Monitoring Cocomposting of Agro-Wastes from Olive Mill By-Products and Poultry Manures

Abstract

The potential of treating agro-wastes by recovering organic amendments for agricultural soils is promising for Mediterranean countries that produce olive oils. In this study, olive mill by-products were cocomposted with poultry manures for periods of up to 6 months. The main physicochemical and microbiological parameters were monitored, and the maturity of the resulting compost and its fertilizer potential were evaluated. Results showed a gradual decrease in organic matter content, total organic carbon (TOC), and carbon per nitrogen ratio during the composting period incubation. Humic acids (HA) were increased from 1.8% to 12.25%, fulvic acids (FA) were decreased, and the humification index (HI) of HA/FA increased from 0.3 to 3.9. Positive and significant correlations were recorded between electrical conductivity (EC), pH, the cation exchange capacity (CEC), the total kjeldahl nitrogen (TKN), HA, and HI. However, all these parameters were negatively associated with TOC, polyphenols, and FA. The most important positive correlation (r = 0.997, p ≤ 0.01) was detected between HA and HI. HA was also highly correlated with TOC (r = −0.978, p ≤ 0.01), EC (r = 0.843, p ≤ 0.05), CEC (r = 0.926, p ≤ 0.01), and polyphenols (r = −0.895, p ≤ 0.05). A high negative correlation was detected between TKN, TOC, polyphenols, and FA. Significant relationships were generally not found with germination indexes (GIs) of Medicago sativa and Sorghum bicolor and physicochemical parameters, with the exception of the negative correlation between potassium and Sorghum bicolor GI (r = −0.925, p ≤ 0.01). Moreover, the resulting compost presented interesting values of GIs of 88%, 102%, and 112%, respectively, for Lepidium sativum, M. sativa, and S. bicolor, which reflects its maturity and its potential qualities as biofertilizer.