Now is the time to be putting down the pre-emergent of your choice.  We at Plant Wisdom, like Ferti-Lome's All Seasons with barricade. 

There is some nitrogen (16-0-8), so it is classified as a lawn food, but you can get more nutrients by just leaving the lawn clippings to decompose on the lawn.

Its real purpose is to prevent the germination of annual weeds.  There are three products that do this safely and sustainably.  Most organic gardeners know the virtues of corn gluten meal.  Put it on during the right times of the year, and it prevents roots from forming during germination.  However, rain and watering will reduce its effectiveness.  It is also not too shelf stable.  Leave a bag in your garage for a few months and you'll know what I mean.

Another product is called Preen.  It is just pelletized corn gluten, making its effects last longer.  It, however, can be expensive.  I'll talk more about it later, but suffice to say, it is best used in gardens as opposed to on your lawn.

Which brings me back to All Seasons.  Is it organic?  No.  This is one of the cases where is doesn't matter, it is a best practice.  The compound in All Seasons, barricade, is derived from a plant.  Callistemon citrinus is an australian flowering shrub that survives with limited resources by stopping the germination of nearby seeds.