Building Soil Nitrogen Capital in Africa

Abstract

The dynamic nature of N cycling dictates that soil N capital useful for supplying N for plant growth must be equated with short- to medium-term, rolling capital (the monthly or annual salary), rather than long-term reserves (gold in the bank). Thus building of soil N capital necessitates a focus on the capacity of soils to store organic matter (the principal reserve of N in the soil) and on management strategies to replenish N reserves. For a given climate, the capacity of a soil to store organic matter is directly related to its texture through both direct effects of clays adhering to organic matter and indirect effects on soil aggregation, which render the organic matter protected from decomposition. Some African soils can contain up to 300 to 400 kg N ha−1 in free mineral form within the top 2 m of soil, which represents a form of vulnerable capital susceptible to leaching. Large amounts of organic inputs are required to build up the soil's capital store of N. The approaches most suitable for this are improved legume fallows, legume-grass leys, or minimum-tillage systems. When soils are cultivated continuously it may be impossible to build up the soil N capital. Even when the capital store of N is replenished, continued use of crop sequences and intercrops with grain legumes and green manures, better integration of crops and livestock, and optimal use of mineral fertilizers are essential to ensure improved yields are maintained. A major challenge is to work together with smallholder farmers to find attractive methods for building the N capital of their soils.