Cover cropping in rainfed olive orchards Conference Paper uri icon


  • Biofertilizers are fertilizing products containing living microorganisms that, when applied to the soil or directly to a seed, colonize the rhizosphere and/or the plant promoting growth by increasing the availability of nutrients. Biofertilizers may improve the bioavailability of nutrients through natural processes such as biological nitrogen fixation, phosphorus solubilization, or through the synthesis of growth promoting substances. Organic fertilizers enriched with heterotrophic free-living nitrogen-fixing microorganisms have recently appeared on the market, seeking to enhance nitrogen fixation by placing the microorganisms directly in the food substrate. However, these nitrogen-fixing microorganisms are ubiquitous in agricultural soils, which allow question of the usefulness of enriching the organic fertilizers in such microbes. Unless the placement of the microorganisms next to the food substrate can give them competitive advantages and increase the biological nitrogen fixation. Field and pot experiments were carried out to compare the performance of two organic fertilizers enriched with free-living nitrogen-fixing microorganisms with other organic and mineral fertilizers. A sequence of horticultural crops (Lettuce-Lettuce-Turnip) was repeated for two years in the same field plots and pots. In the third year, barley was grown without fertilization to assess the residual effect of the fertilizers. Anion exchange membranes were used to monitor nitratenitrogen in the soil and plant dry biomass and nitrogen concentration in plant tissues to assess plant nutrient uptake and nitrogen use efficiency. Biofertilizers displayed results lower than mineral fertilizers but higher than organic fertilizers without microorganisms’ addition.
  • Cover cropping has been advocated as a mean of reducing soil erosion and increasing soil organic matter and functional biodiversity. There are many other good reasons to establish a cover crop in an orchard since it can improve in several ways the physical, chemical and biological properties of a soil. However, herbaceous vegetation competes for resources. In intensivemanaged orchards, without limitation of available water and nutrients, probably this is a minor problem. In rainfed orchards, to balance the potential beneficial effects of cover crops and their strong competition for resources in particular for available water is a great challenge. In this work, we report results from field trials from which we are proposing a model of intervention in rainfed managed orchards consisting of the use of very early maturing self-reseeding annual legumes as a cover crop. These plants are able to enrich the soil with nitrogen and to protect the soil all year round through a green cover of vegetation during autumn-spring and a mulch of dead material during summer. Experimental results also showed that nitrogen nutritional status of the olive trees and olive yields increased in comparison to a plot of natural vegetation fertilized with 60 kg N ha-1 yr-1.
  • Despite the increased acreage of high-density irrigated orchards, traditional rainfed olive groves maintain huge social importance. While in irrigated orchards cover cropping is generalized, in the rainfed groves the ground continues to be tilled or maintained barely by using herbicides. Cover cropping is important as is an effective mean of reducing soil erosion. However, the covers consume water which can severely reduce olive yields. Over the last 18 years, experimental work has been done in searching for a solution for these orchards. This work summarizes the results obtained in several field trials, which included several soil management treatments, such as conventional tillage, bare soil by using residual or post-emergence herbicides, natural vegetation mowed or grazed, legumes of erect habit grown as green manures and self-reseeding annual pasture legumes. The results indicate that withdrawing tillage and allowing the development of the root system significantly increases olive yield. Cover crops of natural vegetation control soil erosion and improve several soil fertility parameters but significantly reduce olive yield through excessive competition for water. Green manures are difficult to manage since they require be sowing and incorporating into the soil by tillage. If the green manures are managed as mulching it causes significant nitrogen losses to the atmosphere. In these orchards, the theoretical model that aggregate the best results is the growing of very early-maturing self-reseeding annual legumes. These plants provide enough protection of the soil, fix nitrogen in rates able to maintain the trees at nutrition levels higher than the application of 60 kg N ha-1 year-1 and ensures high olive yields due to the little competition for water. These covers proved to be the only way to make profitable organic farming, an interesting alternative for these low input agricultural systems.

publication date

  • January 1, 2017