Deep subsoil compaction of two cracking clays used for irrigated cotton production in Australia

Densification of soils can be caused by several mechanisms: of these compaction (whereby soil particles are brought into closer contact by pressure) is the most widely recognised, but clay translocation (whereby illuviated clay fills pre-existing pore space) has also been implicated in suh- soil densification (Greenland, 1977). Most subsoil compaction reported in the literature refers to compaction in the 4MOcm soil layer and the deepest recorded agriculture-related densification in non-cracking soils appears to be down to 1 metre (Hakansson 8i Reeder, 1994). However, McKenzie etaf. (1991) indicated that irrigated cotton production had resulted in densification of a cracking clay soil in Australia down to 115 cm. McKenzie et al. (1991) also observed increased clay dispersion in the surface soil and suggested the observed deep subsoil densification possi- bly was a result of 'vertical translocation of dispersed clay in the cultitated area (filling) macropores' in the deep sub- soil. Given that clay coatings are common in these soils (Sleeman & Brewer, 1984; Sullivan 8i Koppi, 1994), the possibility that deep subsoil densification in these soils is caused by clay translocation is cause for serious concern (Greenland, 1977). The main objective of this study was to determine if the cause of the deep subsoil densification of the cracking clays used for irrigated cotton production in Australia was clay translocation as has been suggested previously.