With the increasing need for energy, energy studies to be obtained from waste gain importance. Overall, results of this study clearly show that these currently produced composts of olive mill pomace are suitable as soil improvers for agricultural purposes, but may not contribute significantly as a N fertilizer for up to 2 y after application. Net N mineralization was negative on average (–20 μg IN g–1) after 1 y, but positive after 2 y of incubation with up to 94% of available N from the total N added and the short-term potential N leaching after compost application was negligible (less than 3.9% of added N) and much lower than the other N fertilizer with up to 80% added N leached. Furthermore, principal component analyses revealed a clear relationship between the quality of the composts and the proportion of manure mixed with the raw materials. Compost pH (8.03), electrical conductivity (2.85 dS m–1), and germination index (65% on average) were adequate for agricultural use. In all the composts tested, organic matter, total C and K were high with 60.5%, 30.7%, and 1.7% on average, respectively, whereas total P was low (0.4%) and with intermediate levels of N (1.5%). Compost of olive mill pomace differed in the proportions of raw materials co-composted with the olive mill pomace, such as olive leaf material, manure, and straw. In this study, the physico-chemical characteristics, net N mineralization, and the potential for N leaching of 7 out of the 11 olive-mill-pomace composts currently produced in the Andalusian olive mills were investigated. Olive mill pomace might be evaluated as an organic fertilizer after composting, however, before wider use of composted olive mill pomace is advocated, characterization of the final product is needed. The agricultural policies of the EU have led to widespread interest in recycling these agricultural by-products.
In S Spain, the Andalusian olive oil industry generates annually 2.5–3.0 million tons of olive mill pomace, a by-product which is comprised of the residues from the two-phase oil-extraction process.
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