Fertilizer Takes a Little Math
- But Not Much
By Joe Wolf
Polk County Master Gardener
The question of how much fertilizer to use in
your yard and on other plants requires a little
math but not much.
Mature trees and shrubs do not need to be
fertilized except in rare circumstances. They have
extensive root systems so if other plants or the
lawn around them are fertilized they will get
enough nutrients. When applying fertilizer the
general rule is to apply one pound of nitrogen
spread evenly over 1,000 square feet of lawn (or
flower bed) per application with two to three
applications per year.
Fertilizer recommendations are given in pounds
of nitrogen not pounds of fertilizer. Use a
fertilizer with a high percentage of slow-release or controlled-release nutrients. These
fertilizers are more expensive but provide nutrients to the plants for months not just a
few weeks. Fertilizer must be spread evenly under the canopy of a tree or shrub. Never
use fertilizer spikes or spread the fertilizer in a ring in this part of Florida.
The American tradition of believing that if one pound is good then two is better just
does not work with fertilizer. One pound of slow release nitrogen will allow the plants
to take up all of those nutrients as needed. Any more will be wasted and drain into the
nearest lake.
How much fertilizer to use to put that one pound of nitrogen on your lawn or in your
flower bed takes a little math, but not much. The first large number on a bag of fertilizer
is the percentage of nitrogen in the bag. Take that number and divide it into 100. For
example: if you have a bag of 15-2-13 divide 100 by 15 (100/15 = 6.7). For this bag of
fertilizer, spread 6.7 pounds of it over 1000 square feet. If the bag is 6-0-6 then divide
100 by 6 and get 16.7 pounds (100/6=16.7). So spread 16.7 pounds of the 6-0-6 over 1000
square feet of your lawn or garden. It you have a 2,500 square foot lawn spread 41.75
pounds on the lawn (16.7x2500/1000= 41.75).
Our two most common plants that don’t follow the above rule are palms and citrus.
Palms require a special slow-release fertilizer - an 8-2-12-4 fertilizer with micronutrients,
which has 8% nitrogen, 2% phosphorous, 12% potassium and 4% magnesium. Let your
fingers do the walking when you are looking for this harder-to-find fertilizer. Spread
1½ pounds for every 100 square feet of canopy. For much more detail on palm
fertilizers see the University of Florida/IFAS publication ENH1009
http://edis.ifas.ufl.edu/ep261 or ENH962 http://edis.ifas.ufl.edu/ep221.
Mature fruit producing citrus (five years old and more) require 1¼ pounds of nitrogen
per year spread under the canopy separated into three applications: one application in
the fall or winter, one in early summer and one in late summer. Apply 2/5 pound of
nitrogen (1.25/3=.42 or 2/5) at each application. A one-year-old tree will want just a small
handful of fertilizer in each of six applications. A two-year-old tree will get about twice
as much in five applications. A three-year-old tree can take about 1/5 pound in each of
four applications, and when the tree gets to be four it will get about 1/3 pound in each of
three applications. More information on citrus fertilization can be found in the UF/IFAS
publication.