Submitted by Chris Hicks – UT Extension

Making sure that plants have the proper nutrition is a critical part of growing just about anything, whether it be grass, vegetables, flowers, or shrubs. If you go to a home and garden supply store to buy fertilizer, you may be overwhelmed by the various options. Let’s take a look at what’s actually in that fertilizer you are buying.

Probably the first thing you will notice on the bag of fertilizer sitting on the store shelf is three numbers which are usually below the fertilizer’s name. The first number is the percentage of nitrogen (N), the second number is the percentage of phosphate (P2O5) and the third number is the percentage of potash (K2O). These three numbers represent the primary nutrients nitrogen (N) – phosphorus (P) – potassium (K).

Remember, these numbers are percentages so a bag of 10-10-10 fertilizer (commonly referred to as “triple ten”) contains 10 percent nitrogen, 10 percent phosphate and 10 percent potash. So a 50 lb bag would contain 5 lbs of each of those primary nutrients (50 lbs multiplied by 10%). 

The remaining weight of the bag is filler or carrier material. This material makes the product easier to spread and may be made up of a number of materials such as sand, clay particles, or diatomaceous earth. 

Fertilizers which contain all three nutrients, such as 15-15-15 or 6-12-12, are known as complete fertilizers. Incomplete fertilizers may only contain one or two of the primary nutrients. Examples would be 46-0-0 (urea), 18-46-0 (diammonium phosphate), and 0-0-60 (muriate of potash).

So which fertilizer do you need? The answer depends on the crop you are growing and the fertility of your soil. Taking a sample of your soil and having it tested for nutrients at a lab will tell you if there are nutrients lacking. 

For example, if your soil sample shows your soil is lacking potash, the lab might recommend using 0-0-60 to raise the levels in the soil. If soils test high in phosphate and potash, the lab might recommend using a fertilizer that only contains N, such as 34-0-0. 

If you’d like to test the nutrients in your soil, the University of Tennessee Extension office has kits available that you can mail to our lab in Nashville. Within a few days of sending your sample, you’ll have results that will tell you exactly how to fertilize your crop. To pick up one of these kits, stop by our office at 125 Gordonsville Hwy in Carthage.