Fertilizer Numbers Explained: What N-P-K Means for St. Augustine

Up, down, and all around

N · Nitrogen · UpLeaf growth, thickness, and deep green color. The nutrient your lawn uses most.

P · Phosphorus · DownRoot growth. Often already plentiful in Gulf Coast soils.

K · Potassium · All aroundHeat, drought, cold, and disease tolerance. The star of fall feeding.

What's actually in the bag

The numbers are percentages by weight. A 40 lb bag of 19-4-10 holds about 7.6 lb of nitrogen, 1.6 lb of phosphate, and 4 lb of potash. The other 26.8 lb is carrier material that lets you spread the nutrients evenly instead of burning the lawn in clumps. Try any bag below.

15-5-10 · wake-up 19-4-10 · main feeding 8-12-16 · fall 21-0-0 · quick boost

N % P % K % Bag (lb) Label rate (lb per 1,000 sq ft)

The right bag for each season

St. Augustine needs different things at different times of year, and the formula on the bag should change with it. Here's what the Turf Method program uses through the year, side by side.

Nitrogen (N) Phosphorus (P) Potassium (K)

Show as a table

Product	When	N %	P %	K %
15-5-10	Optional late-winter wake-up	15	5	10
19-4-10	Main spring and early-summer feeding	19	4	10
8-12-16	Fall root feeding	8	12	16
21-0-0	Occasional quick green-up (ammonium sulfate)	21	0	0

• Late winter (optional): a gentle 15-5-10 once the lawn starts to green. See when to fertilize in spring.
• Spring and early summer: nitrogen-forward and low in phosphorus, like 19-4-10.
• Fall: less nitrogen, more potassium, like 8-12-16, to build roots and cold hardiness without pushing soft growth into large patch season.
• Quick boost: 21-0-0 greens fast but fades fast. Use it sparingly. See iron for St. Augustine.

Slow-release vs. quick-release

Two bags can share the same numbers and still behave very differently. Quick-release nitrogen dissolves right away: a fast flush of green that fades in a few weeks, with a higher chance of burning the grass. Slow-release nitrogen is coated or bound so it feeds a little at a time for weeks. The bag's guaranteed analysis usually lists how much of the nitrogen is slow-release, sometimes as "water insoluble" or "controlled release."

Quick-release Slow-release

Why the middle number matters on the Gulf Coast

Phosphorus builds up in soil over the years, and many Houston-area soils already test high in it. Adding more doesn't help the grass, and the extra can wash into storm drains and bayous. That's why spring formulas here keep the middle number low. A soil test is the only way to know what your yard actually has. The MySoil kit is the easiest way to start.

Turning the bag into a plan

1. Know your lawn size. Measure it in Google Earth.
2. Use the label rate for St. Augustine. The calculator above shows how much nitrogen that rate actually delivers. I weigh product on a digital scale so I spread what the label says, not a guess.
3. Buy the right number of bags with the bag calculator, or get it all done at once in the plan builder.

A note for the detail-minded: the second and third numbers are technically percentages of phosphate (P₂O₅) and potash (K₂O), the standard way fertilizer is labeled, not pure phosphorus and potassium. For comparing bags and following a program, reading them as P and K works fine.

Common questions

What do the 3 numbers on fertilizer mean?

They're N-P-K: the percent of nitrogen, phosphorus, and potassium in the bag by weight. A 50 lb bag of 19-4-10 holds 9.5 lb of nitrogen, 2 lb of phosphate, and 5 lb of potash.

What fertilizer numbers are best for St. Augustine?

A nitrogen-heavy, low-phosphorus blend with slow-release nitrogen during the growing season, and a low-nitrogen, high-potassium blend in fall. A soil test tells you whether you need phosphorus at all.