The Do’s and Don’ts of Fertilizing Outdoor Plants

We get more questions about fertilizing outdoor plants than we do any other topic. Here are some tips on how and when to use fertilizer in your landscape.

Why Fertilize?

Plants need 17 elements to use when they are growing. Carbon, oxygen, and hydrogen are supplied by air and water. Fourteen elements are supplied by the soil the plants are growing in.

Plants need large quantities of nitrogen, phosphorus, and potassium, so they are considered primary macronutrients. Plants need sulfur, calcium, and magnesium in smaller amounts, so they are called secondary macronutrients. The trace elements plants need are boron, copper, iron, manganese, molybdenum, nickel, zinc, and chlorine.

Plants use these elements as they grow, produce flowers, and make seeds. Vegetables and fruits consume the most nutrients, in general, so they may require fertilizing many times during their short life. Turfgrass also uses lots of nutrients from the soil. It is fertilized multiple times a year.

Ornamental plants, including perennials, trees, and shrubs, use less elements each season but will eventually exhaust the elements in the soil. They are fertilized fewer times during the year.

Soil Tests

Different soils have the elements plants need in different proportions. The only way to know what is in the soil is to do a soil test. Ideally, you should do a soil test before you plant anything. Each state in the United States has a soil lab that will do the soil test for you. Canada has a similar system. These tests are not very expensive. You can find the instructions and forms for them on the soil lab's webpage.

Your soil test results will tell you how much of what a plant needs is already in the soil. If you tell the soil lab what you intend to grow in that sample area, you will also receive recommendations for how much of each element you need to add before you plant anything.

The soil test includes the pH of your soil. Most plants like a slightly acidic or neutral soil. Blueberries and strawberries prefer a more acidic soil. Blackberries, however, prefer a more neutral soil. Soil pH affects how much of an element a plant can absorb. For example, acidic soils predispose plants to get iron chlorosis. Even if there is plenty of iron in the soil, the plant can't access it.

Soil tests should be done once a year in your vegetable garden. You only need to test the soil in your lawn and landscape beds every three years.

Picking A Fertilizer

All fertilizers have three numbers on the label. These are, in order, the percent nitrogen, phosphorus, and potassium the fertilizer has. You use fertilizer with different ratios of these three minerals for different plants.

For example,10-10-10 has ten percent nitrogen, ten percent phosphorus, and ten percent potassium. The rest is some carrier to make the fertilizer easier to spread and some preservatives to keep the fertilizer from spoiling. Some fertilizer has secondary macronutrients and trace elements in them. In addition, most fertilizer has some added material to make the fertilizer more effective. The exact composition is a trade secret held by each manufacturer.

Organic versus Manufactured Fertilizers

There are two types of fertilizer. One is organic, which is made from organisms that were once alive. This fertilizer also contains carbon. Inorganic fertilizers were made from chemicals found in petroleum products. They do not contain carbon. Some people think organic fertilizers are better, while others prefer synthetic fertilizers. Organic fertilizers must be broken down into elements by microorganisms before plants can use them. Synthetic fertilizers are absorbed by the plants directly, so they work faster.

When To Fertilize

Trees, shrubs, and perennials are usually fertilized in early spring when they start to grow. They are usually fertilized in late summer or early fall to provide nutrients over the winter and early spring. Do not fertilize in winter when the plant is dormant. If you live in a hot climate, do not fertilize during the summer dormancy, either.

How Much Fertilizer?

Soil test results will tell you how many pounds of a nutrient you need to apply. This is different than the number of pounds of fertilizer you need to apply. How much of a particular fertilizer you need to apply depends on the ratio on the fertilizer label. For example, if you need a pound of nitrogen for your garden row, you would have to use ten pounds of 10-10-10 to get that much nitrogen on your garden row.

Organic fertilizers tend to have lower ratios than inorganic fertilizers. Inorganic fertilizers are usually cheaper than organic fertilizers.

How To Apply Fertilizer

Most fertilizers come in either a liquid, a water-soluble form, usually grains that dissolve in water, or a solid granular fertilizer. All fertilizers have application instructions on them. It is important not to apply more fertilizer than the label instructions tell you to because it can burn or kill your plants. Excess fertilizer may also be washed into storm drains, leading to algae blooms in the creeks and lakes the storm drain empties into. These algae blooms can cause large fish kills.

Liquid fertilizers are often sold as ready to use mixes you attach to your garden hose and spray. Move the spray in a sweeping motion back and forth as you walk. It is important to keep the spray moving so no one plant gets too much fertilizer. Try not to get the fertilizer on leaves, stems, and trunks so it doesn't burn the area. Water soluble fertilizers are mixed with water and applied the same way.

Solid fertilizer is usually spread as a certain number of pounds per square inch of landscape. Tree and shrub fertilizer will tell you how much to use based on how big your tree is. For example, if you have a big chestnut oak tree, you might spread 6 pounds of a given fertilizer around it. Fertilize from the drip line of the tree to three feet from the trunk, creating a donut shaped application with the tree in the center of the hole.

Many fertilizers require you to water the plants the night before application. Most solid fertilizers require you to water after their application to drive the nutrients into the soil where the plants can reach them.

We Can Help

At Garden Plants Nursery, our staff can help you choose the best fertilizers for your garden. Call 931.692.7325 to speak to a garden advisor.