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 post-seeding 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.
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Fischer et al. (1994) studied this question.