Improved soil quality after 16 years of olive mill pomace application in olive oil groves

This report shows notable improvements of soil physical, chemical, and biological properties after long-term soil application of olive mill pomace compost. About four million tons of olive mill pomace is produced annually in Andalusia, Spain. Olive mill pomace is a main by-product of the olive oil extraction industry. Composting is a promising strategy to manage the huge volume of this potentially environmentally harmful pomace. Converting olive mill pomace into a useful soil amendment in semiarid Mediterranean areas of olive oil farms, characterized by low organic matter content and subjected to progressive degradation, would be valuable. There is actually no data on the long-term effects of composted olive mill pomace application on soil physicochemical and biochemical properties. However, this information is needed to encourage the composting of this pomace. Here, a field study evaluated soil fertility and soil capacity to degrade organic compounds after the application of composted olive mill pomace. Olive groves received compost annually for 3, 4, 9, and 16 years. Soils were sampled and compared to olive groves without compost application. Soil physico-chemical properties and soil enzyme activities such as acid phosphatase, beta-glucosidase, protease, invertase, and dehydrogenase, were analyzed. Our results show that soil organic matter, nitrogen, available phosphorus, cation exchange capacity, aggregate stability, and exchangeable potassium were between 1.4 and 3.3 times higher in the compost-treated farms. Soil enzyme activities in soils treated with compost was between 180% and 420% higher than in untreated soils. Moreover, there was a clear trend of increasing soil fertility and enzyme activities with years of compost application. Here, we conclude that the addition of composted olive mill pomace to olive groves markedly improved soil quality.