Where trees and shrubs cost less than fertilizer

Leguminous trees and shrubs grown with maize provide high-quality, nitrogen-rich residues that improve soil fertility, boost yields and provide new sources of income.

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, which has an unusual growth habit. The tree is dormant in the early rainy season and loses its leaves just as field crops are being established; the 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 ha, 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.