Sulphate product containing 20% iron would be mixed with water at the rate of 50 to 150 grams to treat 100 square metres of lawn. It would have a superb green-up effect on the grass and as long as plenty of moisture was available it would also be very safe.
This means you can use iron sulphate at these concentrations pretty much any time of year as long as good soil moisture is available and there is no strong sun or heat.
Increase the concentration to 150 to 200 grams and not only will it have a good green-up effect but it will also start to ‘harden’ the turf making it more resistant to disease and fungal attack.
This is a very common practice on sports turf in late autumn and winter.