Although it resembles the lime, calamansi is the result of a cross between the mandarin and the kumquat. One of the most important factors to consider when making a calamansi tree give fruit is the region in which it is being grown. It requires lots of sunshine and little rain, within Zones 9 and above on the U.S. Department of Agriculture Hardiness Zone Map. If you have a young calamansi tree, keep in mind that it won't bear fruit until it is at least two years old.
Take soil samples from around the calamansi tree and have them tested for pH levels. The calamansi tree requires a soil pH between 5.5 and 6.5 to flower and set fruit.
Raise the pH by adding agricultural lime, at the rate suggested by the soil test results and carefully water it into the soil. Don't apply more than one-half inch of water. Lowering the pH of alkaline soils is quite challenging and may be impossible. Consult with the county cooperative extension office in your area for assistance.
Fertilize the first year calamansi with 2/3 cup of each of urea and 16-20-0 fertilizers, mixed, in spring and again every four months thereafter. When the tree reaches two years of age, and each year thereafter until it is eight, give it three pounds of 12-24-12 fertilizer two months after it flowers and then again immediately after harvest. When the calamansi tree reaches eight years old, supply it with four to six pounds of 12-24-12, according to the same schedule.