Don't blow it this summer over-fertilizing your lawn. They are already green in warmer climates and just turning green in our cooler zones and something weird is happening. They are turning green all on their own. This is because of the natural action of micro-organisms in the soil. They are breaking down organic matter and doling out nitrogen to each other and of course to the lawn. In other words there is always some nitrogen available to lawns right away. Adding fertilizer now before the lawn really needs it may do one of two things:
1) Weaken the lawn by causing it to grow lush (and you just know we are going to have a few more snow falls, or
2) Run off the lawn surface or into the sewer systems further polluting out water.
What to do? Rake your lawn, or have it aerated and compost the "browns" coming off your lawn now. Then top-dress the lawn with organic matter. I just used composted manure and worm castings. Rake it in lightly and then water. I watered it lightly with a solution of kelp mixed with humate to make sure to give the micro-organisms a good start. It has been very dry here but if it had been threatening rain I would not have watered at all.