Slow-release or quick-release lawn fertilizer: which to use

Two speeds of nitrogen

Penn State Extension splits lawn nitrogen into two groups. Quick-release sources are soluble in water, so the nitrogen is available to the plant immediately, and they can burn turf more easily. Slow-release sources release part of their nitrogen over several weeks to several months.

The choice is a trade. Slow-release products are usually safer to use because of their reduced burn potential and last longer, but they cost more per unit of nitrogen and green up the lawn more slowly.

What the nitrogen is made of

The bag’s nitrogen number looks the same for every product, but the source behind it differs. Penn State lists the common groups.

• Quick-release: urea and ammonium sulfate. They dissolve and feed at once.
• Sulfur-coated urea: urea granules sprayed with sulfur. Because the coating varies from granule to granule, release is staggered, and an under-fed lawn can look spotty as individual granules let go.
• Polymer-coated urea: urea in a thin layer of polymer resin. Water passes through the coating, and a thicker coating means slower release.
• Methylene urea: urea reacted with formaldehyde, typically 30 to 40% nitrogen. Some versions contain enough soluble nitrogen to act closer to quick-release.
• Natural organics: made from plant and animal by-products, usually under 10% nitrogen, and release depends on soil microbes.

How to tell which kind is in the bag

Penn State says the guaranteed analysis shows the mix as percentages of total nitrogen. Ammoniacal nitrogen and urea are quick-release. Water-insoluble nitrogen, slowly available nitrogen and controlled-release nitrogen are slow-release.

Its example label reads 20% total nitrogen, made of 10% ammoniacal nitrogen, 3% urea and 7% water-insoluble nitrogen. That is 7 of 20 parts, or 35%, slow-release. A practical tip from us: if the front of the bag says only “slow-release”, check the analysis for the share.

Some states set a floor. The University of Maryland Extension says lawn fertilizers used in Maryland must contain at least 20% slow-release nitrogen, and should supply no more than 0.9 pounds of total nitrogen per 1,000 square feet per application. Check your own state’s rules.

Speed, burn and runoff

Soluble nitrogen is cheaper and greens a lawn up fast, but Maryland Extension says its effect is short. Insoluble nitrogen costs more and releases more evenly as soil microbes and other factors convert it into soluble forms.

Slow release can also mean less loss. Penn State says recent studies show some slow-release products are less likely than soluble ones to leach into groundwater, though others use nitrogen less efficiently. Maryland adds that lawns on sandy soil, or near a high water table, should favor slow-release nitrogen, because soluble nitrogen is lost faster.

Heat and weather

Weather changes how each type behaves. Penn State says nitrogen release from polymer-coated urea increases with higher temperatures and depends little on soil moisture. Natural organics depend on soil microbes, so temperature, moisture, oxygen and pH all change how fast they break down.

For urea, Penn State says some nitrogen can be lost to the air as ammonia if it is not watered in. Hot days, high soil pH, and granules left on the surface or in thatch with no rain or irrigation afterward favor that loss. Always follow the label’s limits on rain and temperature, and check your local extension service for summer advice.

When each one makes sense

These are our own practical matches, built from the facts above and not taken from one source.

• Pick slow-release for a steady lawn, sandy soil, a first-time spreader or a wider margin for error.
• Pick quick-release when you want a fast green-up and are comfortable measuring the lawn and applying the label rate exactly.
• Pick a blend, which many bags are, when you want some quick color and some feeding that lasts.
• Pick a natural organic if you accept a lower nitrogen number and slower, microbe-driven release.