Soil Aggregates and Clod Strength Dependence on Size, Cultivation, and Stress Load Rates

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Abstract
A study was carried out (i) to determine soil clod fragmentation patterns' dependence on specific surface energy, aggregate or clod size, and stress loading rates; and (ii) to examine the hypothesis that the large clods produced during tillage operations performed on air‐dry soils are a result of soil compaction due to overtilling and machinery traffic on a moist soil during the cropping season. It was found that surface energy and specific energy required to reduce aggregate size are dependent on stress loading rates. Under a specific stress loading rate, suspected to be less than or equal to the stress wave velocity in the soil, both common methods—the unconfined compression test and the drop‐shatter test—are assumed to be equally efficient in determining the energy required for soil fragmentation. It was concluded, based on soil strength values obtained, that the fracture strength of the soils tested is attributed to CaCO3 bonding bridges and is practically uniform for all clod and aggregate size ranges tested. The observed huge clods created while tilling the air‐dry soils were size dependent on row spacing and tillage direction. These results are typical of fracturing patterns of material characterized by uniform strength. The fracture patterns for these materials follow the planes of maximal shear stresses unless obstructed by plants of lower strength (e.g., plant rows, major cracks); thus the size of fractured units depend on tillage direction, tillage implement, and row spacing.

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