Organic producers have few sources of plant available nitrogen (PAN) for cash crops. Improving the prediction of PAN from compost at different maturity stages will allow growers to better utilize this N source. We assessed the relationship between compost maturity and PAN during a 4-month replicated greenhouse pot study. Thirteen compost samples were collected from the same windrow over a 100 d period, beginning when compost met National Organic Standards. The compost feedstock included fish waste, sea urchins, spoiled haylage, heifer manure and dry softwood shavings. A sandy loam soil was amended with compost at different maturity stages. An application rate equivalent to 400 kg of total N/ha from compost was added to 420 g of soil (dry weight). Two non-compost controls were included: minus N and plus N (15 mg of N/kg of soil /week). All pots were provided the similar amounts of potassium and phosphorous. Annual ryegrass was planted to a depth of 1.5 cm in each pot. Pots were weighed and watered daily to maintain soil water at estimated field capacity. Natural and artificial lighting was provided a 16 hours of photoperiod. Ryegrass was harvested at 30 and 60 d after planting.Then barley was planted in the pots and harvested after 30 d of growth. Total above ground plant biomass, N concentration, and N removal were calculated. Plant biomass for compost-amended soil was significantly higher than the N minus control and less than the N plus control. There were significant differences in plant biomass between compost treatments, although the relationship between compost maturity and plant biomass was more variable for early plant growth (ryegrass harvests) than for later plant growth (barley). This may have been due to phytotoxicity of the most immature composts. Nitrogen availability was strongly correlated with compost maturity for the last barley harvest.