Long‐term Tillage Practice Effects on Soil Aggregation Modes and Strength

Air dry aggregates of two soils were collected from cultivated fields and a natural reserve, and separated into five diameter groups: 1 to 5, 5 to 10, 10 to 20, 20 to 40, and 40 to 76.4 mm. Tensile strength of aggregates from these size groups was determined by brittle fracturing the aggregates in an unconfined compression unit. Analyses in view of using the brittle fracture theory showed that soil exhibits specific structural states organized hierarchially (e.g., primary, secondary, etc). Each consecutive state has a characteristic internal strength which diminishes and a “cell” structure the size of which increases with respect to the consecutive hierarchial order. This implies that aggregates are made of mixtures of soil materials belonging to the different aggregation states, and the materials of a lower hierarchial order act as cementing or binding material between soil cells of a higher aggregation order (e.g., tertiary aggregation material binds secondary aggregates and the bound mass, consisting of tertiary and secondary aggregates, binds the primary aggregates). Tensile strength of aggregated samples in a cultivated field was greater than that of samples taken from a noncultivated virgin site of the same soil. Internal strength of the basic soil aggregation units was quadrupled under cultivation as compared to that determined for virgin samples. These findings suggest that cultivation and trafficcaused compaction affect the basic smaller structural units of the soil rather than the larger aggregation units.