Arbor Culture 8/24/14: Basics about fertilizing trees — part 1

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Jane Weber’s article last week on planting shade trees is a perfect segue to this month’s article regarding proper fertilization.

While trees produce their own food (starch and sugars) through the miracle of photosynthesis, plants and trees still need supplemental nutrients found in the soil. These various nutrients are dissolved in solution between soil pore spaces and are absorbed by roots. Often, certain minerals needed for growth or various plant functions are deficient in the soil; such deficiencies manifest themselves in the tree through stunted growth, chlorosis (yellowing of leaves), and deformed new growth.

However, before a homeowner fertilizes his or her trees, he/she must understand how to read a fertilizer label. The most important and prominent part of a fertilizer label are the three numbers known as the analysis. These three numbers are indicated as 6-6-6, 12-4-8, etc., and indicate the percentage of nitrogen (N), phosphorous (P), and potassium (K), respectively.

In other words, a 16-4-8 analysis indicates that that particular fertilizer contains 12 percent nitrogen, 4 percent phosphorous, and 8 percent potassium. This is important because one needs to know how many pounds of a particular nutrient one is applying. Therefore, a 50 pound bag would contain 16 percent nitrogen — or 8 pounds if the entire bag of fertilizer were applied. 

Another way of looking at the label is the ratio of the analysis. In other words, the 12-4-8 fertilizer mentioned above has a ratio of N-P-K of 3-1-2. Most trees in Florida can be fertilized using a fertilizer with a 3-1-2 ratio. This is perhaps more important than the actual numbers or percentages of each nutrient. Most Florida soils, at least in this area, contain high amounts of phosphorous (P), thus the reason for the low number for P in the ratio. 

Most tree roots are found within 12 inches of the soil. Therefore, a surface application to the lawn should be sufficient to allow the nutrients to reach tree roots. However, tree roots often extend to a distance of two to three times the distance of the drip line — the tree’s canopy diameter. In other words, if you have an oak tree with a canopy spread on one side of 15 feet, the tree roots may grow 30 to 45 feet beyond the trunk of the tree.

When applying a 3-1-2 fertilizer, the University of Florida recommends that no more than one pound of nitrogen be applied per 1,000 square feet. Because tree roots extend well beyond the canopy, you should apply fertilizer to a distance of one and one-half to two times the drip line. 

To determine the amount of fertilizer to apply to 1,000 square feet, simply divide the percentage of nitrogen of the fertilizer into 100. Therefore, for the 12-4-8 fertilizer above, simply divide 12 into 100 to get 8.3 pounds of fertilizer per 1,000 square feet. A quick measurement of the tree’s canopy can easily determine how many square feet is to be fertilized.

For example, a tree with an average canopy spread (outside to outside) of 30 feet results in a canopy coverage of approximately 700 square feet (15 x 15 x 3.1415 707). Thus, expanding the fertilizer coverage to twice the canopy spread would equal 2,800 square feet (30 x 30 x 3.1415 2,827). Therefore, to apply 8.3 pounds of fertilizer per thousand square feet would require approximately 23 pounds of fertilizer (8.3 x 2.8). So if you bought a 50 pound bag of fertilizer, you could apply half the bag. 

Most shade trees in Florida should be fertilized using no more than four pounds of nitrogen per 1,000 square feet per year and no more than one pound per 1,000 square feet should be applied at one time. Thus, a maximum of four applications per year will achieve the total recommended amount of fertilizer.

Palms require a somewhat different ratio or analysis. A 10-5-10 or 20-5-20 analysis or a 2-1-2 or 4-1-4 ratio is recommended for palms. 

Generally, younger trees are fertilized to help with growth and maturity. However, newly planted trees should not be fertilized immediately after planting. Wait 30 to 60 days before applying fertilizer. Watering is more important during initial establishment. Mature trees generally don’t need much fertilizer as they are well established and usually get enough fertilizer from lawn applications.

However, if you suspect a nutrient deficiency, have the soil tested and fertilize accordingly. Most County Extension offices provide soil testing services. There are also several private labs, which, for a very modest fee, also provide soil testing and analysis. 

Next month’s article, Part 2, will cover more fertilizer information to include micronutrients and nutrient deficiencies, different forms of nitrogen in the fertilizer, organic versus inorganic fertilizers, and slow versus quick release fertilizers.

Eric H. Hoyer is a certified arborist, a certified forester, a registered consulting arborist and a qualified tree risk assessor with Natural Resource Planning Services Inc. He can be contacted at erich@nrpsforesters.com.