For optimal results, gardeners should fertilize fig trees with an all-purpose 8-8-8 or 10-10-10 fertilizer. These fertilizers contain eight percent or 10 percent each of nitrogen, phosphorous and potassium, respectively. The nitrogen encourages foliage growth. Phosphorous encourages the development of roots, flowers and fruits, while the potassium strengthens the tree’s resistance to disease.
The soil should have a pH level between 6.0 to 6.5, as fig trees may not be able to access all of the nutrients in the fertilizer outside of this range. In most environments, gardeners should only fertilize fig trees when the tree shows signs of low nutrients, such as slower growth or lighter coloring in the leaves. The exceptions are in substrates with high levels of sand, which may require annual fertilization to replace nutrients that leach out quickly, or fig trees in dense plant beds that are competing with other plants for nutrients.
Gardeners should fertilize young trees once a month as soon they produce leaves and cease fertilizing before August. Established trees require less feeding, preferably once at the end of winter, once halfway through spring and then a final fertilizing at the height of summer. Gardeners should fertilize in 1-ounce doses for young trees and 1/3 pound for each foot of height for adult trees.