Subsoil macropore space of a transitional red-brown earth after either deep tillage, gypsum or both .II. Chemical effects and long term changes

The management of sodic texture-contrast soils of the riverine plains of S.E. Australia for crops other than rice is difficult because improved subsoil structure often deteriorates after a few seasons of irrigated cropping. This research aimed to improve understanding of the processess which cause deterioration of macropore space (volume and permeability of air-filled soil at -10 kPa matric potential) as well as providing some guidelines for improved management. A transitional red-brown earth was ameliorated by eight different treatments and compared with an unameliorated control treatment in a field experiment. The treatments included surface gypsum application, deep ripping after gypsum application, five designs of gypsum-enriched slots and traffic-free beds after addition of gypsum and deep ripping. The macropore space of the subsoil after initial winter cropping and summer fallow is described from measurements on soil cores collected from the experiment. An index of electrochemical stability (EC/ESP) is proposed which explains the loss of macroporosity of cultivated subsoil in the absence of significant soil stressing by wheels. Considerable loss of macropore volume and permeability after the first summer crop could be associated with loss of soluble gypsum, some increases of exchangeable sodium and compression by lateral swelling of the uncultivated soil from between the slots. These changes were compensated by an increase in the number of vertical fissures, directly observed and deduced from measurements of water entry into whole plots. Thus, management of the balance of soluble salts is essential for the long-term productivity and profitability of such systems of amelioration using deep tillage zones and gypsum application. This is in the absence of significant compaction under wheels or opportunities for addition or organic matter and formation of biopores by roots and soil animals. The relief of lateral swelling of the uncultivated B horizon by the loose slots and formation of extra vertical fissures is a process which may provide long-term benefits and deserves further research.