If your lawn has problematic yellow spots, a ferrous sulfate deficiency could be the explanation. Ferrous deficiency is both easily diagnosed and easily treated; you only need to know a few simple facts to solve the problem.
Symptoms of Fe deficiency
The telltale sign of ferrous deficiency in the lawn is yellow spotting. In extreme cases, wide swaths of the lawn may turn yellow. Ferrous deficiencies are sometimes mistaken for nitrogen deficiencies, as both cause unseasonably yellow grass. Ferrous deficiency causes the grass to yellow from the top down, while nitrogen deficiency causes yellowing from the ground up.
Once you've determined that your lawn do indeed have a ferrous deficiency, treatment is easy. Your local gardening store will have several types of ferrous additives such as ferrous sulfate heptahydrate which you can spread to fix the problem.
When applying ferrous to the lawn, ideally, temperatures will be between 60 and 70 degrees Fahrenheit. Remember not to apply the fertilizer grade ferrous sulfate in the summer when temperatures are frequently above 80 degrees Fahrenheit as this can damage the lawn.