Date Published: July 01, 2018
Author(s): D. van Vugt, A.C. Franke, K.E. Giller.
•N2-fixation depends on genetic, environmental, management and socio-economic factors.•More productive and wealthier farmers benefit most from soybean-maize crop rotation.•Soybean-maize rotations should be promoted with integrated soil and crop management.
In Southern Africa, maize is the most dominant crop and is produced on 47% of cultivated land (FAO, 2014). It is the main crop for smallholder farmers who constitute the majority of the rural population and depend mainly on rain fed agriculture for food and income generation. In 2011 the average smallholder landholding size in Malawi was 0.8 ha and over 80% of this land was cultivated with maize (IFAD, 2011). Fertiliser use is highly variable among African smallholder farmers, but generally resource constrained farmers apply few external inputs, which leads to poor yields and nutrient depletion (Waddington et al., 2004, Vanlauwe and Giller, 2006). As a result nitrogen is widely limiting and farmers find themselves in a poverty trap where increasing nutrient and organic matter depletion may eventually result in non-responsive degraded soils (Tittonell and Giller, 2013). Increasing the share of legumes can contribute to sustainable intensification of maize-based cropping systems by enhancing the input of abundantly available atmospheric N2 through biological nitrogen fixation (BNF) (Mhango et al., 2013). Legumes fix on average 30–40 kg of N2 for every ton of shoot dry matter produced and can contribute to improved soil fertility and enhanced yields of a subsequent cereal crop (Peoples et al., 2009). Crop diversification with legumes can better meet caloric and protein needs of farm households if farmers adopt species that perform well under variable rainfall patterns (Snapp et al., 2014). Legumes also provide nutritional benefits through the addition of proteins to starch-based diets (Bezner Kerr et al., 2007). There is scope for enhancing productivity of edible and marketable grain legumes (Mhango et al., 2013) such as soybean (Glycine max (L.) Merril.) for the expanding market in Southern Africa for livestock feeds, edible oils and human foods (Tichagwa and Rusike, 2009). Soybean fixes on average approximately 50–60% of its nitrogen (Hardarson and Atkins, 2003, Salvagiotti et al., 2008) though ranges of 9 to 91% have been reported (Franke et al., 2017).