Relationships between a tropical grass and lucerne on a solodic soil in a subhumid, summer - winter rainfall environment

Abstract

Summary. Relationships between a tropical grass (Digitaria eriantha) and lucerne (Medicago sativa cv. Hunter River) grown in mixture were studied in a field experiment on a solodic soil on the Far North-West Slopes of New South Wales and a glasshouse experiment in simulated solodic profiles. Temperature and moisture conditions covering the range encountered at the field site were incorporated into the glasshouse study, together with high and low levels of available nitrogen. The relative yield ratio and its components, and the relative competition intensity for each species were used to investigate competitive trends with time. Digitaria growing with lucerne commonly showed a trend to smaller reductions of its yield with time, and eventually higher yields in mixture than in monoculture. This indicated a transfer of nitrogen from legume to grass. When such benefits to the grass coincided with only moderate reductions in lucerne yield, the mixture outyielded both monocultures. This occurred principally in the absence of applied nitrogen, at summer temperatures and at adequate levels of soil moisture. Lucerne was least competitive under very wet or dry conditions at summer temperatures, a situation aggravated by the dominance of digitaria in summer. Except in extreme cases, lucerne recovered its competitive strength when conditions were more favourable, especially in the autumn–spring period. When the time trend in the relative competition intensity of lucerne was near parallel to that of digitaria, the relative yield ratio was near constant, a condition of competitive equilibrium. Such a situation occurred under moderate temperature and moisture conditions most favourable to lucerne growth. This study indicates the importance of a high level of adaptation of a legume to climate and soil, for achieving competitive balance with an associated grass. It also suggests that management should include appropriate means to rectify any imbalance caused by deficiencies in legume adaptation, or summer dominance by the grass.