Why we use fertilizers

Richard Bogren, Gill, Daniel J.  |  5/30/2012 12:47:12 AM

For Release On Or After 06/29/12

By Dan Gill
LSU AgCenter Horticulturist

There is a great deal of confusion among gardeners 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. Plants make their own food through a remarkable process called photosynthesis, which utilizes the energy of the sun to create sugar from carbon dioxide and water. If you need to think of plants consuming something, their food is light.

To be healthy, all plants also require 16 elements that are essential to their ability to carry on their life processes. 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 each essential element is always present. Sometimes, however, an essential element may not be present in sufficient quantities for a plant to grow and function at its full potential – or may even become sick. That’s where fertilizers come in. A fertilizer is a substance added to the plant’s environment that provides one or more essential elements.

Of the 16 essential elements, three of them – carbon, hydrogen and oxygen – are obtained 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 (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 (because they are all equally important) but based on the relative amounts of the elements used by plants. Micronutrients are used in very tiny amounts and include boron, chlorine, copper, iron, manganese, molybdenum and zinc. Micronutrients are also called 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 that are 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. Nitrogen, phosphorus and potassium are represented by the three numbers on a fertilizer’s label, which indicates the percentage of those nutrients contained in the fertilizer.

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 that 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 fertilizer. One – what response do you want from the plant? Two – 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 growth of fruits and roots.

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, phosphorus fertilizer will not help.

It is even more important to discover what 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. Contact your parish AgCenter office to find out how to submit a sample, or 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 nutrients you need.

For a collection of container plants, which are often growing in a variety of 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 use what they need.

Rick Bogren
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