Evaluation of composition and performance of composts derived from guacamole production residues

The utilization of organic wastes to improve soils or for growth media components in local farms and nurseries can reduce the environmental pollution linked to waste disposal while increasing the sustainability of crop production. This approach could be applied to waste products generated from the production of guacamole (an emerging activity in the avocado production areas in mainland Spain), where appropriate treatment of this oily and doughy waste product has not been previously reported. The aim of this work is to study the feasibility of co-composting guacamole production residues (GR) with garden pruning waste (PW) as bulking agent, and the possible use of the compost produced depending on its quality. A windrow composting trial using three GR:PW ratios, 2:1, 1:2, and 1:7 was carried out. Temperature, moisture, organic matter, and C/N ratio were used to follow the evolution of the composting process during 7 months. After an additional 3-month curing period, composts were sieved to less than 10 mm and a set of European quality criteria was used to assess compost quality and intended use. In general, the 3 composting mixtures followed the classical process evolution, with minor differences among them. The 1:2 GR:PW ratio appeared most adequate for combining better process evolution and maximum GR ratio. Except for their high pH that limits their use as growing media component in some particular cases, the obtained composts fulfilled the more stringent European standards for commercial composts. Self-heating tests confirmed the high stability of the composts produced. The germination of cress by the direct contact method was satisfactory for composts GR:PW 1:2 and 1:7, showing no signs of toxicity. Avocado seedlings planted in substrates containing 67% of the GR:PW composts exhibited greater plant growth than those in the control treatment, and with no signs of phytotoxicity. The results open an interesting opportunity for the sustainable treatment of avocado fruit by-products derived from guacamole and avocado oil processing.