EFFECTS OF SUBSURFACE DRAINAGE, CULTIVATION, AND STUBBLE RETENTION ONSOIL POROSITY AND CROP GROWTH IN A HIGH RAINFALL AREA

Wheat, triticale, and rapeseed growth and yield were studied under var ious tillage (conventional, deep ripping, direct drilling) and stubble -handling (burnt, retained) regimes with and without drainage at Hamil ton in south-western Victoria from 1985 to 1987. Grain yield was incre ased from about 2 to >4 t/ha by drainage in both years; however, effec ts of other treatments, although significant, were much less. Soil str ucture (as measured by fractional air-filled porosity at -5 J/kg) dete riorated during winter and recovered during spring and summer. A labor atory experiment showed that this variation in soil structure resulted from saturation per se and redrying. In the field, the decline in por osity was most pronounced with cultivation and the absence of drainage , but overall, the effects of stubble retention and tillage treatments were small. There was a significant positive relationship between yie ld and porosity on undrained areas, but not where drains were present. Drainage reduced soil structural decline during winter, while stubble retention reduced the decline in porosity in the cultivated-undrained treatment in 1987.