SOIL ACIDIFICATION, CHLORSULFURON APPLICATION AND RHIZOBIUM-MELILOTI AS FACTORS IN LUCERNE YIELD DECLINE

Paddock sampling and subsequent pot experiments have identified a numb er of factors which may contribute to low lucerne yields in the mid-no rth of South Australia. Soil acidification appears to be a significant factor as soil pH(w) (1 : 5, soil-water) ranged from 5.0 to 6.8 in th e surveyed paddocks, 0.9-1.7 units below the fenceline values for thes e paddocks; 80% were below pH(w) 6.0. In all cases, soil from these lu cerne paddocks also had low Rhizobium meliloti populations with paddoc k values ranging from <2-114 rhizobia g(-1) soil. Whilst liming produc ed lucerne growth responses in pot experiments, there was little respo nse to inoculation with R. meliloti strain CC169, regardless of liming treatment. The evaluation of 5 Rhizobium strains on 11 lucerne cultiv ars showed a high level of interaction between hosts and strains, with one strain, NA39, outstanding; the poorest was CC169. Another potenti al factor affecting lucerne yields was the use of the herbicide chlors ulfuron in established lucerne crops. in a pot experiment, the equival ent of 5 g ha(-1) of chlorsulfuron severely restricted lucerne growth and nodulation, suggesting that the use of this herbicide on lucerne s hould be discouraged.