The planting of so-called “fertilizer trees,” indigenous tree species that draw nitrogen from the air and replenish the soil, has significantly improved the crop yields in five African nations over the last two decades, researchers say. Since the 1980s, when the World Agroforestry Centre started working with local farmers to identify trees that can help improve soil fertility, more than 400,000 small farmers in parts of Malawi, Tanzania, Mozambique, Zambia, and Zimbabwe have planted these “fertilizer” trees, according to a new study published in the International Journal of Agricultural Sustainability. In some cases, farmers who have planted these tree species — including fast-growing varieties of acacia — had twice the maize yields as those who did not, increasing incomes and food security. In Zambia, for instance, the income for farmers using fertilizer trees were $233 to $327 per hectare, compared with $130 for unfertilized fields. Across the region, the higher yields produced 57 to 114 additional days of food. The trees also improved water efficiency, said Oluyede Ajayi, senior scientist at the World Agroforestry Centre and lead author of the study. “The trees are helping reduce the runoff and soil erosion that is a key factor behind food production shortfalls in Africa,” he said.