Organic amendments initiate the formation and stabilisation of macroaggregates in a high clay sodic soil

Subsoil constraints present a substantial problem for crop production in many agricultural regions. In particular, soils in temperate grain production areas of Australia are often poorly structured due to high content of sodic clay. An alternative to the standard practice of addition of gypsum is to incorporate organic amendments deep into the subsoil. An incubation experiment was performed for 174 days using several organic amendments. These consisted of wheat shoots, lucerne pellets, canola and chickpea stubbles, chicken manure, peat, and sawdust. Gypsum, an inorganic amendment commonly applied to sodic soil, was included for comparison. The change over 174 days in soil structural properties was measured using wet-sieving. Formation of slaking-resistant macroaggregates >2mm was most rapid with 'green plant material', wheat and lucerne, while the 'stubbles' were markedly slower in obtaining the equivalent level of aggregation. However, the largest growth in aggregates after day 56 was shown by the 'stubble' and chicken manure amended soils. The gypsum amendment was not capable of forming large, slaking-resistant aggregates >2 mm; this may be attributed to the inability of gypsum to stimulate soil biological processes. Peat and sawdust failed to initiate slaking-resistant macroaggregates. The study demonstrated that a variety of organic amendments have the ability to improve the physical fertility of sodic subsoil, and in the case of the green plant materials within 1 week of incubation.