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[BLANTYRE, MALAWI] The biodiversity of crop fields could be key to a greener revolution in Africa, where ecosystems are degrading and crop yields are stagnating, says a study conducted in Malawi.

African farmers could halve their fertiliser use and still get the same yields, the study found, with less year-to-year variation in yields and with as much as 70 per cent more protein in grains — by simply rotating their maize crops with shrubby legumes.

The legumes, such as soybeans, groundnuts or pigeon pea, fix nitrogen in the soil to make it richer in nutrients by hosting nitrogen-fixing soil bacteria called rhizobia in their roots. These live in a symbiotic relationship with their host plant ― exchanging nitrogen for nutrients. They may also help enrich degraded soil with phosphorous.

The study, published in Proceedings of the National Academy of Sciences (PNAS) last month (22 November), tested different intercropping and rotation regimes throughout maize fields in Malawi. It concluded that crop diversification to include shrubby legumes could be effective in enhancing food and environmental security on a countrywide scale.   

"Our study, conducted between mid-1990s and 2010, led to the conclusion that legume diversification is the foundation for enhanced nitrogen fixation and carbon sequestration in agriculture," George Kanyama-Phiri, co-author of the study and crop scientist at the University of Malawi, told SciDev.Net. "The incorporation of organic dead plant material also improves soil organic matter, soil structure and texture."

But he cautioned that the rotation with legumes should be viewed as complementary to inorganic nitrogen fertiliser and not a replacement — modest fertiliser use has sent yields skyrocketing in Malawi, he said.

Kanyama-Phiri also said that local differences in the timing of farming practices may limit the benefits of legumes. What works in central Malawi may not work in the southern region, where farmers construct maize ridges later — exposing the legumes to damage from sun and goats.

"Many farmers may want to adopt the technology but they may be limited by the availability of [legume] seed,"  Kanyama-Phiri added.

Sosten Chiotha, environmental scientist and head of Southern and Eastern Africa branch of non-governmental organisation for sustainable leadership, LEAD, said: "Applying fertiliser is a short-term solution for replenishing nutrients in the soil. It can be both expensive and unsustainable in some soils".

"Farmers have to embrace new technologies that encourage them to grow more crops on a piece of land [mixed cropping] so that they achieve food sufficiency at the household level and replenish the soils at the same time.

"Mixed cropping also enables farmers to reduce soil erosion and moisture loss, as the farm land is less exposed to the sun, unlike when a farmer only plants maize which requires spacing of about 90 to 75 cm apart."

"Innovation in agriculture is crucial if farmers are to adapt to climate change," said Chiotha.

Link to article abstract in PNAS


PNAS, doi:10.1073/pnas.1007199107 (2010)