Residual Effects of Rhizobium and Preplant N Fertilizer on Newly Established Alfalfa1

In areas where ineffective nodulation may be a problem, ‘starter’ nitrogen fertilization is often suggested as a useful management tool to sustain ineffectively nodulated seedling alfalfa (Medicago sativa L.) until an effective population of Rhizobium can develop. Although ineffective nodulation has been reported from several locations around the world, there have been no studies examing this possibility. A field experiment was conducted during 1982 and 1983 on a Woodburn silt loam soil (Aquultic Argixeroll) which contained a symbiotically ineffective native population of Rhizobium meliloti Dang. This study examined the residual and interactive effects of preplant N fertilizer (0, 45, 90 kg N ha^?1) and inoculation on the N nutrition and yield of seedling alfalfa during the first year of growth. The sustaining effects of the fertilizer N were transient and resulted in decreased N₂ fixation until 35 days following harvest. It was concluded that during the establishment year, managerial emphasis should be placed on inoculation and not on N fertilization to obtain a consistent, economical source of N for herbage protein and dry matter production.