RELATIONSHIPS BETWEEN A TROPICAL GRASS AND LUCERNE ON A SOLODIC SOIL IN A SUBHUMID, SUMMER-WINTER RAINFALL ENVIRONMENT

Relationships between a tropical grass (Digitaria eriantha) and lucern e (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 N ew South Wales and a glasshouse experiment in simulated solodic profil es. Temperature and moisture conditions covering the range encountered at the field site were incorporated into the glasshouse study, togeth er with high and low levels of available nitrogen. The relative yield ratio and its components, and the relative competition intensity for e ach species were used to investigate competitive trends with time. Dig itaria growing with lucerne commonly showed a trend to smaller reducti ons of its yield with time, and eventually higher yields in mixture th an in monoculture. This indicated a transfer of nitrogen from legume t o 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 summ er temperatures and at adequate levels of soil moisture. Lucerne was l east competitive under very wet or dry conditions at summer temperatur es, a situation aggravated by the dominance of digitaria in summer. Ex cept in extreme cases, lucerne recovered its competitive strength when conditions were more favourable, especially in the autumn-spring peri od. When the time trend in the relative competition intensity of lucer ne was near parallel to that of digitaria, the relative yield ratio wa s near constant, a condition of competitive equilibrium. Such a situat ion occurred under moderate temperature and moisture conditions most f avourable to lucerne growth. This study indicates the importance of a high level of adaptation of a legume to climate and soil, for achievin g competitive balance with an associated grass. It also suggests that management should include appropriate means to rectify any imbalance c aused by deficiencies in legume adaptation, or summer dominance by the grass.