilized natural aggregates during fast wetting (100 mm tension). Both soil organic C content and aggregate structure were key factors controlling aggregate stability and wettability.