It depends upon your soil. Iron tends to be fairly plentiful in most
soils, but its availability to plants is dependent mainly upon soil
pH, and to a lesser degree on the soil temperature and degree of root
development. Before adding a soil amendment, it is always recommended
to have a soil sample tested to determine what, if anything, you need
Soils with a higher pH tend to tie up more iron in the soil, so
supplementing with iron can be useful. If your soil isn't in the
alkaline range, adding iron may make little to no difference to the
lawn. In the long run, you'd see better results by lowering the soil pH.
Adding an iron supplement to an higher-pH soil through a top-dressing
treatment, such as an iron-supplemented fertilizer, works only for a
few weeks because the added iron will quickly be tied up in the soil.
Thus, if soil conditions require it, it may need to be regularly
applied through the growing season. However, as the soil warms and
bacterial activity and root growth increases, the amount of iron
uptake in plants increases. For that reason, a home owner with an
alkaline soil may be satisfied with limiting iron supplementation to
the spring season.
Tl;dr: don't assume you need to add iron until you have had your soil
tested and have determined its pH.
A better (longer-term) solution to your moss problem is to improve
soil drainage by reducing compaction. Core aeration helps. You'll hear
talk about applying gypsum to loosen soils; realistically, that only
works when applied (and mixed into) the soil at a heavy rate.
Top-dressing a lawn with gypsum won't cut it.
As for iron applications and potential harm to the lawn, most types of
iron supplements won't have a harmful effect, with the possible
exception of ammonium sulphate. That's a nitrate fertilizer that will
also lower soil pH; over-apply any nitrogen fertilizer and you risk
burning the lawn.
Remember, iron is the fourth most common element on earth; so it's
almost never an issue of there being insufficient iron in the soil.
It's that other circumstances - usually soil pH - limit the amount of
that iron that plants can take up.
Yeah too much will weaken and thin out the grass. Every 4-6 weeks was
for high intens sports turf on a sandy rootzone which is prone to losing
nutrients. While I agree with the answer given, it is always a good
starting point to know ur soil PH, I don't know of many people who would
do it to determine iron use, most people like yourself only apply it to.
control Moss. If you are applying Iron Sulphate then 15g per meter
squared is adequare to control a minor Miss problem. If you have lots of
Mossi then you can apply iron to weaken it but it will not get rid of
it. Physical removal through scarifying will have to be done..