Fertilizers are a plant's vitamins

I've found that there is a great deal of confusion among gardeners about what fertilizers are, what they do and why we use them.

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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, despite the common use of the terms "plant food" and "feeding," 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.

If you need to think in terms of plants eating something, their energy source is light and they build their bodies from air and water.

In addition to the carbon, hydrogen and oxygen that plants get from air and water, they also require 13 elements that are essential to their ability to carry on their life processes and be healthy.

Rather than thinking of them as food, a better analogy would be to think of them as vitamins. These 13 essential elements are the same for all plants, and they are almost always absorbed by plants from the soil through their roots.

All 13 nutrients are present to some degree in average soil.

Nutrient deficiencies

Sometimes, however, one or more may not be present in sufficient quantities for a plant to grow and function at its full potential.

On occasion, a plant may be so deprived of an essential element that it actually gets sick (as when azaleas or gardenias get iron chlorosis).

That's where fertilizers come in. A fertilizer is a substance added to the plant's environment that provides one or more of the essential elements.

The essential elements obtained from the soil are divided into three groups, based on the relative amounts of the elements used by plants -- but it's important to remember that all the essential elements 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. However, in the alkaline soils south of Lake Pontchartrain, acid-loving plants have difficulty absorbing iron, and iron deficiencies are fairly common in plants like gardenias and blueberries.

The secondary elements -- calcium, magnesium and sulfur -- are used in larger amounts. The soils of the south shore are generally rich in these elements, although sulfur may be added to make the soil more acid. The native soils north of the lake, though, are often low in calcium and magnesium and quite acid. Gardeners on the north shore may need to add dolomitic lime to their gardens to provide calcium and magnesium and make the soil less acid.

Nitrogen, phosphorus and potassium are called the primary elements. They are used in the largest quantities and so are most likely to be in short supply. As a result, we focus on these elements when using fertilizers. Indeed, the three numbers on a fertilizer's label -- called the analysis -- indicate the percentage of nitrogen, phosphate (phosphorus) and potash (potassium) contained in the fertilizer, always in that order.

Choosing a fertilizer

You do not need a different fertilizer for every plant you grow. This isn't the same as buying separate foods for your dog, cat, fish and parakeet.

Despite the bewildering array of fertilizer brands and formulations available, it is not that complicated to choose a fertilizer. Remember, all plants use the same essential elements.

When selecting a fertilizer you need to ask two questions:

What nutrients are lacking, and which are already available in sufficient quantities?

What form of fertilizer do you want to use?

To discover what nutrients are lacking in your soil, you can have your soil tested and get recommendations through the LSU Agricultural Center's Soil Testing Laboratory in Baton Rouge. Contact your local LSU AgCenter Extension office to find out how to submit a sample.

Once you know what your soil is lacking and what it has plenty of, it is simply a matter of checking the analysis on the label and selecting a fertilizer that emphasizes the needed nutrients and has less of the nutrients already present in sufficient amounts.

Adding more of a nutrient when there is already enough present does not benefit the plant. If your soil is very high in phosphorous, for instance, choose a fertilizer that has a relatively low percentage of phosphorous in it, regardless of what you are growing, such as 15-5-10 or 16-4-8 (the actual numbers are not all that important, it is the relative levels).

For a collection of container plants, which are often growing in a variety of mixes, it's not practical to test each pot. Select a soluble or slow-release fertilizer that contains equal amounts of nitrogen, phosphorus and potassium (such as 20-20-20), and let the plants take what they want.

Choose a form

Next, you need to decide what form of fertilizer to use. There are four basic forms: 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 over-apply these fertilizers.

Soluble fertilizers are dissolved in water and applied as a solution. They provide immediately available nutrients to the plant. But because the nutrients leach out rapidly, they must be reapplied frequently, especially to container plants, as they are watered often. Because they are applied in a diluted form, they are not likely to burn plants.

Slow-release fertilizers release their nutrients very gradually over a long period, usually 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. However, they are more expensive. Slow-release fertilizers also 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, fish emulsion, bone meal, 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 typically are not immediately available, so you must put them out earlier in the season than other types. They also release their nutrients over a long time and reduce nutrient runoff.


25_gill_mail.JPGView full sizeThe walking iris, Neomarica gracilis, is not a true iris.

I have a plant in a pot that I am trying to identify; it appears to be an iris. After it bloomed, a growth like a new plant appeared where the bloom was. Please look at the attached photo. I would like to know how to separate and grow this new plant.


This plant is called the walking iris; the Latin name is Neomarica gracilis. it's not a true iris. After it blooms, small plants grow from the flower spike. Moth orchids, spider plants and daylilies also will do this. These small plants can be used to propagate walking irises. Fill a small pot with potting soil. Leaving the small plant attached to the original plant, push it slightly into the soil of the pot, and use a U-shaped piece of wire to anchor it in place. Put the pot next to the original plant. Allow the young plant to root in the pot for about six weeks, and then cut it from the original plant. Walking irises also are planted into the landscape, where they form nice colonies in shady beds. When grown in the garden, the flower stems with proliferations eventually bend over from the weight of the young plant and touch the ground. At that point, the small plant roots and begins to grow independently. As this occurs from year to year, the original planting will spread, and it will appear that the walking iris is "walking" to new areas. This is where this plant gets its common name.


I purchased several gold dust plants, and the card attached said that four to six hours of sun is acceptable. I planted these plants in an area that receives about four hours of sun. The leaves look droopy during the day but look better when the sun goes down. Recently, many of the leaves have turned black, even though I'm watering every other day (they are planted in the ground). Besides the obvious possibility of too much sun, my question is will these plants eventually adapt to the level of sun they receive and thrive? Thanks.

Dan Waters

I disagree with the tag on the plant. Perhaps farther north this plant could tolerate four to six hours of sun, but our sun is more intense down here. This is a shade-loving shrub. Two to four hours of early morning sun in an eastern exposure would likely be OK. If it is late morning, midday or afternoon sun, they are not likely to adapt. So, if this is an eastern exposure with morning sun, the plants may very well adapt. But, it is worrisome that the location is so stressful to the plants that they are wilting every day. The black leaves are likely a disease of aucuba called anthracnose. This disease rarely does much damage to plants growing in shady locations, but can ravage plants growing in stressful locations that are too sunny. So, the appearance of this disease also is an indicator that this location is not suitable.


I planted several eggplants. They have grown nicely and are between 3 and 4 feet tall. They are full of flowers, but they do not produce any fruit.


This is a common problem with the large-fruited eggplants during the hottest part of summer. There's really nothing you can do. I always plant a few of the oriental-type eggplants, like Ichiban and Tycoon, as they set very well in the heat. They are the ones with the long, narrow fruit.


Dan Gill is extension horticulturist with the LSU Ag Center.

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