What is the potential for biogas digesters to improve soil fertility and crop production in Sub-Saharan Africa?

Abstract

Three alternative soil amendments of organic wastes are considered: application of untreated animal manures, bioslurry from biogas digestion, composted materials, and biochar produced by pyrolysis cook-stoves. Application of untreated manures provides high input of available nutrients, which results in an initial flush in crop growth. However, risks of losing nutrients are high because manure is usually applied before sowing to avoid reduced yields due to phytotoxicity, resulting in increased losses by leaching or volatilization. Furthermore, the heterogeneous nature of untreated manures results in immobilization of nutrients by carbon-rich materials. A greater amount of nutrients are potentially available to crops from applied bioslurry. Typically 5-10% of the nitrogen is lost during anaerobic digestion, but bioslurry provides immediately available nutrients that can be applied as needed, so reducing risks of nutrient loss. If, however, bioslurry was applied in a single dose, losses would be similar in magnitude to untreated manures. Risks of nutrient losses are also lower when wastes are applied as composts, but in contrast to bioslurry, this is because the concentration of immediately available nutrients is very low, most nutrients being held in organic form that will become available only slowly over the growing season. Composts provide an option for single dose application, but a larger proportion of nitrogen is lost during composting (26-51%) than during anaerobic digestion (5-10%). Losses of nitrogen during pyrolysis are also very high (70-90%), but biochar can reduce losses of native soil nutrients by providing exchange sites that hold nutrients in the soil.

Methane, nitrous oxide emissions and mitigation strategies for livestock in developing countries: A review

“…In sub-Saharan Africa, the production of compost in pits with a mix of animal faeces, feed and crop residues and domestic waste is extensively prevalent among small households. Householders irrigate the pit, turn the compost, use a cover to limit N losses, and use the compost as natural fertilizer, because it is particularly rich in nutrients (Smith et al, 2014). In Vietnam, parts of both liquid and solid manure produced by pig farms are applied to fish ponds and used to feed fish for local consumption (Vu et al, 2012).…”

A Qualitative Evaluation of CSA Options in Mixed Crop-Livestock Systems in Developing Countries

“…Their applicability in relatively low-input mixed farming systems is likely to remain limited (FAO 2013), as the investment costs, labour demands and technical know-how will be beyond the reach of the great majority of smallholders. Some recent studies indicate that there is potential for communal biogas digesters to improve soil fertility in the developing world (see, for example, Smith et al 2014), but the constraints of unaffordability, water scarcity, inappropriate technology and lack of technical capacity may be insuperable without considerable public sector investment (Mwakaje 2008). The conditions under which such interventions are climate smarter are still largely unknown.…”

Evaluation of the Performance of Different Organic Fertilizers on Maize Yield: A Case Study of Kampala, Uganda

“…Vermicomposting is a process by which worms are used to convert organic wastes into a humus-like substance called vermicompost, which can also be used as a fertilizer and soil conditioner (Munroe, 2007;Rajesh, Reddy, Naidu, & Ramavatharam, 2003). In anaerobic digestion, organic residues such as animal manure are transformed into biofuel (biogas), while the resulting effluent (digestate) can be reused in agriculture to improve the physical properties of soil and supply vital plant nutrients (Smith et al, 2014). Composting is a simple method of controlled aerobic degradation of manure to produce a stable, soil-like substance.…”