Multipurpose Legumes for Soil Rehabilitation

Technologies that support manure production, such as residue mulch systems, agroforestry, and intensified livestock systems, are promoted to build long-term soil productivity in Malawi. Additionally, knowledge of soil erosion control focusing on management practices such as planting on the contour and building bunds to control water flow is urgently needed. Efficacy of these management practices will require widespread education efforts and our lab group is invested in collaborating with Malawi government extension in these areas. We are particularly interested in approaching these through innovative education practices such as farmer field schools, innovation platforms and participatory co-learning in support of farmer experimentation.

At the same time, there are significant challenges to these approaches, such as low livestock numbers in Malawi’s rural communities. Furthermore, there has been very little evidence that farmers are able or interested in investing in agroforestry systems. High labor requirements and/or limited returns may be major impediments. Conservation agriculture in various forms has also been promoted in Malawi, but there are barriers to its uptake and weed management in these forms of agriculture remains one of the major constraints.

Another approach to rehabilitating soils has risen to the fore in recent years. On-farm research initiated in Malawi in the mid 1990s has consistently indicated farmer preference for multipurpose pigeonpea mixtures rather than agroforestry or green manure systems (Chikowo, et al., 2014; Kanyama-Phiri et al., 1998; Snapp et al., 1998). The risk of household food insecurity has been shown to be minimized through adoption of pigeonpea-based organic matter technologies with more mixed results from agroforestry technologies (Sirrine et al., 2010).  One of the most promising multi-purpose legume systems for Malawi mid-altitude zones involves using pigeonpea either grown in mixtures with maize or as a doubled up legume system (pigeonpea and an understory of soybean or groundnut), then rotated with maize. Snapp et al. (2010; 2014) found evidence of superior performance from these systems in country-wide trials.

However, there has not been sustained effort to investigate the potential for soil organic matter rehabilitation through diversification with these plant types. High variability of soil organic carbon and management effects are difficult to detect on-farm (Snapp et al., 2010a). Yet, results from on-farm trials in Malawi indicate aboveground residues produced by leguminous shrubs and vines such as pigeon pea, mucuna, climbing beans and tephrosia are nitrogen-enriched and of mixed quality biochemistry, in the range of 3 to 5 MG biomass per ha (Table 1, Snapp et al., 1998; 2010a). Root biomass measurements have recently been conducted for pigeon pea grown on over 40 fields in Central Malawi, where biomass ranged from 0.9 to 2.0 MG biomass per ha (Gwenambira, 2015). These levels of vegetative biomass and in particular the root organic material are expected to translate into improved soil organic C (Kong et al., 2005; Puget and Drinkwater, 2001).

In terms of maize yield improvements, the maize-pigeonpea intercrop system was the highest performer. It is also well suited to small land holdings and imperfect market conditions where farmers cannot sell legume grain with a reasonable expectation of obtaining sufficient profit or access to maize. Many farmers in the region use pigeonpea as a relish, or secondary food source. At the same time, the doubled up legume rotated with maize system, was especially promising in favorable growing seasons and produced as much maize yield as monoculture maize fertilized at twice the rate (Figure 1).

Two additional benefits from the multipurpose legumes may be climate change resilience and improved water use efficiency through hydraulic lift. Climate resilience gains are particularly marked when maize hybrids are grown with targeted fertilizer, in rotation with doubled up legume and mucuna rotations. This has been shown to not only improve maize fertilizer response by 50% or more, but also to dramatically enhance yield stability (Snapp et al., 2010). Competition for water from pigeonpea shrubby growth types with maize and other food crops may be minimized if pigeonpea is able to facilitate hydraulic lift of water to the surface. There is evidence from Zambia of this phenomena (Sekiya and Yano 2004) and the Snapp research group has recently initiated field experiments in Mali to test this hypothesis.

In summary, growing more biomass is one of the only ways to enhance soil C and N status under on-farm conditions in the sub-humid tropics of East Africa. Livestock integration, conservation practices, organic farming and integrated nutrient management all require a foundation of biomass, particularly high quality legume biomass. Indeterminant, shrubby forms of legumes such as pigeonpea can provide substantial amounts of biomass over 8 to 24 months, and many such species fix copious amounts of N over this time period. In contrast, annual food legumes are grown for about 4 months with about half the plant biomass removed at harvest index, which minimizes residue biomass to insignificant amounts. The biomass potential of multipurpose legumes grown in the often marginal soils of East Africa agricultural lands is understudied and should be a priority.

Kanyama-Phiri, G.Y., S.S. Snapp and S. Minae. 1998 Partnership with Malawian farmers to develop organic matter technologies. Outlook on Agriculture 27:167-175.

Mhango, W. S.S. Snapp and G. Y. Kanyama-Phiri. 2013. Opportunities and constraints to legume diversification for sustainable cereal production on African smallholder farms. Regenerative Agriculture and Food Systems 28:234-244.

Sirrine, D., C. Shennan, S.S. Snapp, G. Kanyama-Phiri, and B. Kamanga. 2010. Agroforestry, risk and vulnerability in Southern Malawi: improving recommendations resulting from on-farm research. International Journal of Agricultural Sustainability 8:290-304.

Snapp, S.S. M.J. Blackie, R.A. Gilbert, R. Bezner-Kerr, and G.Y. Kanyama-Phiri. 2010. Biodiversity can support a greener revolution in Africa. PNAS 107: 20840-20845.

Snapp, S.S., T.S. Jayne, W. Mhango, T. Benson and J. Ricker-Gilbert. 2014. Maize yield response to nitrogen in Malawi’s smallholder production systems. Working Paper 9. Malawi Strategy Support Program. IFPRI.

Snapp, S.S., P.L. Mafongoya and S. Waddington. 1998. Organic matter technologies to improve nutrient cycling in smallholder cropping systems of Southern Africa. Agriculture, Ecosystems and Environment 71:187-202.