Choosing a fertilizer for your landscape is not as difficult as it might seem.
First, fertilizers are not food. Plants make their own food through a remarkable process called photosynthesis. Plants do this by capturing the energy of the sun to create sugar from carbon dioxide and water.
Instead of food, you might compare fertilizers to vitamins for a better analogy. Vitamins are not our food, but we need them to be healthy. Humans even take mineral supplements, such as calcium, potassium or iron. But you would never consider a calcium pill food, and neither is the fertilizer you provide your plants.
And like the vitamins and mineral supplements we take, it does not take much fertilizer to satisfy the needs of plants. Gardeners often apply more than necessary.
Plants require 16 elements that are essential to their ability to carry on their life processes, and they are the same for all plants. Completely deprived of any one of the essential elements in laboratory experiments, plants become very sick or die.
Sometimes one or more essential element may not be present in sufficient quantities in soil for a plant to grow and function at its full potential. A fertilizer is something we add to the soil that provides one or more essential elements.
Of the 16 essential elements, carbon, hydrogen and oxygen are obtained from water and carbon dioxide and are always available to plants under normal conditions. The other 13 are almost always absorbed by plants from the soil through their roots.
The elements obtained from the soil are divided into three groups based on the relative amounts of the elements used by plants – but they are all equally important. The micronutrients (or trace elements) include boron, chlorine, copper, iron, manganese, molybdenum and zinc. They are used in very tiny amounts and are rarely deficient.
The secondary nutrients calcium, magnesium and sulfur are used in larger amounts. Acid soils that are low in calcium are treated with lime to provide calcium and make the soil less acid. Dolomitic lime is used when the soil is also low in magnesium.
Nitrogen, phosphorus and potassium are the primary elements. Although they are no more important to plants, these nutrients are absorbed in the largest quantities and so are most likely to be in short supply.
As a result, gardeners focus on them almost exclusively in their use of fertilizers. The three numbers on a fertilizer label, called the analysis, indicate the percentage of nitrogen, phosphate (phosphorus) and potash (potassium) contained in the fertilizer, always in that order.
You do not need a separate fertilizer for every plant you grow. Remember, all plants use the same essential elements. First, determine which nutrients are already available in sufficient quantities and which are lacking.
To discover which nutrients are lacking in your soil, you can have your soil tested through the LSU AgCenter Soil Testing Laboratory in Baton Rouge. Contact your local LSU AgCenter Extension office to find out how to submit a sample. Many garden centers also have information and boxes you can use to send samples to the lab.
Once you know what your soil is lacking, it is simply a matter of selecting a fertilizer that emphasizes the needed nutrients. We generally choose fertilizers that have some nitrogen in them. Look at the three numbers in the analysis of the fertilizer on the container. If a soil was high in phosphorous and low in potassium, you would choose a fertilizer with a lower middle number (representing phosphorous) such as 15-5-10.
For a collection of container plants, which are often growing in a variety of mixes, select a soluble or slow-release fertilizer that contains similar amounts of nitrogen, phosphorus and potassium.
You also need to decide what form of fertilizer to use. There are four basic forms of fertilizers: granular, soluble, slow or controlled release and organic.
Granular fertilizers are inexpensive and easy to use, and so are popular. They provide quick nutrient release and then generally release all their nitrogen in six to eight weeks. It is easy to burn plants if you overapply these fertilizers.
Soluble fertilizers are dissolved in water and applied as a solution and are less likely to burn the plant. They provide immediately available nutrients to the plant, but because the nutrients leach out rapidly, they must be reapplied frequently, especially to container plants because they are watered often.
Slow-release fertilizers release their nutrients gradually over a long period, generally a number of months. They are labor saving and handy to use because one application at the beginning of a season provides regular nutrient release for the growing season. But they are more expensive.
Organic fertilizers are derived from natural sources, such as finely ground minerals or animal byproducts. Blood meal, bone meal, sulfur, green sand and manure are commonly used organic fertilizers. Manufacturers are even blending materials to create more balanced organic formulations. The nutrients in organic fertilizers are generally not immediately available, so you must put them out earlier in the season that other types.
Dan Gill is a horticulturist with the LSU AgCenter. He can be reached at DGill@agcenter.lsu.edu