Legumes used as cover crops to reduce fertilisation problems improving soil nitrate in an organic orchard

The organic olive orchard represents an economic opportunity for certain rural areas that are not very agriculturally productive. Those are usually mountainous areas, with high slopes and acid shallow soils with little organic matter. The implementation of a legume cover crop in these organic olive orchards would reduce erosive processes and increase the amount of nitrogen in the soil. The objective of this study was to evaluate, during three cropping seasons (2010-2013), the possibilities of implantation and behaviour of different species of leguminous plants as a cover crop in the organic olive orchard, estimating their capacity to protect the soil, increasing the amount of nitrogen in soil when the tree most needs it and determining the proper management of the plant residues of these covers during the decomposition process that maximizes this contribution. The results obtained indicate that Vicia ervilia was the legume that presented the best behaviour in the increase of the nitrate content in the soil. Regarding the soil management system, the use of the residues obtained by the process of field mowing increased the soil nitrate either under no-tillage and tillage system. Nevertheless, the nitrate content was higher in tilled soils than in those where covers were only left on the surface. In May of the second season, V. ervilia accumulated 85 kg ha−1 of nitrate in the first 20 cm of soil in tillage, being 40 kg ha−1 in the no-tillage system. Regarding the protection of soil against erosive agents, none of the studied species were effective in protecting the soil until the next sowing.