Fertilizers from composting of olive mill wastewaters

The aerobic bioremediation of olive-mill wastewaters produces a high quality compost, characterized by a considerable presence of nutrients, mainly organically-bound nitrogen (1.5-3%), a good level of humification (degree of humification=78%; humification index=0.28), and by the absence of phytotoxicity. The agronomic value of a compost thus obtained was assayed both by the 'crop test' and following the plant-soil system as influenced by compost supply. Field experiments performed on maize showed that compost, when supplied before sowing in amounts of 60-90 Mg ha(-1) (equivalent to a manuring on the basis of organic matter) is able to reduce the need for chemical fertilization. The same quantity supported the nutritional need of rye grass and horticultural plants. A good rest-effect was also recorded. Compost supply enhanced both soil oxygen consumption and nitrogen fixation in the open field. An improvement of activities in the plant-soil system was made evident by pot trials.