Purpose The region of Souss-Massa generates huge quantities of organic horticultural wastes estimated up to 1,307,465 tons·year⁻¹. Fruit and vegetable producers are asked to process the organic wastes because of the GlobalGAP certification requirements. As a result, there is a need to assess the existing composting plants at regional scale, to improve their efficiency, produce a high-quality soil amendment, free from any pathogens, and weed seeds. Methods The survey consisted of a diagnosis of the quantitative and qualitative status of the organic horticultural wastes (axillary buds; pruned leaves), and an agro-environmental assessment of nine composting units. Results The analysis of macronutrients (nitrogen, phosphorus, and potassium) in the organic wastes reveals that 13 million euro worth of N, P, and K can be generated by composting all horticultural wastes (12,000 tons of nitrogen, 6000 tons of phosphorus and 14,000 tons of potassium · year⁻¹), constituting for soils an important source of fertilization backup. Results of field investigations showed that 55% of the composting plants have an area equal to one hectare. The proximity between mature piles and raw materials in 55.5% of cases could highly contaminate them by root knot nematode inoculum. Only 11% of composting units were operating on impermeable surface. The test of circular chromatography showed that 80% of sampled composts are immature, even with high operational costs. Conclusion Much effort is required to control and optimize the maturation process. The composting activity should be organized through a professional skilled organization, supported by scientific research and government subsidies.

The environmental issues related to olive oil by-products and turkey manure are continuously increasing. This ecological hazard could be mitigated by the composting process. This study aimed to investigate the effect of the initial carbon to nitrogen ratio (C/N) and aeration rate (turning frequency (TF)) on mineral contents (P, K, Ca, and Na) and their leaching in the soil surface. Olive pomace (OP) and turkey manure (TM) were co-composted to prepare six mixtures at three levels of initial C/N (20, 22, and 28) and two levels of TF (once and twice a week). The results revealed a substantial effect of TF, twice a week, resulting in a loss of 36.5%, 36%, and 27% for K, Ca, and Na contents, respectively, whereas TF, once a week, preserves the maximum of nutrients in heaps with good compost maturity.The initial C/N of 28 had significantly reduced P and K by 14% and 13%, respectively, and had increased Ca and Na contents by 85% and 30 %, respectively. The leaching of almost all the studied minerals has been demonstrated after composting. Exchangeable cations have been leached more under heaps with higher C/N ratios with TF of once per week, whereas P leaching has been recorded at the maximum level under the heap with an initial C/N of 20 and turned twice weekly. The resulting composts have generally displayed good mineral quality.