Iron for lawns: How often is it okay to apply?
It kind of depends.
For fescues, overapplication or application too often will send the grass a very strange gray-green-black color. It's perfectly healthy, it just looks like zombie grass and takes forever to fade back. I'd say 2 oz of spray iron per every month is about the maximum, but play it by ear. If it starts to turn strange colors, stop.
Rye isn't quite so persnicketty, but still looks strange if you apply too much or too often. I don't have direct experience with rye.
Kentucky bluegrass is iron-demanding and seemingly impossible to overdo. I couldn't send the color off applying 4 oz of ferrous sulfate sprayed per week, but didn't keep after it for too long as it's annoying to do it that often.
For soil-applied iron, the numbers differ. The maximum rate is 10 pounds per thousand square feet of ferrous sulfate per year, and the each-time application maximum is around 3 pounds per thousand. Applying more than this sends the soil chemistry off in strange directions, and iron is both a phosphorus and manganese antagonist.
Even at lower rates, I strongly recommend a soil test yearly to check your pH. Ferrous sulfate is about one eighth as acid-reacting as pure sulfur, doesn't rely on bacterial work, and works perfectly well surface-applied (as sulfur does not).
Grass-type(s): Kentucky bluegrass / perennial ryegrass
Applied pelletized-sulfur at the rate is 35 pounds per 1000 square feet. Application was a bit too heavy as I have some burn-spots. I'll will consider ferrous-sulfate in the future.
Flowers of sulfur (pure sulfur, in other words) at 35 per K? While 75% of that is wasted eventually due to outgassing as sulfur dioxide (smog), 25% of that is still going to hit the soil eventually.
I hate to say it, but you just touched off a world of problems. Biochemically speaking, the surface soil just became a killing ground.
I fear the same. I did it on the advice of a respected educated professional. Do you make house(turf) calls? :-)
>>respected educated professional
Any respected educational professional who recommended 35 pounds per thousand square feet of elemental sulfur all at once is, bluntly, none of the three.
20 pounds per thousand per year is the maximum generally accepted amount to apply, and none of the soil test interpreters I know would recommend it at those levels. It's kind of a kick to the upper soil layers, plus the effectiveness over time is questionable.
And never in August on a northern lawn, it's asking for burnout.
>> Do you make house(turf) calls? :-)
Not 250 miles away. :-) Fortunately, it's not really required here. What's done is done, and now we let it perk in and do what it's going to do, and work with it later once things settle down.
Will you lose your lawn? In a few patches, you might, but it's not likely there will be widespread damage. The damage from this is more subtle, and to the soil chemistry and bacterial/fungal populations.
You're likely to see a lot of burns here and there over the short term. Keep an eye on them and make sure they come back in September. Water as necessary--and letting the lawn dry out is not a good idea right now.
You can try to seed any lost patches, but keep in mind that sulfur impacts seed sprouting and growth. Sulfur levels over 200 PPM tend to kill seed, and right now the surface layer is higher than that. Fortunately, some will still grow, and it takes extremely high sulfur levels to kill all the seed.
That having been said, don't seed more heavily than recommended. If you get lucky and it all grows, you then have an overgrown patch that has to thin itself out.