Soil acidification, chlorsulfuron application and Rhizobium meliloti as factors in lucerne yield decline

Paddock sampling and subsequent pot experiments have identified a number of factors which may contribute to low lucerne yields in the mid-north 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 the surveyed paddocks, 0.9–1.7 units below the fenceline values for these paddocks; 80% were below pH(w) 6.0. In all cases, soil from these lucerne paddocks also had low Rhizobium meliloti populations with paddock values ranging from <2–114 rhizobia g−1 soil. Whilst liming produced lucerne growth responses in pot experiments, there was little response to inoculation with R. meliloti strain CC169, regardless of liming treatment. The evaluation of 5 Rhizobium strains on 11 lucerne cultivars showed a high level of interaction between hosts and strains, with one strain, NA39, outstanding; the poorest was CC169. Another potential factor affecting lucerne yields was the use of the herbicide chlorsulfuron in established lucerne crops. In a pot experiment, the equivalent of 5 g ha−1 of chlorsulfuron severely restricted lucerne growth and nodulation, suggesting that the use of this herbicide on lucerne should be discouraged.