The potential of using free-living nitrogen fixing bacteria (diazotrophs) as a source of nitrogen nutrition for crops has not been realised in most parts of the world, largely because of the inability of the organisms to multiply effectively in temperate agricultural soils. In recent papers (Keeling et al, 1994, 1996) we showed how populations of diazotrophs in green waste compost-treated soil can be stimulated to increase by soil glucose treatment after sward development, and measured an associated increase in grass sward yield of 36% over a 12 month period Here, we describe in detail the changes in bacterial populations over a 12 month period and relate them to grass growth patterns and nitrogen uptake, for compost amended-soil treated with both glucose and soluble starch. It was found that diazotrophic populations were enhanced 300% over the long term, and nitrogen uptake by plants increased by over 100% in the first 2 months post 15 gl(-1) glucose treatment in compost-grown swards, while soluble starch-treated sward growth was inhibited. Further a typical field soil similarly treated with glucose failed to respond to the treatment; rather; a nitrogen immobilizing effect was observed It was concluded that significant nitrogen fixation and plant N availability was stimulated by the glucose treatment of compost, but the mechanisms of these processes require more extensive investigation.