Shear strength of aggregates compared with bulk soil of two haploxerolls from Chile

Shear strength of aggregates and the bulk soil matrix in two Haploxeroll of central Chile was assessed in order to determine: (1) the effect of previous tillage on shear parameters between aggregates and in the soil matrix and (2) the effects of structure development on differences between dry-wet mechanical parameters. Undisturbed soil cylinders and single aggregates (0-10 cm layer) were collected from untilled and conventionally tilled sites in the Santiago (STG) and Mapocho Soil Series (MPC). General soil properties and shear parameters were determined. The shear tests were conducted on confined and free-draining aggregates and on soil cores, equilibrated at field capacity and air-dried, to obtain data on cohesion and angle of internal friction in the samples. Dry aggregates from the tilled sites showed 40 and 70% greater cohesion compared to untilled sites in STG and MPC soils, respectively, associated with a decrease in coarse porosity in the STG soil and a decrease in organic matter in the MPC soil, thus reducing its friability. In air-dried condition, aggregates from untilled sites developed 6-7 fold strength compared with soil matrix and the difference increased (9-13 fold) in samples from tilled sites. Undisturbed soil cores showed higher friction values when air-dried (68o in average, compared with 43o in individual aggregates), but at -33kPa water tension the tendency was reversed. The results of this study confirm that smaller differences between wet and dry behaviour are associated with better structural development and lower tillage intensity.