Organic Matter and Aggregate-Size Interactions in Saturated Hydraulic Conductivity

The objective of this study was to investigate the effect of the interaction between organic matter (OM) content and aggregate size on the mechanisms that degrade the soil structure and on the saturated hydraulic conductivity (K_s). Two samples of sandy loam (Humic Dystrudept) containing 2.5 and 3.5% OM, referred to as low-OM soil and high-OM soil, respectively, were collected from adjacent fields. Dry samples were sieved to obtain aggregate sizes of <2, 2 to 4, and 4 to 6 mm. Slaking, swelling, and dispersion values were measured for each soil and aggregate size. The saturated hydraulic conductivity was determined in disturbed soil columns by means of a constant-head device. For the <2- and 2- to 4-mm aggregate sizes, K _s of the high-OM soil was, in general, significantly higher than that of the low-OM soil. The average K_s for the entire leaching run in the <2-mm and 2- to 4-mm aggregate sizes was 7.7 and 183 mm h ^-1, respectively, for the low-OM soil and 13 and 412 mm h ^-1, respectively, for the high-OM soil. Moreover, there was a significant interaction between aggregate size and OM content in their effects on K_s. For the low-and high-OM soils, the slaking values were >93 and <6.7% respectively, and the clay dispersion values in deionized water were >2.9% and <2%, respectively. This suggests that the larger decrease in K_s of the low-OM soil than in the high-OM soil during wetting and leaching was mainly a result of more intense aggregate slaking and dispersion in the former soil.