Plant perennials to save Africa’s soil
Integrating perennials with food crops could restore soil health and increase staple yields, say Jerry D. Glover, John P. Reganold and Cindy M. Cox
In the 1990s soil degradation limited annual maize yield to less than 1 tonne per hectare in Africa—a common enough yield on the continent, but only one-tenth of that seen in the Corn Belt of the US Midwest. However, farmers in some parts of Africa are turning these meagre harvests into bountiful ones with the help of perenniation, the integration of trees and perennials, into fields of food crops. Perennials such as pigeon peas and groundnuts add nitrogen to the soil; trees and shrubs such as Faidherbia albida and Gliricidia sepium, improve long-term soil fertility, and some farmers now report up to four tonnes of maize per hectare in a good year.
Three perenniation approaches that show particular promise in sub-Saharan Africa are: evergreen agriculture, doubled-up legume systems and ‘push–pull’. Of the three, evergreen agriculture—planting nitrogen-fixing trees, is the best known and most widely adopted. The leguminous trees in these systems, such as Faidherbia albida, can triple maize yields while improving the soil. In doubled-up legume systems, farmers grow perennial pigeon-peas along with annual legumes such as soya beans or groundnuts. After harvesting the legumes, farmers plant maize in or beside the rows of pigeon peas and then harvest both.
Perennial plants can also help to manage pests and diseases. More than 30,000 farmers in East Africa have adopted push–pull systems to manage stem-borer moths and African witchweed, both widespread in sub-Saharan Africa. In this method, silverleaf, a perennial leguminous animal-feed crop, is interspersed among maize plants. The silverleaf produces chemicals that repel or ‘push’ pests away, and perennial napier grass grown around the edges of the fields ‘pulls’ the pests in by providing attractive leaves for egg-laying. Push–pull systems can more than double maize yields by reducing pests and increasing the amount of nitrogen in the soil.
Each of these three soil-building systems can be adapted to specific types of farming, such as conservation agriculture, organic or conventional management, or production of both crops and livestock. Even though farmers have taken some perenniation approaches well beyond the proof-of-concept stage, many questions remain: which species are best suited to which types of land, and how to maximize productivity in different areas? Perenniation, along with technologies such as improved seed, fertilization and irrigation, should be a priority for the international agricultural research-and-development community. This means scaling up the use of approaches known to work, and backing research in cultivars and techniques that farmers have not yet tested widely.
Some efforts to expand perenniation are already under way. The World Agroforestry Centre (ICRAF) , which has led the development of evergreen agriculture, has launched ‘Trees for Food Security’. During this 4-year project the centre, in partnership with the governments of Ethiopia, Rwanda, Burundi and Uganda, will plant millions of trees on farmland throughout sub-Saharan Africa. This is in line with the CGIAR’s Collaborative Research Project on Trees, Forests and Agroforestry (CRP6), in which ICRAF collaborates with 3 other CG centres, to improve smallholder systems and outputs—with a focus on forestry and agroforestry. Apart from ICRAF, a number of other institutions are promoting this work and USAID is investing US$9 million annually in ‘Africa Research in Sustainable Intensification for the Next Generation’, a programme that includes support for the study of perenniation strategies.
Sub-Saharan Africa’s population is expected to reach 1.5–2 billion by 2050. Already the population is ballooning; in many areas, the risk of drought and flood is increasing; most soils are poor; and richer nations are buying up Africa’s arable land for their own food or fuel security. African farmers have demonstrated the promise of perenniation. It is now time to scale up
its use and put it firmly on the research-and-development map.
Read the full article:
Plant perennials to save Africa’s soils. 20th September issue of Nature (vol. 489, page 359-361). Jerry D. Glover, of the USAID Bureau for Food Security, John P. Reganold of Washington State University and Cindy M. Cox of the International Food Policy Research Institute (IFPRI).