Effect of grazed lucerne on the moisture status of wheat-growing soils

Abstract

The effects of varying durations of lucerne ley and subsequent wheat cropping on the moisture status of contrasting wheat growing soils are reported from two experiments in northern New South Wales. Lucerne leys of 5 1/2, 3 1/2 and 1 1/2 years on a red brown earth and 3 1/2, 2 1/2, and 1 1/2 years on a black earth were compared with a wheat-cowpea rotation, long fallow and continuous wheat. Lucerne extracted available soil water within eight months of sowing to a depth of 100 cm, with partial extraction of water to 150 cm. Within 18 months available water to a depth of 200 cm had been extracted. Subsequent growth of lucerne was dependent on incident rainfall and water extraction from depths greater than 200 cm. Wheat extracted water from the soil only to a depth of 150 cm, and extracted less water to this depth than did lucerne. Lucerne extracted water beyond the -1 5 bar suction level. On the black earth, soil moisture at 150 cm had not been recharged before time for sowing the third wheat crop, 28 months after lucerne plough-out. On the red brown earth, after a drought enforced long fallow immediately following lucerne plough-out, soil water at 200 cm was recharged within 18 months. Available soil moisture after three to five months fallow following plough-out of the lucerne leys was much less than for a wheat cropping system, with a consequent severe reduction in the grain yield of the following wheat crop. On the black earth grain yield was reduced by 7.7 kg ha-1 for each reduction of 1 mm in available moisture in the first crop after lucerne plough-out. Data indicate that lucerne must be ploughed out before January if a severe reduction in yield of the following wheat crop is to be minimized.