Coberturas vegetais: a estratégia decisiva na gestão sustentável dos olivais de sequeiro
The olive and olive oil is a strategic sector for the Agricultural Department, due to the economic and social relevance that assumes in many inland regions of Portugal. In spite of the fact that irrigated areas are currently increasing, most of the olive orchards are cultivated under rainfed conditions. In Trás-os-Montes, only 5% of the total area harvested is irrigated, and this picture will not change in the near uture, due to natural limitations of water resources. Thus, notwithstanding the importance of some high-density and super-high-density orchards that have been planted in the South of Portugal, it is still crucial for the olive sector, and in particular for many inland regions, that the rainfed orchards are managed in a profitable and ecologically sustainable way.The soils of the rainfed olive orchards are usually deeply eroded, due to steep slopes, rainfall erosivity and wrong soil management practices. The eroded soils have low water-holding capacity, due to their shallow depth, which limits the efficient use of water provided by winter rains. Erosion reduces soil fertility and causes several adverse environmental impacts, such as ground-water eutrophication and the deposition of sediments in the estuaries and shallow lakes. To control soil erosion, the rational option is cover cropping.The productivity of rainfed orchards is low, due mainly to scarce rains in the summer. The soils present low fertility levels due to thehigh slopes and low depths. The maintenance of a minimal production level is also greatly dependent on the use of commercial fertilizers. However, the huge increase in the price of fertilizers in the last few years, together with the stagnancy of olive oil prices makes the use of fertilizers increasingly unviable. The situation in organic orchards is even more complex. The fertilizers allowed for organic farming are scarce, which makes its price high and often speculative. This makes organic farming paradoxically a high-input system in terms of capital. Legume-based cover crops however may significantly reduce the use of commercial fertilizers. Cover cropping is widespread in fruit farming in temperate climates and in irrigated olive orchards, where water is not a limiting resource. Reported data on the subject is therefore abundant and conclusive. The introduction of cover crops in rainfed olive orchards however is a high-risk option, since the competition for water and nutrients can be severe. In addition, available information on cover crop management in rainfed olive orchards is still scarce, making further research necessary. According to the proposals of this project, the ground of the rainfed olive orchards must be covered by green vegetation during the autumn and winter, and dead material in spring and summer. The species used as cover crops should present minimal competition for water and make a positive contribution to soil fertility. Some annual legume species can provide both these characteristics: they are comparatively less competitive for water than grasses and can fix atmospheric N2. Nitrogen fixed by legumes may thereafter assist in the nutrition of the tree crop in particular in the orchards grown under organic regimes. In addition, cover crops increase carbon sequestration and may contribute to Portugal’s targets set down in the Kyoto protocol.Two different kinds of legume species are proposed as cover crops in the project: self-reseeding pasture species with a short annual growth cycle, such as subterranean clover; and a tall standing species, such as white lupine, often used in Portugal as a green manure. The main problem in managing self-reseeding pasture species as cover crops is the fact that olive growers have neither flocks nor herds. This kind of legume species is used in pastures around the world, as the abundant literature on the subject shows. However, knowledge about the management of these species in rainfed olive orchards without grazing, but only by mowing or using post-emergence herbicides, as is proposed in this project, is practically non-existent. Regarding white lupine, although some experience exists on its use as a green manure in traditional rainfed orchards, it is necessary to acquire knowledge on the management of the cover in order to maximize the biomass yield and N fixation, and keeping the competition for water at an acceptable level. In practice, it is necessary to establish a threshold limit above which the benefits of N fixation are cancelled out by the loss of water by transpiration from the cover crop. These soil management techniques, when properly studied and their technical and economical feasibility demonstrated will have widespread application to many farmers. They are also practices that, with a few modifications, can be used in other rainfed perennial crops, such as vineyard, almond and chestnut trees. There is an urgent need to improve the profitability and the sustainability of rainfed olive orchards. Cover cropping may make a decisive contribution.