Lucerne in crop rotations on the Riverine Plains. 1. The soil water balance

Abstract

Dryland salinity, caused largely by insufficient water use of annual crops and pastures, is increasing in southern Australia. A field experiment in north-eastern Victoria (average annual rainfall 600 mm) assessed the potential for lucerne grown in rotation with crops to reduce the losses of deep drainage compared with annual crops and pasture. Soil under lucerne could store 228 mm of water to 1.8 m depth. This compared with 84 mm under continuous crop (to 1.8 m depth), except in 1997–98 where crop dried soil by 162 mm. Between 1.8 and 3.25 m depth lucerne was able to create a soil water deficit of 78 mm. The extra water storage capacity was due to both the increased rooting depth and increased drying abiliy of lucerne within the root-zone of the annual species.

Large drainage losses occurred under annuals in 1996 and small losses were calculated in 1997 and 1999, with no loss in 1998. Averaged over 1996–1999, drainage under annual crops was 49 mm/year (maximum 143 mm) and under annual pastures 35 mm/year (maximum 108 mm). When the extra soil water storage under lucerne was accounted for, no drainage was measured under this treatment in any year. Following 2 years of lucerne, drainage under subsequent crops could occur in the second crop. However, with 3 or 4 years of lucerne, 3–4 crops were grown before drainage loss was likely. Our calculations suggest that in this environment drainage losses are likely to occur under annual species in 55% of years compared with 6% of years under lucerne. In wet years water use of lucerne was higher than for crops due to lucerne’s ability to use summer rainfall and dry soil over the summer–autumn period. During the autumn–winter period crop water use was generally higher than under lucerne.

The major period of increased soil water extraction under lucerne was from late spring to midsummer, with additional drying from deeper layers until autumn. Under both lucerne and crops, soil dried progressively from upper to lower soil layers. Short rotations of crops and lucerne currently offer the most practical promise for farmers in cropping areas in southern Australia to restore the water balance to a level which reduces the risk of secondary salinity.