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This study evaluated the interactive effect of biosolids, time of application, and soil water on plant N concentration and uptake by Bouteloua gracilis (blue grama) and Hilaria mutica (tobosagrass) grown in pots. Biosolids were surface-applied to the soil of the pots either in the spring or the summer at rates of 0, 7, 18, 34, and 90 dry Mg ha(-1). All of the pots were irrigated weekly to achieve 40% or 80% of field capacity soil water. The maximum increase in B. gracilis tissue N concentration due to biosolids application with respect to control was 41% in plants grown under the higher irrigation regime and only 15% in those plants grown under the lower irrigation regime. In H. mutica, the higher irrigation level produced an average of 20% higher tissue N concentration than the lower irrigation level across all biosolids application rates. Both species had a much higher N uptake (3.5- to 6.3-fold) under the higher than under the lower irrigation regime due to the large increase in shoot biomass caused by the higher irrigation level, regardless of rate of biosolids. At low biosolids rates, soil NO3--N was higher under the lower irrigation regime than under the higher irrigation regime due to the lower N uptake in plants. Spring application of biosolids produced higher soil NO3--N levels than summer application only when irrigation level was high. The fertilizing effect of biosolids depended heavily on soil water availability.