Dryland salinity, caused largely by insufficient water use of annual crops
and pastures, is increasing in southern Australia. A field experiment in no
rth-eastern Victoria (average annual rainfall 600 mm) assessed the potentia
l for lucerne grown in rotation with crops to reduce the losses of deep dra
inage compared with annual crops and pasture. Soil under lucerne could stor
e 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 defici
t of 78 mm. The extra water storage capacity was due to both the increased
rooting depth and increased drying ability 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 annu
al pastures 35 mm/year (maximum 108 mm). When the extra soil water storage
under lucerne was accounted for, no drainage was measured under this treatm
ent in any year. Following 2 years of lucerne, drainage under subsequent cr
ops 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 sugg
est that in this environment drainage losses are likely to occur under annu
al 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 abili
ty to use summer rainfall and dry soil over the summer-autumn period. Durin
g the autumn-winter period crop water use was generally higher than under l
ucerne.
The major period of increased soil water extraction under lucerne was from
late spring to midsummer, with additional drying from deeper layers until a
utumn. Under both lucerne and crops, soil dried progressively from upper to
lower soil layers. Short rotations of crops and lucerne currently offer th
e most practical promise for farmers in cropping areas in southern Australi
a to restore the water balance to a level which reduces the risk of seconda
ry salinity.