Aggregate sizes and stability in cultivated South Dakota prairie Ustolls and Usterts

Soil structural stability often decreases as the intensity of cultivation increases. The effect of three different management systems (grass, no-till, and till) on soil aggregate stability and sizes were studied in six Ustolls and two Usterts on central South Dakota farms. Soil structure was morphologically described throughout the profile. Stability of dry and wet aggregates in the topsoil was tested by dry and wet sieving. Most structural changes were observed in the top 0 to 0.20 m. Granular structure was dominant under grass, whereas plates, blocks, and compacted layers were most common in conventionally tilled and no-till soils. The largest mean weight diameters (MWD) of dry aggregates were found in no-till soils (10 mm vs. 7 in till and 6 in grass). Wet aggregate stability was higher in grass (87%) than in cultivated soils (70%). After about 10 yr of no-till management, no-till soil aggregates were significantly more stable (5% for wet and 32% for dry aggregates) than till aggregates only in the top 0 to 0.05 m. The structural stability of cultivated soils was greater in Usterts than in Ustolls.