Five years after sewage sludge application, the residual effect of the decomposed sludge on fertilizer N uptake by-maize ( Zea mays L.) was studied. Maize grown from 1975 to 1978 without N fertilization had reduced the N mineralization potential of sludge-treated soil to that of the original soil. In 1979, N uptake from fertilized and non-fertilized sludge plots, and from N fertilized control plots, was compared using ( 15NH 4) 2SO 4. Uptake of soil N was not affected by fertilization. Uptake of fertilizer N was lower in sludge-treated plots (27%) than in control plots (41%), while the proportion of fertilizer N immobilized was higher in the sludge plots (32%) than in the control plots (18%). Stronger soil biological response paralleled a lower N uptake efficiency by maize grown on sludge plots. Fertilizer NH 4-N nitrified more rapidly in the sludge soil (31 g NH 4-N (kg soil) −1 day −1 than in the control plot soil (13 g NH 4-N (kg soil) −1 day −1. Dehydrogenase activity was twice as high as in fertilized sludge soil (140 mg Formazan kg −1) as in control soil (60 mg Formazan kg −1). Maize grain yields were linearly related to N uptake, whereas fodder yields were curvilinearly related, across all treatments. This response indicated that N fertilization induced no toxic side effects in sludge-grown maize.