University of FloridaSolutions for Your Life

Download PDF
Publication #SL266

Soils & Fertilizers for Master Gardeners: The Florida Gardener's Guide to Landscape Fertilizers1

Amy L. Shober2

This article is part of a series entitled Soils and Fertilizers for Master Gardeners. The rest of the series can be found at http://edis.ifas.ufl.edu/topic_series_soils_and_fertilizers_for_master_gardeners. A glossary can also be found at http://edis.ifas.ufl.edu/MG457.

Introduction

While the soil generally provides most of the nutrients needed for optimum plant growth, there are times when fertilizers are needed. Fertilizers contain one or more essential plant nutrients and can be applied to landscapes to improve plant growth and quality or to correct a nutrient deficiency. There are many fertilizers available to consumers at local lawn and garden centers. With so many choices, it's easy to get confused. This publication provides information about fertilizers and associated fertilizer terminology to help consumers make educated decisions when purchasing fertilizers and using them in the home landscape or garden.

What Fertilizers are Available?

There is no easy answer to this question because there are many different fertilizers available to the homeowner or gardener for use on landscape plants, lawns, and vegetables. Fertilizer selection will vary depending on location and store. For the most part, garden centers sell fertilizer blends, where several nutrient sources are mixed together to create a fertilizer for a specific purpose. The materials used to create the fertilizer blend are listed on the fertilizer label (Figure 1) and are identified by the words "derived from". Blends are created to achieve a specific fertilizer grade. The fertilizer grade is the percent (by weight) of nutrients in the fertilizer. For example, a 10-5-5 fertilizer has 10% N, 5% P3, and 5% K. In a 50 lb bag of 10-5-5 fertilizer, there are 5 lbs N, 2.5 lbs P, and 2.5 lbs K.

Figure 1. 

All fertilizers sold in Florida must carry a label like this generic one.


[Click thumbnail to enlarge.]

In addition, most of the products sold for use on ornamentals are complete fertilizers. In order for a fertilizer to be classified as "complete", it must contain nitrogen (N), phosphorus (P), and potassium (K). For example, an 8-2-12 blend is considered a complete fertilizer. Complete fertilizers can also contain secondary plant nutrients and micronutrients. Secondary plant nutrients include calcium (Ca), magnesium (Mg), and sulfur (S). The micronutrients, manganese (Mn), boron (B), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo) and chloride (Cl), are required by plants in smaller amounts than the primary and secondary plant nutrients. Some fertilizer materials contain one or more of these secondary plant nutrients, which will be listed on the fertilizer label.

Fertilizers available to consumers for use in the landscape will also have variable properties depending on brand and formulation. Many fertilizer materials are inorganic or synthetic. Examples of inorganic nutrient sources that may be used in blends include ammonium nitrate, ammonium phosphate, and potassium chloride. Other fertilizer materials are organic, including animal manures, composted materials, and plant or animal residues.

In addition, fertilizer materials may be water-soluble (soluble or quick-release), slow-release, or controlled-release. Water-soluble fertilizer material dissolves immediately in water. Once dissolved, nutrients in the fertilizer are available for uptake by plant roots. Most of the inorganic fertilizer materials are soluble, unless they have been formulated to be slow- or controlled-release. Slow-release fertilizers will release nutrients slowly for a period of time ranging from weeks to months. Examples of synthetic slow-release fertilizers are sulfur-coated urea and Nitroform®. Organic fertilizers also contain slow-release nutrients; however, not all nutrients in organic fertilizers are slow-release. Soil microbes must convert these slow-release fertilizer nutrients to forms that can be used by plants. In contrast, controlled-release fertilizer materials are soluble inorganic fertilizers that have been modified to allow nutrients to be released over a specific time period. In a controlled-release fertilizer, the water-soluble fertilizer materials are encapsulated in a plastic or polymer coating. Nutrients are released with time; the amount of time it takes for nutrients to be fully released depends on the nature of the coating, contact with moisture, and temperature. Typically, nutrient release from controlled-release fertilizers increases with increasing temperature and soil moisture. Examples of controlled release materials are Osmocote®, Nutricote®, and Polyon®.

Reading the Fertilizer Label

Now that you know the types of materials available to you, let's discuss how to read the label. You should always read the label thoroughly so you can make the most informed decision possible. This document will go through two examples of materials that can be purchased at a typical "big-box" chain store.

Example 1: All Purpose Fertilizer

The first example is an "all purpose" ornamental fertilizer listed for use on all flowers, trees, shrubs, vegetables, and houseplants (Figure 2). The label on the box lists that is it a water-soluble fertilizer, so immediately we know that the material will dissolve in water and all nutrients will be in a plant available form. The grade is 24-8-16, which means that, by weight, it contains 24% N, 8% P, and 16% K. It has a fertilizer ratio of 3 parts N to 1 part P to 2 parts K. (To determine the fertilizer ratio from the fertilizer grade, divide all numbers on the grade by the lowest number. For example, for a 24-8-16 fertilizer, divide all numbers by 8 to get a 3-1-2 grade.)

Figure 2. 

Fertilizer label for an All-Purpose ornamental fertilizer.


[Click thumbnail to enlarge.]

According to the label, 3.5% of the total N in the box is ammoniacal N (ammonium) and the remaining 20.5% is urea (3.5% + 20.5% = 24% total N). In the "derived from" statement, ammonium sulfate, urea, and urea phosphate are the sources of N. (Note: Fertilizers containing ammoniacal N or urea should be watered in to prevent N loss to the atmosphere). The sources of P are potassium phosphate and urea phosphate. The sources of K2O are potassium phosphate and potassium chloride.

This fertilizer also contains the following plant micronutrients: 0.02% boron (B) as boric acid, 0.07% copper (Cu) as copper sulfate, 0.15% iron (Fe) as iron EDTA, 0.05% manganese (Mn) as manganese EDTA, 0.0005% molybdenum (Mo) as sodium molybdate, and 0.06% zinc (Zn) as zinc sulfate. The term EDTA in iron and managenese EDTA indicates that the nutrient is in chelated form. Chelated forms of nutrients exist as a complex molecule containing carbon and hydrogen that have a prolonged period of plant availability. The other elements are listed as water-soluble.

Example 2: Controlled-release Shrub Fertilizer

Our second example fertilizer is a complete shrub fertilizer with minors (Figure 3). The label indicates that the material is polymer resin coated, indicating that it is a controlled-release material. The grade is 18-6-8, which means that, by weight, it contains 18% N, 6% P, and 8% K. It also has a fertilizer ratio of 3 parts N to 1 part P to 1.3 parts K.

Figure 3. 

Fertilizer label for a controlled-release shrub fertilizer.


[Click thumbnail to enlarge.]

According to the label, 8.6% of the total N is ammoniacal N (ammonium) and the remaining 9.4% is nitrate N, both of which are water soluble forms of N (8.6% + 9.4% = 18% total N). In the "derived from" statement, ammonium nitrate, ammonium phosphate, and potassium nitrate are the sources of N. However, the normal quick-release action of water-soluble N is tempered by the fertilizer coating in this case. Note the statement (below the "derived from" statement) indicating the percentage of nutrients that are coated, and thus in controlled-release form. In this fertilizer, all the nutrients are controlled-release because the percentages exactly match those on the upper part of the label. The sources of P2O5 are ammonium phosphate and calcium phosphate. The source of K2O is potassium nitrate.

This fertilizer also contains the following plant nutrients: 1.2% magnesium as magnesium sulfate, 0.02% boron (B) as sodium borate, 0.05% copper (Cu) as copper sulfate, 0.20% iron (Fe) as ferrous ethylenediamine tetraacetate (EDTA), 0.06% manganese (Mn) as manganese sulfate, and 0.02% molybdenum (Mo) as sodium molybdate. In this case, only the Fe is in a chelated form, the rest are water soluble (but are all coated).

Choosing a Fertilizer

Before you decide to use a fertilizer, you should have your soil tested. Most county Extension offices offer a pH test, while the University of Florida Extension Soil Test Laboratory (http://soilslab.ifas.ufl.edu) offers tests for pH, lime requirement, P, K, and Mg. (For more information about soil testing and how to take a soil sample, see Landscape and Vegetable Garden Test Information Sheet, http://edis.ifas.ufl.edu/SS187). The soil pH test is sufficient if you plan to use a complete fertilizer. This is because complete fertilizers are usually applied to meet the N requirements of plants and N rate recommendations are based on research on plant response to N fertilizer rather than the amount of N measured in a soil sample. However, the results of a full soil test will help you decide what level of P and K, if any, will benefit your plants.

An example of the soil test results for woody ornamentals growing in the landscape is shown in Figure 4. (Before selecting a fertilizer, it is important that you read the entire soil test report, including the footnotes. The footnotes will provide important information about application methods and rates.) This sample soil test report indicates that plants would respond to applications of P and K. However, more K is needed than P.

Figure 4. 

An example fertilizer test report for woody trees grown in the landscape.


[Click thumbnail to enlarge.]

In most cases, the complete fertilizers available to consumers for use on landscape plants will not be available in the exact ratio needed to meet the N, P, and K requirements of landscape plants. When most available complete landscape fertilizers are applied to meet the N recommendations based on the soil test, P and K will be over-applied. The concern here is that if applied excessively to the point of leaching or runoff, P can contribute to water quality problems. Please note that fertilizers labeled for use on turf will have a different ratio of N, P, and K than the fertilizers labeled for ornamental plants. This is because the Urban Turf Fertilizer Rule limits the amount of N and P that can be applied to turf. For more information, see the 2007 fact sheet on this topic published by the Florida Department of Agriculture and Consumer Services: http://www.flaes.org/pdf/Urban_turf_fact_sheet.pdf

Final Thoughts on Landscape Fertilizers

Knowing fertilizer terminology and understanding the fertilizer label will help you make informed choices if you decide to use fertilizer in your home landscape or vegetable garden. Since improper fertilizer use can contribute to water quality problems, always follow best management practices (BMPs) described in UF-IFAS lawn and landscape publications. Have your soil tested to determine the need for fertilizers and always follow University of Florida fertilizer recommendations. For more information about fertilizer recommendations for Florida lawns, landscape plants and palms, see these EDIS publications: Fertilization and Irrigation Needs for Florida Lawns and Landscapes (http://edis.ifas.ufl.edu/EP110), and Fertilization of Field-grown and Landscape Palms in Florida (http://edis.ifas.ufl.edu/EP261).

Footnotes

1.

This document is SL266, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date July 2008. Revised January 2009. Reviewed February 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Amy L. Shober, assistant professor, Gulf Coast REC, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611

3.

Phosphorus and potassium in fertilizers are always listed on the fertilizer label in their oxide forms (P2O5 and K2O, respectively). For simplicity, P2O5 and K2O are listed as P and K in this publication; however, the numbers in the fertilizer grade actually represent the percentage by weight of these nutrients in their oxide forms.

All chemicals should be used in accordance with directions on the manufacturer's label.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.