PERSISTENCE AND WATER-USE EFFICIENCY OF A TROPICAL GRASS AND LUCERNE ON A SOLODIC SOIL ON THE FAR NORTH-WEST SLOPES OF NEW-SOUTH-WALES

The persistence and water use efficiency of Digitaria eriantha spp. er iantha and Hunter River lucerne were compared on red solodic soil with a hard-setting surface and poor internal drainage, on the North-West Slopes of New South Wales. After prolonged watering, the profile was w et to a depth of 48 +/- 1.5 cm, with an available moisture store of 90 mm. Over 3 years, persistence of digitaria was excellent. The populat ion of lucerne was reduced following flooding at summer temperatures. Dry matter production of nitrogen (N) fertilised digitaria per mm warm season rainfall was similar to that of tropical grasses adapted to co mparable rainfall environments in subtropical Queensland. Lucerne dry matter per mm rainfall was only about half that of digitria (3.2 v. 6. 3 kg). Lucerne grew well in mixture with digitaria except under prolon ged wet soil conditions in summer. Artificial solodic profiles were co nstructed in the glasshouse to compare digitaria and lucerne in monocu lture and mixture under varying temperature, moisture, and N regimes. Lucerne showed sensitivity to both high and low moisture levels at sum mer temperatures but performed very well at spring temperatures and mo derate moisture levels where the mean evapotranspiration ratio was 400 g water per g dry matter. Water use efficiency was higher in digitari a than in lucerne, except at spring temperatures without added N. Wate r use efficiency of the mixture was always similar to that of the most efficient monoculture of the particular treatment.