Where trees and shrubs cost less than fertilizer
Food security in Malawi and Zambia depends on maize production. However, in both countries, yields are low, on average 1.2 tonnes per hectare. Since maize is almost entirely rainfed, the crop is highly vulnerable to fluctuations in rainfall and temperatures. This vulnerability is likely to increase with climate change.
One of the main obstacles farmers face in increasing maize production is low soil fertility. Many maize farmers can neither afford mineral fertilizer nor obtain sufficient amounts of organic fertilizer, such as animal manure. Decades of intensive cultivation without fertilization have drained nutrients, particularly nitrogen, from the soil.
To address the problem, the Zambia National Farmers’ Union has explored ways of integrating nitrogen-fixing trees into maize production systems. The most promising candidate was found to be Faidherbia albida, an African acacia species with an unusual growth habit. The tree is dormant in the early rainy season and loses its leaves just as field crops are being established. Its leaves only grow back at the end of the wet season. Maize can be grown directly under the leafless Faidherbia canopy, as the trees do not compete with the crop for light, nutrients or water while the maize is growing. Thanks to the decaying leaves, the soil under the trees contains up to twice as much organic matter and nitrogen as soil outside the canopy. There is also a marked increase in soil microbiological activity, and an increase in water holding capacity. Numerous studies have noted increases in yields when maize is grown in association with Faidherbia, and those increases tend to be higher where soil fertility is low. In Zambia, maize planted outside the tree canopy produced average yields of 1.9 tonnes per hectare, compared to 4.7 tonnes when the crop was grown under the canopy. In Malawi, maize yields increased by 100 to 400 percent when the crop was grown with Faidherbia. Both countries promote Faidherbia as part of conservation agriculture systems that offer smallholder farmers a means of increasing maize productivity and earning higher incomes. National recommendations are to grow 100 trees per hectare in a 10 m x 10 metre grid pattern.
Although Faidherbia is one of the fastest growing acacia species, it is not a quick fix for low soil fertility. In a survey of 300 Zambian farmers, one-third said that yields increased over a period of one to three years, while 43 percent said that it took up to six years before they saw benefits in production. Planting leguminous coppicing trees, such as Gliricidia sepium, which take less time to establish, is another way of increasing maize production sustainably. On small landholdings in southern Malawi, the World Agroforestry Centre is promoting a system in which farmers plant Gliricidia in rows in their maize fields, prune them back two or three times a year, and mix the leaves into the soil. Findings from a decade-long study indicate that on unfertilized plots where Gliricidia is intercropped with maize, yields average 3.7 tonnes per hectare, and reach 5 tonnes per hectare in good years. On unfertilized plots without Gliricidia, average yields were only 0.5 to 1.0 tonne per hectare.
In areas where land holdings are larger than 1 hectare, growing leguminous shrubs such as Sesbania sesban on fallow fields is another option for revitalizing the soil and increasing maize yields.
Leguminous trees and shrubs add from 100 to 250 kilogram of nitrogen per hectare to the soil in fields that are left fallow for two or three years. Even though fields are unproductive for two out of every five years, returns on investment are higher when maize is grown in rotation with nitrogen-fixing shrubs and trees. In eastern Zambia, one study found the average net profit was USD 130 per hectare when farmers cultivated maize without fertilizer; USD 309 when it was grown in rotation with Sesbania; and USD 327 when it was intercropped with Gliricidia. For each unit of investment, farmers who integrated trees with maize benefited from higher returns than those who used mineral fertilizer, subsidized or unsubsidized, for continuous maize production. The study confirmed that maize agroforestry is both socially profitable and financially competitive, compared to maize production using only mineral fertilizer.
The trees also provide fuel for rural households. In Zambia, farmers were able to gather 15 tonnes of fuelwood per hectare after the second year of fallow with Sesbania and 21 tonnes after the third year.
Integrating multipurpose trees into crop production does not require large investments. Low-income farmers are often quicker to embrace it than farmers who are better off. Although more labour is required initially, farm labour can be used more efficiently once farmers master the new practices. However, incorporating trees into crop production is a knowledge-intensive activity. Policy support, continued research and rural advisory services that engage smallholder farmers are crucial for the long-term expansion of farming systems that integrate maize, shrubs and trees.
Source: adapted from FAO, 2015e.