Soil properties and crop yields after 21 years of direct drilling trials in southern Spain

A long-term experiment to study the influence of management systems on fertility-related soil properties has been carried out in southern Spain since 1982. The experiment introduced conservation tillage systems in dry-farming agriculture in the clay soils region. Two tillage systems were compared: conventional tillage (CT), and direct drilling (DD), in a wheat–sunflower–legume rotation.Conservation tillage systems appreciably improved the fertility level of the soil in the organic matter as compared to conventional tillage, increasing the organic matter of the profile 18 Mg ha−1 down to 0.52 m in the profile, in the DD treatment. Nitrogen, and available phosphorus and potassium contents, are greater in conservation tillage too, from 7 to 24 ppm and from 400 to 760 ppm, respectively. In the two different samplings the increasing trend of the direct drilling treatments was maintained.A stratification of the nutrient contents in the soil profile under direct drilling has been observed, possibly due to the natural compaction of the soil and to the absence of mixing mechanisms other than the swelling-shrinkage due to changes in moisture content between dry and rainy seasons. Organic matter and nutrient concentrations tend to accumulate in the surface horizons of the soil under DD and they are uniformly distributed under CT.The mean yields obtained with the different treatments are not significantly different to each other. It has been detected that DD have greater sunflower yields than CT in dry years where the annual rainfall is below 490 mm. There are no appreciable differences in years in which the annual rainfall is higher than that amount.