Slow-release fertilizers can provide season-long nutrients to the soil. (Photo by Dan Gill, LSU AgCenter)

Fertilizers come in a variety of analyses. Choose the one that’s right for the plants and time of year. (Photo by Dan Gill, LSU AgCenter)

For Release On 09/11/15

By Dan Gill
LSU AgCenter Horticulturist

One of the most common questions I receive from gardeners is, “What kind of fertilizer should I use?” These gardeners generally assume there must be an easy answer for this if they just tell me what kind of plant they’re growing. There is a common misconception that the type of fertilizer you choose to use is based on the plant. In other words, each type of plant needs its own specially formulated fertilizer. This just isn’t the case.

Surprised? Well, to see why this is wrong, it’s important to look at what fertilizers are and why we use them. To be healthy, plants need certain mineral elements they absorb from the soil. These “essential elements” include nitrogen, phosphorous, potassium, calcium, magnesium, sulfur, iron, boron, chlorine, molybdenum, zinc, copper and manganese. All of the plants you grow use the same essential elements. If any of these are in short supply, the health or performance of the plant will be affected to some degree – the more serious the deficiency, the more obvious the symptoms.

But these essential elements are not food. They are not what plants “eat.” And they are not at all equivalent to the food we feed ourselves or our pets. A better analogy would be to compare them to vitamins. I don’t think anyone would consider that little pill you take in the morning your food supply for the day. Plants make their own food from air and water through photosynthesis.

A fertilizer is something we add to the plants’ environment that provides one or more essential elements. The role of fertilizers is to supplement the mineral nutrients that are already present and available in the soil. If a plant is already getting enough of an essential element from its environment, adding more of that nutrient will not benefit the plant in the least. It is wasteful and may contribute to environmental problems such as non-point source pollution. A fertilizer will only help a plant if it provides a nutrient that is in such short supply that the health or performance of a plant is affected.

Because all the plants you grow use the same essential elements, the idea that each type of plant, such as roses, tomatoes, lawn grasses, flowers, fruit trees, requires a separate and different fertilizer is simply not the case. Remember, fertilizers are not plant food. Fertilizing plants in your landscape is not all the same as feeding your pet.

That said, the type of plant can influence which fertilizer we purchase and how we use it. A fertilizer for acid-loving plants, for instance, would be appropriate for plants like azaleas, gardenias and blueberries that like acid soil. And some plants require higher nutrient levels in the soil to do their best and are fertilized as higher rates than those that require lower nutrient levels.

The sole purpose of applying fertilizer is to supplement nutrients in the soil so the plants are healthy and perform up to their full potential. While the fertilizer material we use can be influenced by the plants we are growing, the choice is determined primarily by nutrient levels in the soil. Choosing a fertilizer for your landscape does not depend so much on what you are growing – it is determined primarily by which nutrients are in short supply in the soil and need to be supplemented and which don’t.

You cannot simply look at the soil and know what the nutrient levels are. The key to proper fertilizing – whether you use commercial or organic fertilizers – is a soil test. You can get your soil tested through the LSU AgCenter Soil Testing and Plant Analysis Laboratory (www.lsuagcenter.com/stpal) in Baton Rouge. A routine test costs $10.

The test results, which you will generally receive in about three weeks, will tell you the texture of your soil. You will also learn the soil pH, which reveals how acidic or alkaline it is. A pH of 7 is neutral; lower numbers indicate an acid soil condition while higher numbers mean the soil is alkaline. Generally, a pH between 5.5 and 7.5 is acceptable for most plants. If necessary, the pH of the soil can be adjusted higher by adding lime or lowered with sulfur.

The soil fertility is indicated in the test results by the levels of phosphorus, potassium, calcium, magnesium, copper, sulfur and zinc. The levels are shown in parts per million and are interpreted for you as very low, low, medium, high or very high. Ideally, the levels of essential elements should be medium to very high. Fertilizer recommendations provided with the test results are based on these levels and the type of plants you indicated you are growing or intend to grow where the soil sample was taken.

A soil test will only resolve issues that relate to soil characteristics such as fertility, pH or sodium levels. If you suspect that these soil characteristics are causing the problems, a soil test will help you determine if, in fact, they are. Soil tests are not useful if the plants are having problems with insects, diseases or cultural practices or for determining pesticide or chemical residues.

Soil testing can be done any time of the year, but fall is an excellent time. When you get your tests back and see the nutrient levels of your soil, you will be better informed when the spring fertilizer season arrives next year. Getting your soil tested helps you choose fertilizers that emphasize the nutrients in short supply and deemphasize the nutrients already at appropriate levels.