Daniel Gill, Merrill, Thomas A.
By Dan Gill
LSU AgCenter Horticulturist
Choosing a fertilizer for your landscape is not as difficult as it might seem – particularly if you look at it the right way.
First, you need to realize that fertilizers are not food. Plants make their own food through a remarkable process called photosynthesis, which uses the energy of the sun to create sugar from carbon dioxide and water.
As for what fertilizers are, you might compare fertilizers to vitamins for a better analogy. For example, if you look at how vitamins serve humans, vitamins are not our food, but we need them to be healthy.
When it comes to plants, they require 16 elements that are essential to their ability to carry on their life processes, and those elements are the same for all plants. If plants are completely deprived of any one of these essential elements, laboratory experiments show 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 a substance added 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. That means those three elements always are available to plants under normal conditions. The other 13 are almost always absorbed from the soil through the plants’ roots.
The elements obtained from the soil are divided into three groups based on the relative amounts of the elements used by plants – although they all are 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. Acidic soils that are low in calcium are treated with lime to provide calcium and make the soil less acidic (or dolomitic lime is used when the soil also is low in magnesium).
Nitrogen, phosphorus and potassium are the primary elements. Although they are no more important to plants than the others, these nutrients are used in the largest quantities and therefore 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’s label indicate the percentage of nitrogen, phosphorus and potassium contained in the fertilizer – always in that order.
Choosing a Fertilizer
You do not need a separate fertilizer for every plant you grow. Remember, all plants use the same essential elements.
First, determine what nutrients already are available in sufficient quantities and which ones are lacking. To discover what nutrients are lacking in your soil, you can have your soil tested and get recommendations through the LSU AgCenter’s Soil Testing Laboratory in Baton Rouge. Contact your local LSU AgCenter Extension Office to find out how to submit a sample. Visit www.lsuagcenter.com to find a listing of offices or check your local telephone directory.
Once you know what your soil is lacking, it is simply a matter of selecting a fertilizer that emphasizes the needed nutrients. Look at the three numbers in the analysis of the fertilizer on the container and pick one that emphasizes the nutrients you need.
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, so they are popular. They provide quick nutrient release and generally release all their nitrogen in six to eight weeks. But it is easy to burn plants if you over apply these fertilizers.
Soluble fertilizers are dissolved in water and applied as a solution. They are less likely to burn plants. They provide immediately available nutrients to the plant, but since the nutrients leach out rapidly, they must be reapplied frequently –especially to container plants since these plants are watered often.
Slow-release fertilizers release their nutrients very gradually over a long period – generally a number of months. They are labor-saving and handy to use, since one application at the beginning of a season provides regular nutrient release for the growing season. But slow-release fertilizers are more expensive. On the other hand, slow-release fertilizers are not likely to burn plants, and they help reduce nutrient runoff into surface water.
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. Manufactures 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 than other types of fertilizers. But they also have the advantage that they release their nutrients over a long time and reduce nutrient runoff.
Regarding plants where soil testing isn’t particularly practical – such as 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.
Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.