Olive Mill Wastewaters Spreading on Agricultural Land: Results and Practical Management

Rich in water (88 to 95%), organic matter (6 to 14%) and mineral matter (1.5 to 4%), the olive mill wastewater or liquid effluent from the olive processing industry represents a certain fertilizer source. Four treatments (Control, 50, 100 and 200 m³ ha^-1 of OMWW) with an annual spreading of fresh OMWW, are applied to evaluate the effects of direct spreading of this by-product on the soil and the plant. From the first application, the level of organic matter changed significantly compared to the control soil without OMWW. In fact, three months after spreading, the respective levels of this substance increased from 0.42% in the control plot, to 0.91%, 1.02% and 1.47% respectively, for the spreading doses of 50, 100 or 200 m³ ha^-1. After several years, these same rates are only 0.84%, 0.91% and 0.95% for the respective doses of 50, 100 and 200 m³ ha^-1, indicating an intense activity of mineralization of organic matter under the effect of the soil microflora whose number has increased. The improvement in mineral status consequently improved the growth of olive groves with application doses of 50 and 100 m³ ha^-1. A decline in olive production was recorded with the dose 200 m³ ha^-1, which could be explained by the inhibitory effect probably resulting from a slight toxic action of polyphenols accumulated in the soil. Finally, it should be noted that the results of this research led to the development of Decree No. 1308/2013 of February 26, 2013, authorizing the spreading of 50 m3 ha 1 of fresh vegetable water.