Even if its aromatic green leaves and bumpy, green fruit rind weren’t crucial ingredients in Thai cooking, the kaffir lime (Citrus hystrix) would be well worth growing for its deliciously fragrant white blossoms. Also called Thai lime, the frost-tender tree grows to 25 feet tall when planted outdoors in U.S. Department of Agriculture plant hardiness zones 10 through 12. Elsewhere, grow it like a compact houseplant and summer it outside. Like all citrus varieties, kaffir lime needs regular applications of high-nitrogen fertilizer.
The Way Nitrogen Helps
A kaffir lime requires nitrogen to encourage lush, healthy foliage. Without lots of leaves to photosynthesize its food, then it does not have enough energy for flowering, fruiting and crushing disease and pest damage. To get a tree grown for its edible leaves, adequate nitrogen is important.
How to Determine Nitrogen Content
Look at the first of the N-P-K statistics on a fertilizer’s label. An “N” number of 10 means that the fertilizer contains 10 percent nitrogen, therefore a 50-pound bag could have 5 lbs of actual nitrogen. To provide your lime with 1 pound of actual nitrogen, you would use 10 pounds of the fertilizer. If the “N” number were 20, the bag could contain 20 percent, or 10 lbs, of actual nitrogen. Five pounds of the fertilizer would provide your lime with 1 pound of actual nitrogen. The greater a fertilizer’s “N” number, in other words, the less of the fertilizer the tree requires.
Fertilizing a Young Lime
In the first year after planting, fertilize your kaffir lime monthly from early spring to late summer. Use 1 tbsp, or the brand’s recommended number, of granulated 21-0-0 ammonium sulfate fertilizer. Scatter it evenly over the root ball and then water the tree well. Starting in the second season, fertilize every four to six months between spring and late summer. A second-year kaffir lime gets 1/2 cup of 21-0-0 granules per program; raise the 1 cup in the third year and 2 cups in the fourth. Spread them a few inches from the back to 1 or two feet beyond the dripline, where rain falls from the outermost branches to the ground. Water the soil slowly to a thickness of 1 inch, or about 6 gallons of water per 10 square foot.
Fertilizing an Established Lime
A proven kaffir lime needs between 1 and 1 1/2 lbs of actual nitrogen per year. That amounts to between 5 and 5 7 1/2 lbs of 21-0-0 ammonium nitrate. Divide the fertilizer into four equal portions and implement it every four to six weeks beginning in February. Keep the granules away from the back and sprinkle them evenly over the roots. Extend the coverage 2 to 5 feet beyond the dripline and water the top 1 inch of soil.
Recognizing Nitrogen Deficiency
If your lime’s leaves have started yellowing from the lower branches up and out of their hints toward their stems, it’s nitrogen deficient. In a Mediterranean climate, a lack is most likely to happen when winter rains leach sulfur in the soil.