Africa has a wide range of soil conditions, most of which are not very favourable for agriculture. The central, more humid part of the continent is dominated by deeply weathered, acidic soils with high levels of iron oxides and lacking in essential plant nutrients. In drier conditions, weathering processes are less intense which, together with inputs of wind-blown dust, give rise to soils with increased clay content and slightly higher pH.

Soil acidity, poor fertility and low water holding capacity are seriously limiting crop yields, hindering economic progress especially for smallholding farmers. Soils are threatened by over-exploitation and increased weather uncertainty due to climate change. Soil carbon contents are declining, and weather conditions are increasingly variable and often unreliable.

Organic fertilisers are more important than ever.

Many farmers already include management practices aimed to maintain and build healthy soil but not every farmer has the financial or technical means to apply sufficient sustainable agricultural practices. Training and value chains must be accessible, particularly to the 500 million smallholders, many of whom are women and youth, who have the greatest difficulty in accessing these resources.

In the OFVI-ABC project, soil pH and soil organic carbon (SOC) are used as indicators to monitor soil quality and assess added value of organic fertiliser (bio-slurry and BEC) application. A pH level around 6.5 is considered optimal for many key crops including corn and soybeans. Efforts by farmers to boost yields by applying traditional fertilisers such as nitrogen and potassium in greater quantities can cause soil acidification, reduced soil organic carbon concentrations and, finally, diminishing yields.

Soil organic carbon (SOC) is the basis for soil fertility and crop production. Carbon is the main component of soil organic matter which helps give soil its water-retention capacity, structure, and inherent nutrient fertility. Loss of soil organic carbon content can limit the soil’s ability to provide water and nutrients for sustainable plant production. This may lead to lower yields and affect food security. Less organic carbon also means less food for the living organisms present in the soil, thus reducing soil biodiversity.

Anaerobic Digestion (AD) and biodigesters can play an essential role in improving conditions for farming in Africa.

Next to provide clean cooking fuel, they can transform manure and crop residues into an improved organic fertiliser (bio-slurry or digestate). Bio-slurry is generated by decomposition of organic materials by microbes. During digestion, most of the degradable organic compounds are converted into biogas. During the process, complex organic nitrogen compounds are mineralized to ammonia; some are used by microorganisms for growth.

Bio-slurry has a higher nitrogen content than other compost sources and is thus particularly interesting for farmers, especially as it contributes to soil organic matter and improves soil structure.

Bio-slurry can improve soil characteristics in target countries.
Application of bio-slurry, either direct or via the production of Bio-slurry Enhanced Compost (BEC), can help transform agricultural production systems, making them more robust and resilient.

Manure, compost and bio-slurry are valuable sources of nitrogen, phosphorus, potassium and micro-nutrients, and their application generally is beneficial to the soil and the crop.

The RVO ABC (African Bio-digester Component) programme aims to support and stimulate bio-digester production and utilisation. The Organic Fertiliser Valorisation Implementer (OFVI) project recognises and promotes the value of bio-slurry as a unique source of nutrients and organic matter that can help to improve soil quality and halt soil deterioration. This will help to improve the business case for the ownership of bio-digesters, either by allowing owners to profit directly from enhanced yields from bio-slurry – and compost made with bio-slurry (BEC), or from sales of excess bio-slurry and/or BEC.