What to Actually Buy During Palm Beach County’s Fertilizer Blackout

Fertility & Soil Health

By Floridist

A label-by-label buyer’s guide to potassium, magnesium and iron — and the four products South Florida homeowners waste money on every summer.

If you’ve read our Palm Beach County fertilizer ban guide and our summer blackout treatment schedule, you know the shape of the problem: through the summer, nitrogen and phosphorus are off the table. Potassium, magnesium, sulfur and iron are not.

This guide is the next step down — not what your summer program should look like, but which bag to pick up, how much of it to put down, and what the research actually says about whether it works. Fair warning: a couple of these answers will cost a garden center a sale.

Step one: the Guaranteed Analysis is the only thing that matters

Ignore the front of the bag. Turn it over and find the Guaranteed Analysis panel — Florida law (FDACS Rule 5E-1.003) requires it.

If the first number is anything other than 0, put it back. A “1-0-2” kelp meal contains nitrogen. The ordinance restricts fertilizer containing nitrogen or phosphorus — not fertilizer containing a lot of it. There’s no de minimis exception. (Straight micronutrient products are the exception to the exception: many carry no N-P-K grade at all, just a guarantee for iron or sulfur. No nitrogen line means no nitrogen.)

Then read the “Derived From” line. Florida requires manufacturers to list the actual source materials there, and prohibits brand names in that section — so it’s the honest part of the label.

  • Green light: potassium sulfate, sulfate of potash magnesia (langbeinite), potassium thiosulfate, magnesium sulfate, ferrous sulfate, iron EDDHA
  • Red light: urea, ammonium sulfate, potassium nitrate, calcium nitrate, bone meal, blood meal, feather meal, fish emulsion, poultry litter

Two traps. Potassium nitrate (13-0-44) sits next to the potash and looks like a potassium product — it’s 13% nitrogen. And most dry kelp meals run 1-0-2, which puts them out too. Liquid seaweed and humic concentrates are frequently 0-0-0 and are fine.

Potassium: buy sulfate of potash, not muriate

Potassium is the workhorse of a blackout program, mostly because it’s the only macronutrient you’re allowed to apply. Two sources dominate the shelf, and the difference matters more in July than it would in January.

SourceAnalysisSalt index (per unit of K₂O)Notes
Sulfate of potash (SOP)0-0-50, ~17% S0.85Gentlest per pound of potassium; free sulfur
Muriate of potash (MOP)0-0-60, ~46% Cl1.94Cheapest per pound; highest burn risk
K-Mag / Sul-Po-Mag0-0-22, ~11% Mg, 8–22% S1.97Only worth it if you also need magnesium

Normalized to the potassium it actually delivers, SOP is roughly 2.3× gentler than muriate. On low-CEC sand, in 90°F heat, with a homeowner-grade spreader that may or may not be calibrated, that’s the difference between a maintenance application and a streak of burned turf. At the rates below, the premium works out to well under a dollar per application on a typical lawn.

K-Mag’s headline salt index looks low, but that’s an artifact of its low nutrient concentration — you have to apply 2.3× more of it. Per pound of K₂O delivered it’s about as salty as muriate.

Rate: 1.0 lb of 0-0-50 per 1,000 sq ft, every 4–6 weeks. That delivers 0.5 lb K₂O. Water it in with just enough irrigation to moisten the top half inch of soil — enough to rinse the granules off the blades, not enough to flush potassium past the roots. Two or three applications across the blackout.

Light and frequent beats one big application here. UF/IFAS is blunt that “potassium is highly mobile in most of Florida’s sandy soils” — and between low CEC and 6 to 9 inches of rain a month through a Palm Beach County summer, a single heavy slug of potassium is largely a donation to the surficial aquifer.

The part nobody selling a summer program mentions: UF/IFAS turf specialists write that “applications of K above a 1:1 N:K ratio are not evidence-based practices on Florida fairways, tees, and approaches” — no gain in drought resistance, quality, or recuperative potential. That’s golf-course turf, so treat it as directional for a home lawn. But it lines up with the other half of the story: the “potassium hardens your lawn against disease” pitch distorts real research showing that deficient potassium increases disease susceptibility. Extra potassium above sufficiency doesn’t buy protection. Summer K is maintenance — replacing what leaches, holding the plant at sufficiency through its hardest four months. It isn’t a performance enhancer, and you shouldn’t pay for one.

Magnesium: only if a soil test says under 20 ppm

Magnesium deficiency yellows the older, lower leaves first — which is exactly how homeowners misdiagnose it as nitrogen hunger and start looking for something illegal to spray.

The threshold is clean: above 20 ppm Mehlich-3 magnesium, UF/IFAS says a response to added Mg is unlikely. Many Florida soils are already there.

If your test comes back below 20 ppm and you were going to apply potassium anyway, K-Mag makes the magnesium free: 2.3 lb per 1,000 sq ft delivers 0.5 lb K₂O plus about 0.25 lb of magnesium, and sulfur on top of that. (Sulfur guarantees on langbeinite products range from 8% to 22% depending on the brand — read your bag.) Epsom salt only wins if you specifically want magnesium without more potassium, or want to spray it.

Do not use dolomitic lime to correct magnesium here. It’s a liming material. On calcareous South Florida fill at pH 7.5–8.3, you’d be making the underlying problem worse.

Iron: spray it, don’t spread it

This is where the research is most decisive, and where most homeowners get it exactly backwards.

Granular iron mostly doesn’t survive our soil. UF/IFAS notes that iron “remains soluble for no more than a few minutes in aerated solutions of pH 7.0 or higher,” and that as much as 95% of soil-applied iron becomes unavailable within one hour. In UF/IFAS turf trials, granular iron — tested as EDTA chelate and as iron humate — produced a response in only 1 of 9 studies. Their summary: granular Fe “increases the cost of granular fertilizer and does not increase turfgrass quality.”

Foliar iron worked in every UF/IFAS trial — six studies across three turfgrass species, two Florida sites, two years. Color holds up to four weeks. (Those sites were Citra and Jay, in central and north Florida; nobody has run this on Palm Beach County marl fill, so we’re reasoning from the closest evidence available.)

Rate: 2 oz of ferrous sulfate in 3–5 gallons of water per 1,000 sq ft — UF/IFAS’s own homeowner recipe, at the low end of the effective window. A ready-made liquid is easier; Southern Ag’s Chelated Liquid Iron (5% Fe, no guaranteed nitrogen or phosphorus) is the one most commonly stocked down here. Follow its label rather than deriving your own rate.

On chelates: Utah State’s position is that EDDHA is “the only chelate that works consistently under high soil pH conditions,” and that everything else on the market is ineffective above pH 7.2. That matters enormously for soil-applied iron on palms and shrubs — and not at all for a foliar spray, where the chelate never has to survive soil chemistry. (The Southern Ag product above uses a lignosulfonate chelate, a weak one — fine on a leaf, not as a soil drench.) For your lawn, skip the EDDHA premium.

Four rules that make or break the application:

  1. Before 9 a.m., or evening. Foliar iron “can cause temporary leaf burning” applied in hot conditions; extension guidance is to spray in cool weather or evening. The 90°F cutoff printed on most liquid turf product labels is a good general rule.
  2. Do not water it in. Let it dry completely first. Rinsing it off is the single most common mistake with liquid iron.
  3. Sweep the hardscape first and after. Iron stains concrete, pavers and travertine reddish-brown, and because the stain forms chemically it’s very difficult to remove.
  4. Understand what you bought. Iron gives you color, not growth — UF trials found quality and color improvements with no yield response. During a blackout that’s the feature: green without a growth flush and without the disease pressure a summer nitrogen application invites.

Four things to skip

Manganese. UF/IFAS tested it at two Florida sites — granular as sulfate and sucrate, foliar as sulfate and glucoheptonate — and got no turf quality improvement from any of it. The one exception is diagnosed take-all root rot, where foliar sulfate-form manganese is part of the protocol. Otherwise it’s the most oversold micronutrient in South Florida lawn care.

Elemental sulfur to lower pH. UF/IFAS: “There is no way to permanently lower the pH of soils formed from high Ca materials, such as marl or limestone.” The carbonate buffers indefinitely. And on established turf mowed at 2 inches or more, Purdue caps safe application at 5 lb sulfur per 1,000 sq ft per application and 10 lb per year — at which rate, they write, “it will take many, many, many years” to move the pH of a soil that’s highly buffered with calcium. Free diagnostic: put a few drops of household vinegar on a dry soil sample. If it fizzes, you have free carbonate, and acidification is a money pit. Manage around the pH instead — that’s what foliar iron is for.

Gypsum and “base saturation balancing.” UF/IFAS is unusually direct: calcium deficiency in Florida turf is “extremely rare,” and balancing soil cations by base saturation ratio “is not supported by evidence and has been widely rejected by land grant institutions.” If someone pitches you gypsum for a Palm Beach County lawn, that’s your answer.

Humic acid, honestly. The theory — raise CEC, chelate micronutrients — is exactly what low-CEC calcareous sand supposedly needs. But the one multi-year study run in calcareous sand found no improvement in water holding capacity, phosphorus uptake, chlorophyll or color; the humic acid was actually hydrophobic. Elsewhere the effects are small and inconsistent year to year. Probably harmless, possibly useless — don’t pay much for it, and confirm it’s genuinely 0-0-0.

Get the soil test — but test for the right things

A UF/IFAS Extension Soil Testing Lab standard fertility test runs about $10. Pull 10–15 cores from the top 4 inches across the lawn, mix, submit a pint, mail it to Gainesville.

Read it for pH — which determines your entire iron strategy and tells you whether acidification is futile — and for magnesium, the one nutrient with a usable threshold. Don’t let the potassium number drive your program. UF/IFAS says it plainly: “Soil test values and recommendations for K are of little value in Florida due to the high solubility of K and the low cation exchange capacity of Florida’s sandy soils.”

Your blackout shopping list

BuyRateWhen
Sulfate of potash 0-0-501.0 lb / 1,000 sq ftEvery 4–6 weeks, watered in lightly
(Substitute) K-Mag 0-0-222.3 lb / 1,000 sq ftOnly if soil test Mg < 20 ppm
Ferrous sulfate, or a liquid iron with no guaranteed N or P2 oz in 3–5 gal water / 1,000 sq ftAs needed for color; before 9 a.m.; don’t water in
Soil testOnceAnytime — read it for pH and Mg

One rule that catches everyone: Palm Beach County’s 10-foot fertilizer-free zone around ponds, lakes, canals, seawalls and wetlands applies to all fertilizer — not just nitrogen and phosphorus. Your 0-0-50 is subject to it too. (Three feet if you’re using a deflector shield or drop spreader.) And skip any application when 2 or more inches of rain is forecast in 24 hours, or when a flood, tropical storm or hurricane watch or warning is up.

Not sure whether your address falls under a September 30 or an October 31 end date — or whether your city has a calendar blackout at all? The rules genuinely differ from one municipality to the next in Palm Beach County. Get in touch and we’ll tell you exactly what applies to your property, or call (561) 941-GROW.

Sources

Floridist Footer — Trimmed