DIRECT DRILLING EFFECTS ON EARLY GROWTH OF WHEAT - STUDIES IN INTACT SOIL CORES

Tillage experiments with wheat (Triticum aestivum L.) were conducted at 2 sites in south-eastern Australia: one cool and wet (Murrumbateman), the other warmer and drier (Yanco). Following crop residue removal, direct drilling treatments with complete surface disturbance to 3 cm depth of the medium-textured topsoils (D), or with disturbance only in the narrow seeding slots (DN), were compared with drilling into a surface prepared by cultivating to about 7 cm depth (control). At the 4.5 leaf stage, seedlings from D and DN treatments had less shoot dry weight than the control except at Murrumbateman under very wet postseeding conditions. Simulation of the above tillage treatments in intact soil cores from both sites kept under controlled temperature and moisture conditions confirmed that DN treatment, with only 11% of the volume of soil disturbance of the control, reduced early growth, especially at Yanco and if post-sowing conditions were drier. Reduced shoot growth was associated with relatively larger effects on the extent of root exploration. Treatment D, with 43% of the volume of disturbed soil, did not reduce shoot growth relative to the control under any.circumstances. Although results suggest a minimum volume of disturbed soil for maximum growth, interactions with biotic stresses as reported recently cannot be ruled out.