Trees that use Nitrogen from the air and "fix" it in their leaves help farmers increase yi
"It can be done by applying livestock manure or by growing green manures. Farmers can also establish a “fertilizer factory in the fields” by planting trees that take nitrogen out of the air and “fix” it in their leaves, which are subsequently incorporated into the soil. That, in essence, is the result of planting Faidherbia albida, a nitrogen-fixing acacia species indigenous to Africa and widespread throughout the continent. Since this tree goes dormant and sheds its foliage during the early rainy season at the time when field crops are being established, it does not compete significantly with them for light, nutrients or water during the growing season; yet it allows a significant increase in yields of the maize with which it is combined, particularly in conditions of low soil fertility. In Zambia, unfertilized maize yields in the vicinity of Faidherbia trees averaged 4.1 t/ha, compared to 1.3 t/ha nearby, but beyond the tree canopy. Similar results were observed in Malawi, where this tree was also widely used. The use of such nitrogen-fixing trees avoids dependence on synthetic fertilizers, the price of which has been increasingly high and volatile over the past few years, exceeding food commodity prices, even when the latter reached a peak in July 2008. In this way, whatever financial assets the household has can be used on other essentials, such as education or medicine."
Oliver De Schutter, "Report submitted by the Special Rapporteur on the right to food", Human Rights Council, A/HRC/16/49: 10.