How to Read a Fertilizer Bag for a Florida Lawn

A lot of the fertilizer that goes on Central Florida lawns is chosen by the picture on the bag. The label carries everything that matters, and reading it takes about a minute once you know what the parts mean. Here is what we look for, and why sandy Florida soil changes the answer from what a bag designed for the Midwest would suggest.

The three numbers

Every bag carries three numbers, such as 15-0-15 or 16-4-8. They are the percentages by weight of nitrogen (N), phosphorus (P) and potassium (K), in that order.

• Nitrogen drives green color and leaf growth. It is what most people think of as fertilizer, and it is what is most often overdone.
• Phosphorus supports root establishment. Most established Florida lawns need little or none of it, and many Florida soils already have enough. Extra phosphorus washes into lakes and feeds algae, which is why so many lawn products here carry a zero in the middle.
• Potassium supports root strength, drought tolerance and disease resistance. It is the number Florida lawns are most often short of and the one most bags underdeliver.

For an established St. Augustine lawn on sandy soil, a bag with a low or zero middle number and a potassium number close to the nitrogen number is usually the right shape. A soil test replaces the guesswork.

Slow-release matters more than the numbers

Below the three numbers the label breaks nitrogen down by source, and somewhere it states the percentage that is slow-release or controlled-release. This is the line that matters most in Florida.

Sandy soil holds almost nothing. Quick-release nitrogen dissolves with the first watering, feeds the lawn hard for a couple of weeks, and the rest leaches straight through the root zone into the groundwater. The lawn surges, softens, invites disease and pests, and then runs out. Slow-release nitrogen meters out over weeks, feeds evenly, and far less of it is wasted. A bag with a high slow-release percentage costs more per pound and delivers more of what you paid for to the grass.

Iron and micronutrients

Florida lawns yellow from iron shortage more often than from nitrogen shortage, especially on high-pH soils and lawns irrigated with hard well water. A label that lists iron, and ideally manganese and other micronutrients, is addressing a real Florida problem. Our post on why Florida lawns go yellow explains how to tell the two apart. Adding iron to a lawn that is short of it greens it without pushing the soft growth that nitrogen does.

Weed-and-feed products

Bags that combine fertilizer with weed killer force two things onto one schedule that rarely line up. The right time to feed is not always the right time to treat weeds, and some of the herbicides in those products damage St. Augustine and centipede grass. We apply fertilizer and weed control separately, each when it is actually needed.

Application rate and coverage

The bag states how many square feet it covers at the recommended rate. Applying more does not produce a greener lawn; it produces burn, runoff and disease. Measure the lawn, set the spreader for the rate on the bag, and apply half in one direction and half at right angles for even coverage.

Local rules

Many Florida counties and cities regulate lawn fertilizer, including restrictions on phosphorus and on summer application. Check what applies where you live before buying, and follow it. Our fertilization schedule covers timing through the year.

Or let us handle it

Our treatment programs are built from a soil test and use products chosen for sandy Florida soil. Ask for an estimate and we will tell you what your lawn is actually short of.