Okay, in response to questions and comments: 1) I haven't been watering at all this spring thus far. Haven't needed to, as the rains have been quite regular and plentiful (for once). 2) The bare spots aren't huge. Mostly, they seem to be the gaps between the sod rolls that were laid down. There are a few assorted spots where for one reason or another the grass didn't take well. Those are the ones I'm most concerned about. I'm just trying to figure out if it's worthwhile to overseed those bits now, or just apply PE and fertilizer and let the grass do what it will over the summer, then overseed this fall if necessary. The sod was installed late October last year. I watered the living pi$$ out of it for as long as directed and then a bit longer than that, stopping when the snow started. There were some areas that didn't take so well - those were the low-lying areas, actually, that saw the most water due to the way water flows around the house. Starter fertilizer was applied at install, and none thereafter. None has been applied thus far this spring; AIUI, it's a bad idea to do so too early, as it can cause a flush of unhealthy top growth and just wastes fertilizer. Spring here has been quite late in coming; it was unusually cold and snowy throughout March, and quite cold and rainy through April. We've had frost as recently as a couple days ago. Daytime highs are just now consistently above 60F. I apply fertilizer and pre-emergent separately, as I am a control freak, and the appropriate times for each are usually a few weeks apart. Constructive advice, comments, criticism is appreciated. Smarta$$ed commenters can GFT.

Answer depends on what kind of grass you have and what kind of weeds you have. The grass you should be able to answer but you might want to take pictures of the weeds. If you can take them when it is shady that is much better. The organic weed control is vinegar. Unfortunately it is non selective and will kill the grass, too.

What you are likely seeing is a product consisting of corn gluten meal. It has some preemergent qualities, but it is important to understand that how well it works depends greatly on your climate and the weather you have. It also is fairly short lived, acting from 4-5 weeks, maybe 6 weeks max. It is also a fertilizer. A search for corn gluten meal on Gardenweb or Google will give you plenty of opinions.

Fertilize with everything you got there- water it in, and then apply weed preventer - a couple of days later. You don't want anything interfering with the chemical barrier created by weed prevent.