Vegetable Gardening: Selecting a Fertilizer

All plants need nutrients for growth. Gardeners can provide supplemental nutrients to plants by applying fertilizers in the form of composted organic material, synthetic fertilizer, or a specific mineral such as iron. In the vegetable garden, unless very large amounts of organic fertilizers are applied, synthetic fertilizer is usually needed. (Typically, specific mineral applications aren't needed in the edible garden.)

Fertilizer Components

Most fertilizers available for use in the home landscape or garden are blends of several elements mixed together to achieve a specific formulation of plant nutrients. 

Macronutrients

Macronutrients are nutrients required by plants in relatively large amounts for optimum plant growth. The three main nutrients contained in fertilizers are nitrogen (N), phosphorus (P), and potassium (K), represented by three numbers that appear on the bag. A complete fertilizer will contain all three of the major plant nutrients. Other macronutrients include calcium (Ca), magnesium (Mg), and sulfur (S).

Many Florida soils have adequate phosphorus, in which case the fertilizer used should contain only potassium and phosphorus. An easy soil test will tell you if phosphorus is needed.

Micronutrients

Micronutrients are nutrients most plants need in small quantities and are sometimes referred to as trace elements or minor elements. These nutrients—which include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn)—are often available in sufficient quantities in the soil, but are also present in many fertilizers. Micronutrients are also sold as individual nutrients.

Types of Fertilizer

A wide range of fertilizers is available for gardeners. You can select from different combinations of nutrients that come in a variety of forms. The key to selecting a fertilizer is understanding what nutrients your plants need.

Inorganic Fertilizers

Inorganic fertilizers are materials that are mined or synthesized from non-living materials. Many inorganic fertilizers contain nutrients that are immediately available to plants. Others are formulated to allow nutrients to be released over a period of time. If you use an inorganic fertilizer in your landscape, choose one with some or all of the nutrients in slow- or controlled-release form, so that the plants will be able to take up the fertilizer as it is gradually released.

Organic Fertilizers

Organic fertilizers are materials that are derived from plants and animals; one of the most common forms is manure. Organic matter incorporated into the soil before planting will help fertilize your vegetable plants, but you'll need to add additional fertilizer after planting.

The quick availability of nutrients, especially nitrogen, is very important in vegetable growing. Therefore, you may want to supplement any organic fertilizer you apply with some inorganic fertilizer for quick feeding. Many gardeners use a combination of fertilizers and techniques in the garden.

Gardeners who wish to avoid chemical fertilizers can also use fish emulsions or manure teas. Fish emulsion, which is usually high in nitrogen but low in phosphorus, is mixed with water and sprinkled around plants every two to three weeks, or as needed. It typically has a 5-1-1 analysis.

Manure tea is made by seeping manure in a barrel or tub of water. Place several shovelfuls of manure in a porous cloth sack, then soak the sack of manure in the water until the water becomes the color of weak tea.

Read "Organic Matter" to learn more about this subject.

Understanding Fertilizer Contents

When selecting a fertilizer, look at the three numbers on the bag. They will read something like 15-0-15 or 16-2-8. The first number represents the percentage of nitrogen in the bag, the second refers to phosphorus, and the third number is the amount of potassium. For example, a 50-pound bag of 16-2-8 is 16 percent nitrogen (8 pounds total); 2 percent phosphorus (1 pound total); and 8 percent potassium (4 pounds total). The remaining weight is usually comprised of inert ingredients. Nitrogen and phosphorus cause the most problems with regard to water pollution.

Most vegetables will do best with a balanced fertilizer—something like 6-6-6. But there are some exceptions. Leafy vegetables may need only nitrogen to grow large, tender foliage, and root crops such as sweet potatoes, potatoes, beets, carrots, and turnips often benefit from an extra dose of potassium.

Slow- & Controlled-Release Fertilizers

Slow- and controlled-release fertilizers provide nutrients to plant roots over an extended period of time. This allows you to fertilize less frequently—and to prevent nutrients from leaving your landscape and entering waterways, contributing to harmful algal blooms and other water quality problems.

In Florida, any fertilizer that is labeled "slow-release" or "controlled-release" must contain 15 percent or more slow- or controlled-release nitrogen. The label will indicate the percentage of slow- or controlled-release nutrients in the fertilizer, and it's best to choose a fertilizer with higher amounts of slow-release nitrogen.

For the vegetable garden, it's a good idea to add quick-release fertilizer and a slow-release source to the soil at planting so that your plants have nutrients readily available to them when they're young, and then gradually receive the nutrients they need as they grow.

Read "Applying Fertilizer" for more information on this topic.