Low-cost and labour-efficient innovations in household recycling of organic wastes for soil improvement

Abstract Organic matter plays an important role in the health and productivity of soils, but its depletion is a common problem in households in low-income countries. This is due to lack of and competing uses for organic resources, and limited information on recycling methods. Therefore, here we review low-cost and labour-efficient innovations to improve recycling of organic wastes, stabilising residues so that soil organic matter can be increased with less inputs and enhancing nutrient content to produce a more effective organic fertiliser. Composting, anaerobic digestion and pyrolysis are all processes that stabilise organic matter. Innovations in treatments are needed to improve stabilization and control the release of nutrients so that they are available to crops in the right amounts and at the right time. This can be achieved by maintaining appropriate treatment conditions: for composting, carbon to nitrogen ratio 25–35, carbon to phosphorus ratio ∼50, pH 5.5–8.5 and 50%–60% moisture content; for anaerobic digestion, carbon to nitrogen ratio 20–35, bulk density 0.6–0.8 g cm −3 , lignin content < 7.5%, pH 6.8–7.4 and moisture content 85%–95%; and for pyrolysis, carbon to nitrogen > 40 and moisture content < 20%. Different methods to achieve these ideal conditions are discussed, including appropriate choice of treatment method, co-composting/co-digestion for ideal nutrient content, enhancing nutrients using collected urine, nitrogen-fixing plants, bioslurry or by inoculating with bacterial communities, absorbing excess nutrients on biochar, adjusting pH using wood ash or biochar, pre-treatment to break down lignin and cellulose, and designs to achieve ideal moisture and temperature. Innovations should also ensure that treatment processes do not overuse or compete with other important household resources, such as finances, water or labour. We draw together findings to identify methods with most potential to improve soils in low-income countries, providing decision tables to guide selection of approaches for different contexts.