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The effect of long-term reduced tillage on the infiltration of water into soil in the Victorian Wimmera and Mallee

M.J. Bissett and G.J. O'Leary

Department of Agriculture, Victorian Institute for Dryland Agriculture, Private Bag 260, Horsham VIC 3400

Surface conditions play a key role in controlling the amount of water entering the soil. Stubble retention has been shown to increase soil water accumulation and yield on clay soils. The increases are often attributed to reduced evaporation by the covering mulch.


Infiltration measurements were made in two consecutive years on two long-term tillage experiments using a disk permeameter. The first experiment was located at the Wimmera Research Station, Dooen, on a friable grey cracking clay (Northcote, Ug 5.2) and had commenced in 1980. The climate is semi-arid, with a450 mm/yr mean annual rainfall. A second experiment was established in 1982 in a drier area at the Malice Research Station, Walpeup on a sandy loam soil (Northcote, Gc 1.2) with a 330 mm/yr mean annual rainfall.

Results and discussion

At Dooen on the grey clay soil, the saturated hydraulic conductivity (K.) was highest in a chemical fallow system (213-216 mm/h) and lowest in a fully cultivated fallow (14-36 mm/h) where the stubble had been removed by burning and a blade-ploughed fallow system gave intermediate rates (62-86 mm/h).

At Walpeup on the sandy loam, K. was highest for the direct drilled systems irrespective of rotation and also highest for the rotations which included fallow irrespective of tillage system. After an eight-year period of reduced tillage increases in the rate of infiltration of between 50 and 100 mm/h occurred in the rotations which included a fallow compared to conventional tillage. The continuous rotation of pasture and wheat had a significantly lower K. of 24 mm/ h which was increased to 50 mm/h with direct drilling.

After 8 to 10 years distinct patterns are emerging as to the physical effects of reduced tillage on soil structure. Reduced tillage and stubble retention clearly increased infiltration rates on both soil types. What is surprising is the massive increase observed on the grey clay soil which supposedly has low conductivities and exceeded the sandy loam by about 100 mm/h. Presumably a larger proportion of macropores were preserved. On the grey clay, sub-surface tillage was beneficial but despite retaining initially similar quantities of stubble on the surface only maintained about one third the rate of the uncultivated soil. In years of low rainfall chemical fallows should benefit by higher yields from increased soil water supply brought about by improvements in infiltration rates.

On the sandy loam improved infiltration, however, has not resulted in increased water conservation or growth and yield of crops (Incerti, unpublished data). Given the infrequency of storms in excess of 24 mm/h it is likely that the benefits of reduced tillage on these soils will not be realised in terms of additional water available for dryland crops.

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