Residues and by-products of olive-oil mills for root-zone heating and plant nutrition in organic vegetable production

Residues and by-products of the olive-mill agro-industry (leaves and pomace) provided rhizosphere heating for cucumber plants during their prolonged in situ composting process carried out under the plant rooting systems. The plants were cultivated following organic production standards in the greenhouse, throughout the early spring production period. The composting of the above materials below the root-zone, resulted in an increase in the rhizosphere temperature by 10-15 °C, resulting in an optimal 20-30 °C ambient root-zone temperature during the whole cultivation period. No phytotoxicity was observed on any of the four different cucumber cultivars that were tested. The mature compost (MC) derived from the above materials was also assessed as a soil amendment for providing nutrients in organic greenhouse production of cucumber, in comparison to the application of other organic substrates available at low cost in Mediterranean regions. All organic substrate applications including MC increased soil organic matter content, as well as P and K availability. Sheep manure application nearly doubled cucumber production compared to the control (non-amended soil), whereas plain seaweed application reduced it by 40%. Spent mushroom (Pleurotus ostreatus) cultivation substrate and MC amendments showed no significant effect on cucumber production. High MC application rates, however, in a more fertile soil, resulted in yields comparable to sheep manure treatment. It is concluded that residues and by-products of olive-mills may provide effective root-zone heating at greenhouse production scale via an on-going composting process set below the plant root-zone, but only high application rates of mature composts derived from the above materials could satisfy nutrient demands during plant growth.