Richard Bogren, Gill, Daniel J. | 9/20/2018 8:13:56 PM
By Dan Gill
LSU AgCenter Horticulturist
(09/21/18) A common misconception is that the type of fertilizer you choose to use is based strictly on the type of plant you are growing. In other words, each type of plant has its own specially formulated fertilizer, and that’s what should be used. However, this is not the case.
Surprised? Well, to see why this is wrong it’s important to look at what fertilizers are and why we use them. Plants need certain mineral elements they absorb from the soil to be healthy. These essential elements include nitrogen, phosphorous, potassium, calcium, magnesium, sulfur, iron, boron, chlorine, molybdenum, zinc, copper and manganese.
All 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.
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. Plants make their own food from air and water through photosynthesis. It is impossible for you to literally feed the plants in your landscape.
A better analogy would be to compare plant fertilizers to mineral supplements that humans take, such as calcium, iron or potassium. I don’t think anyone would consider that pill you take in the morning your food supply for the day. But you need the minerals to be healthy.
A fertilizer is simply something we add to a plant’s environment that provides one or more essential element. The role of fertilizers is to supplement the mineral nutrients that are already present and available to a plant. 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. 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 and fruit trees, require a separate and different fertilizer is simply not accurate. The idea that you need different fertilizers for your plants the way you need different foods for your pets is simply wrong.
That being said, the type of plant can influence what fertilizer we purchase and how we use it. A fertilizer for acid-loving plants, for instance, would be appropriate for plants that like acid soil, like azaleas, gardenias and blueberries. Also, some plants require higher nutrient levels to do their best and are fertilized at higher rates or more frequently than those that require lower nutrient levels.
While the fertilizer we choose can be influenced by what plants we are growing, it is governed 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 in Baton Rouge.
A soil sample should be submitted for testing from each unique area of your landscape, especially if the soils may be different due to past treatment or location. For instance, one sample could be submitted from your front lawn area and another from a flower bed in the front yard because the soils would have been treated differently over the years. A vegetable garden will require a separate test from other areas.
Small boxes and forms to submit samples are available at your parish LSU AgCenter office and at many local nurseries. Go to the Soil Testing Laboratory website www.lsuagcenter.com/soiltest to find where soil test boxes are available in your area. Once you are ready to submit your samples, you can mail them directly to the lab in Baton Rouge.
The test results you receive will tell you the texture of your soil. You will also learn the pH of your soil, which reveals how acid 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 from 5.5 to 7.5 is acceptable for most plants. If necessary, the pH of the soil can be adjusted higher by the addition of lime or lowered by the addition of sulfur.
The fertility of the soil 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 should be medium to very high. Fertilizer recommendations you receive with the test results are based on these levels. The amount of fertilizer needed and the number of applications in a season are influenced by the type of plants you indicated you are growing or intend to grow where the soil sample was taken.
Soil testing can be done anytime 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.
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
LSU AgCenter soil testing kits, like these on display at the Botanic Gardens at Burden, are available for consumers to use to have their soil tested. They are available at AgCenter parish offices and research stations and at many retail garden centers and nurseries. Photo by Olivia McClure/LSU AgCenter
Samples of soil 6 inches deep from several locations in a garden or lawn can be tested to provide information on soil pH and fertility. Test results come with recommendations based on the type of plants you intend to grow. LSU AgCenter file photo by Tom Koske