Why we use fertilizers

Richard Bogren, Gill, Daniel J.  |  6/29/2018 2:01:04 PM

By Dan Gill

LSU AgCenter Horticulturist

Gardeners often exhibit a great deal of confusion about what fertilizers are, what they do and why we use them. To put things in perspective: using fertilizers properly is an important part of gardening, but it is almost never a matter of life and death.

First, fertilizers are not “food.” When you apply fertilizer to your plants, you are not “feeding” them. 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. There is no way to literally provide food for your plants — they must make it for themselves. If you need to think of plants consuming something, their food is sunlight.

To be healthy, plants also require 16 elements that are essential to their ability to carry on their life processes. These 16 essential elements are the same for all plants. Completely deprived of any one of them in laboratory experiments, plants become very sick or die.

That, of course, never happens in the garden because at least some of all the essential elements are always present. At times, however, an essential element may not be present in sufficient quantity for a plant to grow and function at its full potential, or it may even become sick. That’s where fertilizers come in. A fertilizer is a substance added to the plant’s environment to provide one or more essential elements.

Of the 16 essential elements, three of them — carbon, hydrogen and oxygen — come from water and carbon dioxide. These elements are always available to plants in abundant quantities under normal conditions, and we don’t have to worry about them. The other 13 are almost always absorbed by plants from the soil through their roots, although some epiphytic and aquatic plants are exceptions.

The 13 essential mineral elements obtained from the soil are divided into three groups, not based on their importance to plants — they are all equally important — but based on the relative amounts of the elements the plants use.

Micronutrients are used in very tiny amounts and include boron, chlorine, copper, iron, manganese, molybdenum and zinc. Micronutrients are also called the trace elements.

The secondary elements calcium, magnesium and sulfur are used in larger amounts, and as a result, deficiencies are more common. Gardeners with acid soils low in calcium and magnesium can add dolomitic lime to their gardens to provide calcium and magnesium and make the soil less acid.

Nitrogen, phosphorus and potassium are the primary elements. These mineral nutrients are used in the largest quantities and are most likely to be in short supply, so we focus on them the most. Nitrogen, phosphorus and potassium are represented by the three numbers on a fertilizer label, which indicates the percentage of those nutrients contained in the product.

Choosing a fertilizer

You do not need a separate fertilizer for every plant you grow. Despite the bewildering array of fertilizer brands and formulations available, it is not terribly complicated to fertilize properly. Remember, all plants use the same essential elements. This isn’t the same as buying separate foods for your dog, cat, fish and parakeet. When selecting a fertilizer, the answers to two questions will help you select the proper product. What response is desired in the plant? What nutrients are already available in sufficient quantities?

As for responses, nitrogen is needed for vigorous growth and dark green, healthy-looking foliage. Lawn fertilizers are commonly high in nitrogen. Compared with the other essential elements, nitrogen is absorbed from the soil in greatest quantities and is the most commonly deficient nutrient. Phosphorous is associated with flowering, the formation of seeds and fruit, and root growth. Potassium contributes to the overall vigor of plants and helps plants deal with the effects of adverse growing conditions.

Looking for a desired response can only be taken so far. Remember, a fertilizer can only help a plant when it provides a nutrient lacking in the soil. You cannot, for instance, “force” a plant to bloom with high levels of phosphorus. Phosphorus fertilizers will only help if the plant is not blooming as a result of a phosphorus deficiency. If it is not blooming because it is too young, it’s the wrong season, it’s not getting enough light or any one of many other reasons, a plant will not be helped by phosphorus fertilizer.

It is even more important to discover which nutrients are lacking in your soil. For a nominal charge you can have your soil tested and get recommendations through the LSU AgCenter Soil Testing Laboratory in Baton Rouge. Your parish AgCenter office and your local garden center can help you find out how to submit a sample. Or you can go online to www.lsuagcenter.com/soiltest.

Once you know what your soil is lacking or has plenty of, it is simply a matter of selecting a fertilizer that contains the appropriate quantities of the needed nutrients. For a collection of container plants, which are often growing in a variety of potting mixes, it’s not practical to test each pot. Generally, select a soluble or slow-release fertilizer that contains similar amounts of nitrogen, phosphorus and potassium, and let the plants take what they want.

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Nitrogen, phosphorus and potassium are represented in this order by the three numbers on a fertilizer label, which indicates the percentage of those nutrients contained in the product. Photo by Rick Bogren/LSU AgCenter

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For a collection of container plants, generally select a soluble or slow-release fertilizer that contains similar amounts of nitrogen, phosphorus and potassium, and let the plants take what they want. Photo by Rick Bogren/LSU AgCenter

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