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The N content of the sandy soils in the semi-arid region of Paraiba State cultivated with potato (Solanum tuberosum) is low. Annual applications of farm manure (FM) and mineral N, determined on an empirical basis, have led to accumulation and I or loss of soil nutrients. Thus, the objective of this study was to determine the efficiency of ammonium sulphate (N-15) combined with FM applied to potato as well as the residual effects on subsequent millet. A field experiment on an Alfisol compared the combination of a rate of 16 t ha(-1) FM + 80 kg ha(-1) of mineral N, as most frequently used in the region, with a rate of 11 t ha(-1) of FM combined with 0, 40 and 80 kg ha(-1) of N in single or split applications. The ammonium sulphate was enriched in N-15(2.5 % N-15 abundance). The thy weight of potato and millet aboveground mass was determined in all treatments, as well as the N isotopic composition of both materials. Fertilization at the traditional rate proved to be excessive for this soil, since crop yields were not reduced when the rates were reduced to 11 t ha(-1) manure + 40 kg ha(-1) mineral N. However, yields were lower with applications of manure alone. Millet recovered much of the mineral N added to potato and was a good alternative for forage production in the area.