Observations of pores and aggregates during aggregation in some clay-rich agricultural soils as seen in 2D image analysis

Observations by 2D image analysis on four clay-rich agricultural soils in laboratory wetting and drying experiments indicate that the aggregation process changes the size distributions of soil materials in producing more irregular, larger particles which form through the accumulation of both small and large particles. The effect is more important with depth in laboratory and field samples. The distribution pattern of agricultural soil aggregates in 2D cuts is frequently initially approximately lognormal in nature. Aggregation shifts the modes of such distributions and at the same time creates irregular distributions of very large particles. Pore distributions associated with the aggregates do not necessarily follow the aggregation process in a systematic manner. They can increase, remain the same or decrease in mode size as aggregation proceeds. Cracks form in large aggregates in the lower portion of the samples. Comparison of images of sections of field cores in an agricultural loessic Mollisol (10 cycles of wetting and drying) with experiments on the same material in the laboratory indicates that the laboratory observations represent field processes reasonably well. In field samples, the upper portions of soils (1–2 cm) show less aggregation than at depth (6–8 cm) where cracks can be seen after 8 to 10 cycles of wetting and drying. In conclusion, in general it appears that change in pore distribution and aggregate size distribution are not necessarily related.