Soil properties
The main soil properties of the soils tested are shown in Table 1. The results indicated the presence of basic soils as a consequence of the presence of calcareous parent material. The EC was higher than 4 dS m-1 in some samples, reflecting soil salinity problems according
to USDA criteria. The OM content was low as a result of intensive use of these soils and the rapid mineralization of OM under semiarid conditions. Carbonate content was also high and in 90% of the plots it was higher than 40%. All soils had an adequate CEC for agricultural use and none displayed a low capacity for nutrient storage (CEC < 10 cmol(+) kg-1). The soils were generally silty clays or silty clay loams.Therefore, 89% of the soils were fine textures.