Under semiarid conditions the response of crops to synthetic fertilizers is often reduced. Organic fertilizers can be used to provide a continuous source of nutrients for the crops. The soil nitrogen and crop yield in a rotation of durum wheat ( Triticum durum)-fallow-barley ( Hordeum vulgare)-vetch ( Vicia sativa) were studied during 4 years when synthetic fertilizer (chemical), compost (organic) or no fertilizer (control) were applied in a field with high initial contents of soil NO 3-N (>400 kg N ha -1), phosphorus (22 mg kg -1) and potassium (>300 mg kg -1). Changes in soil organic matter, phosphorus and potassium were also measured. During the crop period, chemical fertilization significantly increased the content of soil NO 3-N in the first 0.30 m of soil with respect to organic fertilization and the control. The yield of wheat and barley was not increased after applying chemical or organic fertilizer with respect to the unfertilized plots. The estimated losses of nitrogen were similar for the three types of fertilization, as well as the uptake of nitrogen for the total biomass produced. The initial levels of organic matter and phosphorus were maintained, even in the plots that were not fertilized, while the potassium decreased slightly. Thus, the rotation and burying of crop residues were enough to maintain the crop yield and the initial content of nutrients.
