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Fallowing for moisture conservation in the Victorian Mallee

M. Incerti

Mallee Research Station, Department of Agriculture and Rural Affairs, Walpeup, Victoria, 3507

In the Victorian Mallee, winter fallows are an integral part of traditional cereal producing systems. Recent investigations at the Mallee Research Station have shown that factors such as soil nitrogen levels and control of cereal root diseases are more important than moisture conservation in improving cereal yields due to fallowing. This paper examines some of the causes of poor water use of wheat on winter fallows in the Victorian Mallee.


The experiment was located at the Mallee Research Station (avg. rainfall 338 mm) on a sandy loam soil which has a clay content increasing from 14% in the top 10 cm to 39% at 100 cm. Soil water content under wheat sown on winter and autumn fallows was measured since 1985 with a CPN503 neutron moisture probe to 140 cm. Cultivated fallows were commenced on pure medic pastures or its residue with each treatment replicated three times in a randomised block design. Medic pastures were maintained grass free with selective herbicides to control cereal root diseases and ensure adequate soil nitrogen.

Results and discussion

0n average (1984/5-1987/8) 30 mm of the extra rainfall received during the winter fallow period (127 mm) was stored at sowing when compared to that stored in the autumn fallow. This was poorly translated into increased yield. The remainder of this water (97 mm) being lost through evaporation and drainage. Although significant gains in water storage can be achieved by winter fallowing it is a relatively inefficient process in relation to rainfall received.

In 1988 the total amount of water stored at any time throughout the growing season was greater following a winter fallow compared to an autumn fallow, however, the extra amount available in the root zone was relatively low (Table 1). Most of the additional stored appears to have moved beyond the root zone during the growing season possibly explaining why it was not reflected in increased yield.

Table 1. Additional water (mm) stored throughout the 1988 growing season from a winter fallow compared to an autumn fallow.

The poor rooting depth of wheat in this environment combined with the relatively low water holding capacity and high permeability of Mallee soils, compared to more heavily textured soils, limit the use of fallow stored water. A more efficiemt use of rainfall could be achieved by substituting legume crops or pasture and autumn fallow for winter fallow. More rainfall will then be utilised for crop production in the year in which the rain is received.

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