Understanding your fertilizer bag

  • Follow Gardening

It can be confusing to know which fertilizer is best to use on the lawn because there are so many to choose from.

Then there comes the question of the type of nitrogen contained in the fertilizer. Do you fertilize with slow release or quick release? And do you use a high or low percent of nitrogen? Or do you use a weed-and-feed with a pre- or post-emergent herbicide?

Today I will try to make some things a little clearer on the types of fertilizers from which to choose. We need it more than ever since the price of fertilizer has gotten pretty high since oil prices shot up a few years ago!

The best and only way to know for sure which fertilizer is best is to take a soil sample. This sample will tell you what nutrients are available in the soil and what you need to put out based on those soil needs.

All fertilizer bags will come with a label telling you what is in the bag: 16-4-8 contains 16 percent nitrogen (N), 4 percent phosphorus (P), and 8 percent potassium (K). A premium grade fertilizer will contain what we call micronutrients such as sulfur, iron, zinc and boron to name a few. These nutrients are important but are not needed as much as N-P-K.

Nitrogen is the element that probably causes the most confusion. You’ll find lawn fertilizers containing 10, 15, 16, 28, 29, 30 percent of nitrogen. And this nitrogen label will be divided on types like quickly available and slow release. The quickly available nitrogen is immediately available to plants provided it has been watered in. This type of nitrogen generally is less expensive, can cause a flush of growth, have a short residual, can leach rather quickly, and have high burn potential. Quickly available nitrogen materials include ammonium nitrate, urea, ammonium sulfate, and potassium nitrate.

Slowly available materials release nitrogen more gradually and over a long period of time. The rate of nitrogen release depends on microbial activity, and to a certain extent, environmental conditions such as temperature and moisture. High temperatures and lots of rainfall or irrigation increase microbial activity and nitrogen release. The slowly available materials are generally more expensive, require fewer applications, reduce losses to leaching, and have low burn potential. Examples of slowly available materials are urea (ureaformaldehyde), methylene, and sulfur coated urea. A high quality slow release lawn fertilizer should contain at least 30 percent nitrogen in a slow release form.

Don’t forget the use of manures, such as cricket or chicken manure as a quick and slow release way to fertilize the lawn. Bags are available for just this purpose.

How many pounds of fertilizer do you apply to the lawn during one application? You would normally base it on how much it would take to apply one pound of nitrogen per 1,000 square feet. To do this you would divide the percent nitrogen of the fertilizer into 100 (for 100 square feet, divide the nitrogen into 10). An example would be how much of 16-4-8 to use on a 3,000 square foot lawn. Dividing 100 by 16 gives you 6.25 pounds. Since there is 3,000 square feet to be fertilized, multiplying 6.25 by 3 equals 18.75. So this is how much fertilizer should be applied to a 3,000 square foot lawn. For a 29-3-4 fertilizer, it would take about 3 ½ pounds of fertilizer.

Centipede is probably the exception to the “one pound per thousand” application. Lightly fertilizing, applying about ½ pound of nitrogen per thousand seems to work better.

If the fertilizer you have chosen has 50 percent or more of the nitrogen in the slow release form, you would apply roughly twice the amount I recommended, but only half as often.

Given equal prices and equal kinds of nitrogen, why would anyone choose 16-4-8 over 29-3-4? Because you may need more phosphorus and potassium than what would be included in the 29-3-4.

I know for a fact that most lawns in the Augusta area are deficient in potassium. I would say 29 out of 30 soil sample results that go through my office show low potassium levels. I think this is one reasons we are having such a big problem with take-all patch and other lawn funguses. Potassium is a very important element in growing grass and is known for helping with disease resistance. In the absence of a soil test, you probably need to be applying a fertilizer with at least 8 percent potassium at each application.

Reach Sid Mullis, the director of the University of Georgia Extension Service office for Richmond County, at (706) 821-2349 or smullis@uga.edu.

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