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What do those numbers on a bag of fertilizer mean? If you're not a farmer or a home gardener, you may not know what I am talking about. 

First of all, fertilizer contains the "food" that we feed our plants. Although fertilizer does not contain all of the nutrients that plants need, it does contain the big three that plants extract from the soil as they grow.

When you buy a bag of fertilizer, there are laws in place to ensure that you're getting what you paid for. The three numbers on the bag indicate the levels of nitrogen, phosphorus and potassium in the bag. A 10-10-10 bag of fertilizer, for example, contains 10% nitrogen, 10% phosphorus and 10% potassium. A 100-pound bag of fertilizer contains 10 pounds of each of those three nutrients. The remaining 70 pounds is filler, such as ground limestone.

The other question you may be asking is, “Are these the only nutrients my plants need?” The short answer is no. Just as humans must get adequate amounts of all essential nutrients to be healthy, so do plants.

Knowing the nutrients required to grow plants is only part of what's needed for a successful garden or landscape. Optimum yield or performance also requires knowing the rate, method, and time of application; the source of nutrients to use; and how the elements are influenced by soil and climatic conditions.

We'll focus here on the 16 essential nutrients required to grow crops or plants. Three essential nutrients — carbon, hydrogen, and oxygen — are taken up from atmospheric carbon dioxide and water. The other 13 are taken up from the soil and are usually grouped as either primary nutrients, secondary nutrients or micronutrients.

The primary nutrients — nitrogen, phosphorus and potassium — are commonly found in blended fertilizers such as 10-10-10, or equivalent grades. Plants and crops use primary nutrients in the largest amounts; therefore they are applied at higher rates than the secondary nutrients and micronutrients.

The secondary nutrients — calcium, magnesium and sulfur — are required in smaller amounts than the primary nutrients. The major source for supplementing the soil with calcium and magnesium is dolomitic lime (ag lime), although these nutrients are also available in a variety of fertilizer sources. Sulfur is available in fertilizers such as potassium and magnesium sulfate, gypsum (calcium sulfate), and elemental sulfur.

Micronutrients — iron, manganese, zinc, copper, boron, and molybdenum — are required in even smaller amounts than secondary nutrients. They are available in manganese, zinc and copper sulfates, oxides, oxy-sulfates and chelates, as well as in boric acid and ammonium molybdate.

In our next article, we will learn about a tool that farmers and home gardeners can use to determine if their soils have adequate amounts of nutrients.

To find how much of each of the essential nutrients are removed by a given crop, check out the following link https://go.ncsu.edu/nutremovbycropnc. For additional information about essential plant nutrients, check out the following link https://go.ncsu.edu/soilandplantnutrients.

Alton Wood Jr. is an agricultural agent with the Pasquotank Center of NC Cooperative Extension.

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