Comparison of olive pomace and biowaste composts in a vegetable cropping system

The main objective of this paper was to study the growth and the yield responses of different vegetable crops to pomace compost and biowaste (source-separated municipal organic fraction) compost and to the increase in their rates. A secondary aim was to assess the efficiency of nitrogen (N) supplied to the crops by the compost rate integrated or not with N fertilisers. Finally, the ability of the two composts to improve the soil organic carbon content was also compared. The research was carried out from July 2009 to June 2011. A comparison was made of treatments resulting from the factorial combination of two composts, two rates of application, and two levels of nitrogen fertiliser. A non-fertilised control was also analysed and a standard mineral fertilisation completed the group of treatments. Cauliflower and potato were harvested after the first compost distribution, and onion and lettuce after the second. Our results indicated that the higher the quantity of olive pomace compost applied the greater the slow release of NO₃–N for crop needs. This has to be related to the high carbon:nitrogen ratio of the olive pomace compost.The halved rate of N fertiliser added to compost was sufficient to overcome the competition between soil microorganisms and roots for nitrogen, only on the second crop in the annual sequence. The biowaste compost without N fertiliser integration also reduced crop yields, but this was to a lesser degree than that achieved with olive pomace compost and was independent of the rate applied. The halved rate of N fertiliser supplied was able to overcome the problems of nitrogen availability. As a consequence, the nitrogen utilisation efficiency showed a higher recovery of nitrogen from biowaste compost than from olive pomace compost, as well as from the 10 t ha^–1 dose (rate 10) of dry matter than from the 20 t ha^–1 dose. On the other hand, the soil organic carbon content increased significantly only when the composts were added at doses of 20 t ha^–1