An assessment of a new model of dynamic fragmentation of soil with test data

The fragmentation of larger soil aggregates to smaller fractions is a fundamental physical process that may occur naturally or be imposed. Soils that naturally re-structure themselves following structural damage from a homogeneous mass into a heterogeneous matrix of fine, granular and stable aggregates and pores are of great benefit. A key driving force is shrink–swell processes, particularly wet–dry cycles. Models of dynamic fragmentation have been devised. A promising model based on a threshold limiting size distribution of aggregates, with temporal- and scale-invariance, has been developed recently. We sought to assess this model with soil fragmentation test data.We sampled 12 soils from a 2 km transect (Bedfordshire, UK), subjected them to physical damage by shearing and kneading, and then imposed up to 3 or 5 wet–dry cycles. We measured the aggregate size distribution and mean weight diameter (MWD) of both the whole soil fraction and the fine-scale (<5 mm) aggregate fraction, selected as a key threshold size, as they fragmented. A subset of four soils was examined in more detail to assess the model predictions.Clayey soils fragmented considerably more than sandy soils, and resulted in an accumulation of fine-scale aggregates. This suggested that fragmentation was induced by shrinkage and swelling stresses impacting upon the clay fraction and that the wet–dry cycles were not able to reverse the structural damage imposed on the sandy soils. In the clayey soils the form of the distribution of fine-scale aggregates showed clear signs of temporal invariance over the wet–dry cycles. In addition, as the cumulative distributions of the fine-scale aggregates followed a power law relationship, this suggested invariance in scale. In fragmenting soils, the MWD of the fine-scale aggregates was a convincing asymptote for the MWD of the whole soil fraction.We demonstrated temporal- and scale-invariance in clayey soils that fragment upon wetting and drying and the accumulation of a resistant set of fine-scale aggregates. Such soils are likely to be better-able to recover naturally from any physical damage to their structure and will require less remedial work, and hence a lower energy input, compared to non-fragmenting sandy soils.

► Fragmentation in soils subjected to wet–dry cycles was measured and modelled.
► Clayey soils fragmented more than sandy soils.
► Temporal- and scale-invariance were found in fine-scale (<5 mm) soil aggregates.
► Fine-scale aggregates represented a resistant set of structural units.