How much fertilizer does my satsuma tree need?
It has recently come to my attention that many gardeners in the area are not sure when and how much fertilizer to apply to their Satsuma trees. The best tool for fertilization is to send in a soil sample to the UF/IFAS Soil Testing Lab. Soil test results provide specific recommendations for both lime and fertilizer applications. While you are at it, why not send in separate soil samples for your lawn and vegetable garden? You can pick up sample bags and forms for submission at your local extension office, or you can use a Landscape and Vegetable Garden Test Form to mail with samples to the soil testing lab in Gainesville. As you fill out the form, you have to identify the crop to get specific fertilizer and lime recommendations. Use code 62 for citrus trees, and 90 for vegetable gardens. Lawn codes depend on the type of grass in your lawn, so the code for centipedegrass is 75.
The problem many gardeners face is that winter is the best time to send in soil samples, yet spring is the time when fertilizer needs to be applied. So what are the general recommendations for fertilizing a satsuma tree? I found a nice fact sheet recently that was created a number of years ago by the Auburn Horticulture Department: Satsuma Basic Production Guide. In this guide, they provided basic fertilizer recommendations that I have modified slightly, to make the easy to read chart below. While soil testing is the best option, this chart provides a general guide for how much fertilizer to apply to each satsuma tree in your yard. It makes good sense to fertilize based on the age and size of the tree. A small, two-year-old tree only needs 0.4 lbs. of nitrogen (N) each year, while a mature tree requires 1.5 lbs. N annually. It is not recommended, however, to apply all of the fertilizer in the spring. Instead, split this total into three applications, so the tree has access to nutrients throughout the growing season. When you buy fertilizer, it is not pure nitrogen (N), phosphorous (P), and potassium (K). Every bag of fertilizer has a label that tells the percentage of N-P-K in the bag , such as 10-10-10. Triple 10 is a common fertilizer used for trees, and the numbers represent 10% of each N-P-K. You would not apply the same amount of 8-8-8 or 13-13-13, so this chart gives the single application rate for each.
What if you want to use an organic fertilizer, or other type of fertilizer? How much should you use? You have to read the label to find out the nutrient concentrations (%N-P-K). Then divide the recommended N lbs. by the percentage in the fertilizer. For example an organic fruit tree fertilizer, available at local garden stores, is 4% N – 5% P – 4% K is sold in 11-pound bags. To meet the nitrogen requirements of a mature tree would require 37.5 lbs. per year, or 12.5 pounds when split into three applications (1.5 lbs.N ÷ 0.04 ÷ 3 applications =12.5). If you just apply just one 11-lb. bag of this product in the spring around your tree, the tree is not getting enough nutrients for a single application, much less for the entire growing season.
When to fertilize?
Another common question is when should you fertilize your satsuma tree? You want to avoid fertilizing from September through mid-February, because you don’t want to encourage new growth that could be injured by a freeze. You want to fertilize just ahead of the greatest need of the plant. According to Auburn’s guide for Satsumas, the best times are:
- Bud-break in the Spring (March)
- Fruit Swell (May)
- >1″ diameter fruit (June)
Once you have purchased fertilizer, and calculated how much your satsuma tree needs, it is time to spread it around the tree. Spread the desired fertilizer evenly around the tree in a diamteter 1.5 the size the width of the dripline (edge of canopy). There is a temptation, especially on very young trees, to sprinkle the fertilizer around the trunk of the tree. The taproot of the tree is right under the trunk and is primarily an anchor to hold the tree up, but the lateral roots with feeder roots that branch out from them take up nutrients needed by the tree for growth and fruit production.