When you check the gazillion piles of lawn fertilzers at the Home Center or the Garden Center, each product has a different set of NPK numbers. With lawn fertilizers, the N or nitrogen number is likely to be quite a bit higher than the other two.

Don't worry about it!

The issue with lawn fertilizer is whether you use a slow release nitrogen product or a quick release nitrogen product. As you would see readilng our files on fertilizing lawns, we are very biased in favor of the slow release products. So here is some help in understanding that issue.

After reviewing this file about how to choose lawn fertilizers, you may wish to go to the file How To Fertilize Your Lawn in Yardener’s Helper. Then go to the file Lawn Fertilizer Products in Yardener’s Tool Shed to learn about the various brands of lawn fertilizers available at your local garden center or home center. Then you will be an educated customer.

To learn about fertilizer spreaders go to Hand Tools For Spreading in Tool Shed.

Slow-Acting vs. Quick-Acting Products

The nitrogen in all fertilizers acts in one of two different ways. In some products the nitrogen acts slowly; it is in a form that is released slowly into the soil over time--from 4 to even 12 weeks. All or most of their nitrogen is water insoluble (WIN). Because it does not dissolve in water, it is available to plants gradually, just as naturally occurring nitrogen in the soil is. In other products the nitrogen acts quickly; it is in a form that dissolves readily in water is immediately available to plants and is used up by them within two weeks. While there are situations where this quick action may be desirable, slow-acting products are easier on the soil and the plants.
When the nitrogen in quick-acting fertilizer products is released it acidifies the soil temporarily. This change in the soil climate interferes with the normal activity of earthworms and microbes which keep soil healthy. Slow-acting fertilizers, either synthetic or natural, do not affect the life in the soil.

Three Kinds Of Slow-Acting Lawn Fertilizers

There are three types of slow-acting fertilizers.

· "Natural" the nitrogen in these products is derived from organic matter or some sort--feather meal, used coffee grounds, manures, leaves, peanut hulls or other natural materials. It is released slowly into the soil by microbial activity. When it is hot, the activity increases and the nitrogen is released more quickly. When the soil is cool, the activity slows, so the release of the nitrogen does too.

· "Synthetic" products feature nitrogen that is synthesized from chemicals and is then encapsulated, or coated, so it, too, is released into the soil slowly over time as soil moisture gradually penetrates or wears away the coating. These high technology products are relatively new to the lawn care market.

· "Bridge" products contain some "synthetic" uncoated nitrogen for quick release into the soil for initial greenup of lawns and plants in the spring. The majority of the nitrogen in their formulation is organic and is released slowly over many weeks.

The basic difference between the groups is price; the synthetic coated fertilizers are much cheaper than the products made mostly or entirely from natural materials.

Recognizing Slow-Acting Lawn Products

Slow-acting fertilizers are also sometimes called "timed release", "continuous-acting" or “long-lasting” on the front of the package of various brand named products. In the nutrient analysis on the back of the package where the percentages of the NPK nutrients are shown, nitrogen is often listed twice. First the total percentage is listed, then the percentage of that number that is "water insoluble nitrogen" (WIN) is listed. A lawn fertilizer is considered to be "slow-acting" if at least 30% of the total nitrogen in the bag is water insoluble. A higher percentage of WIN assures steady, consistent delivery of this critical nutrient to plants over time. To assure healthy soil and vigorous plants buy organic fertilizer products or look for granular synthetic fertilizer products that feature from 40 to 60% WIN.

How Slowly Is The Nitrogen Released?

How Slowly Is The Nitrogen Released?
Some slow-acting fertilizers last longer than others. Package labels indicate the period of release. Most products last at least 8 to 10 weeks, or two and one half months. With this time frame two applications would be necessary to cover the season--one in early spring to cover about eight weeks to midsummer and then another in midsummer to cover the eight weeks through fall. As the technology for coating or encapsulating the nitrogen in synthetic fertilizer products develops, release periods will be extended. Some products now on the market advertise up to 16 weeks or more. While these products cost more in dollars, they save valuable time and energy.