Iron for St. Augustine: Fixing Yellow Grass That Fertilizer Won't
Why St. Augustine turns yellow even after you fertilize, how to spot iron chlorosis, how to apply liquid iron safely, and when ammonium sulfate makes sense.
The short version
- If St. Augustine is growing and fertilized but still yellow, iron is the likely cause.
- Alkaline soil locks up iron so the grass can't use it, even when it's there.
- Liquid iron usually greens it up within days.
- Iron stains concrete. Keep it off driveways and sidewalks.
What iron chlorosis looks like
The grass is yellow or pale, often more yellow between the veins with green stripes along them. It's actively growing, so you're mowing regularly, and it's had fertilizer recently. Adding more nitrogen makes it worse: you push new growth that the plant can't color in.
It's common across the Gulf Coast because many soils here are alkaline. At a high pH, iron in the soil turns into a form roots can't absorb. It shows up most in spring growth spurts and in lawns watered with alkaline water.
Rule out the look-alikes first
- Pale and slow in spring, not much growth: probably hungry. See spring feeding.
- Patches browning in hot, sunny spots: check for chinch bugs.
- Yellowing that fertilizer and iron never fix, with rotted roots: could be take-all root rot.
Not sure? Use the yellow or brown lawn diagnosis guide.
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How to apply liquid iron
- Choose a chelated liquid iron labeled for lawns. Chelated iron stays available to the grass longer. The one I use is Southern Ag Chelated Liquid Iron.
- Spray in the cool part of the day, early morning or evening, at the label rate.
- Protect hard surfaces. Iron leaves rust-orange stains on concrete, pavers, and stone. Don't spray near them, and rinse off any drift immediately.
- Watch for results within a few days to a week. Repeat during the growing season as needed, following the label's interval.
Iron is a color fix, not a replacement for fertilizer. Keep your regular feeding schedule.
A quick color boost: ammonium sulfate
Ammonium sulfate (21-0-0) is a fast-acting nitrogen source that greens a lawn quickly, and it can help slightly acidify soil over time. Use it only on actively growing grass, at the label rate, and water it in right away so it doesn't burn the blades. Skip it in extreme heat or on stressed grass. Think of it as an occasional boost, not your main program.
The long-term fix
A soil test confirms your pH. Topdressing with compost and sphagnum peat moss can slowly lower pH over seasons, and that helps the grass take up iron on its own.
Everything in my St. Augustine program
The fertilizers, weed killers, fungicides, and tools I actually use on my own lawn, with links to buy.