Both citrus trees and avocado trees are tolerant of acidic and alkaline soils, but if your soil is prone to being too alkaline you can make it more acidic slowly by adding vinegar to the soil. The vinegar will not overly acidify your soil. Also, adding vinegar to the water you apply to your avocado or citrus trees will clean the water and produce more vigorous growth in certain instances, according to Howard Garrett, the "Dirt Doctor."
Measure out how much vinegar you want to use on your avocado and citrus trees. If you want to increase the acidity of the soil, add about 1/2 cup of vinegar for every gallon of water you give your trees each time you water them. If you just want to clean the water, add 1 ounce of vinegar for every gallon of water.
Mix the water and vinegar together into a solution.
Apply the water and vinegar solution around the base of each citrus or avocado tree. The vinegar and water solution will soak into the soil. Be sure to add enough of the solution that it will soak down to the tree's roots.