Ferrous Ammonium Sulfate

...and some basic chemistry.

Thanks to Gary in Chicago and some chemical calculations, the lawn has darkened considerably (even more than before):



You can make your own ferrous ammonium sulfate very easily from two separate chemicals:  ferrous sulfate and ammonium sulfate.

I have some rough measurements and calculations that work for me.  The solution should be relatively close to equimolar (containing an equal number of moles of each chemical.  A mole is around 6 × 10²³ molecules, but as long as the molecule count is the same, it doesn't matter how many there actually are).

Fortunately you don't need a very powerful magnifying glass and the ability to count to extremely high numbers!  A little basic chemistry will tell you that there are 151.91 grams of material in 1 mole of ferrous sulfate, 132.14 grams in a mole of ammonium sulfate.  As long as you use 1.15 times as much ferrous sulfate by weight as you do ammonium sulfate, the solution will be close to pure ferrous ammonium sulfate.

It isn't that critical to be precise, so a 1 to 1 ratio is more than good enough.  Ammonium sulfate can strip some calcium from plants, so tilting the balance toward the ferrous sulfate is best--use more than 1.15 times the weight of ferrous sulfate as ammonium sulfate.

One thing to note is that this is by weight.  My ferrous sulfate doesn't seem to pack as well as the ammonium sulfate, so I require about twice as much by volume.  Your mileage will almost certainly differ.

Two to four ounces per thousand square feet in three gallons of water worked best for me.  Let it sit on the grass for 24 hours, then irrigate in well.  This solution should never be applied if temperatures will be over 80 degrees F.

With a good eye, you may notice a slight color change in an hour (I did).  However, if the lawn requires the iron, you'll certainly notice it within a day or two.  Color durability is much better than plain ferrous sulfate--post the next mow, I still saw very dark green color that hasn't faded at all.