Categories of soil structure based on mechanical behaviour and their evaluation using additions of lime and gypsum on a sodic Vertisol

Measurements of soil aggregate strength were made using a simple crushing (indirect tension) test. The resulting values of strength were used to examine aspects of the internal structure of the soil aggregates. This was done using 2 methods: firstly, by studying the dependence of aggregate tensile strength on aggregate size; and secondly, by studying the variability of strength measurements made on aggregates of one size. Combination of the results from the 2 methods enables some new categories of soil behaviour to be defined. The new categories were evaluated using soil samples collected from a field experiment in which additions of lime and gypsum were made to a sodic Vertisol. The use of the new categories leads to the conclusion that the added compounds resulted in larger aggregates being weaker than the smaller aggregates. This was partly due to a greater amount of micro-cracking in the larger aggregates, and partly due to a greater weakening of the matrix within the larger aggregates. Both of these changes are consistent with the soil being more friable after the additions of the calcareous amendments. It was not possible to distinguish qualitatively between the effects of lime and gypsum. It is suggested that the amendments did not modify the soil structure directly, but that they increased the tendency of the soil to self-mulch in response to wetting and drying cycles.