What We Mean When We Talk about Fertilizers: Introduction

(News article for June 5, 2021. This article is part of a series. Click to see articles about nitrogen fertilizers; phosphorus and potassium fertilizers; calcium, magnesium, and sulfur fertilizers; and micronutrients.)

Like we do, plants need certain nutrients to live. Having nutrient concentrations within optimum ranges help them to resist diseases and insect pests, tolerate drought stress, and produce flowers, fruits, and seeds.

It’s not only important that plants have enough of each essential element but also that they don’t have too much of any one. If too much of a nutrient is present, it may be directly toxic to the plant or reduce uptake of other nutrients.

Excessive amounts of nutrients can also leach into ground water or run off into waterways. The “dead zone” in the Gulf of Mexico is an example of a negative effect of fertilizer runoff.

There are approximately 17 elements that are essential for plant growth and reproduction. (I say “approximately” because there is some disagreement about whether certain elements are essential or just beneficial.)

Plants get the elements they need in the largest amounts – carbon, oxygen and hydrogen – from air and water. Carbon dioxide (made up of carbon and oxygen) is taken in through leaves, while water (containing hydrogen and oxygen) is, for the most part, taken up through roots.

Besides carbon, oxygen, and hydrogen, six elements are present in plant tissue in relatively large concentrations (greater than 0.1%, typically): nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. These are called “macronutrients.” Iron, chlorine, manganese, boron, zinc, copper, nickel, and molybdenum are “micronutrients,” or elements present in plants in very small concentrations.

Plants usually get these macronutrients and micronutrients from the soil. In many cases, they will naturally be present in soil in adequate quantities. When plants need more than is available from the soil, we can add more in fertilizers. Fertilizers can be natural or synthetically produced.

We don’t typically apply nutrients in the pure, elemental forms but use fertilizers that contain other elements. For example, when providing nitrogen to plants, we could use a natural source like manure or cottonseed meal, or a synthetic fertilizer like ammonium sulfate, calcium nitrate, or urea.

We need to consider the concentration of the nutrient in the fertilizer when deciding how much of a fertilizer to use. Most fertilizer packages indicate the concentrations of nutrients in the product. To determine how much fertilizer you need, divide the amount of the nutrient you want by the concentration of the nutrient in the fertilizer.

For example, if we wanted to apply nitrogen at a rate of 1 pound per 1000 square feet and had ammonium sulfate fertilizer (21% nitrogen) on hand, we could divide 1 by 0.21, which would give us 4.8. We’d need 4.8 pounds of ammonium sulfate to get 1 pound of nitrogen.

One way to get an estimate of plant nutrient needs is to send a soil sample to the LSU Soil Testing and Plant Analysis Lab (STPAL) or another soil testing laboratory. Default recommendations are often available for certain types of plants but do not account for the levels of nutrients already in the soil.

Sometimes, nutrients are not taken up in adequate amounts by a plant even when nutrient levels in the soil are sufficient. This can happen if soil is too wet or too dry, if roots are diseased, if soil pH is too low or too high, or if there is an excessive amount of a competing nutrient in the soil.

So, when crops show symptoms that we think indicate a nutrient deficiency, we can take a plant tissue sample to find out what the nutrient concentrations in the plants are. Many soil testing laboratories, including the LSU STPAL lab, also provide plant tissue testing services. It’s very important to follow lab instructions when taking and sending plant tissue samples in order to get results that are comparable to established standards and therefore useful.

In following articles, I’ll provide more information about specific nutrients and fertilizers.

Contact Mary Helen Ferguson.

7/22/2021 6:30:27 PM
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