Earthworms, and all the other members of the Soil Food Web, need organic matter in the soil to live on, so what is the level of organic matter in your soil? One application of a synthetic fertilizer will not drive the earthworms out, but a lack of organic matter will.

kimmq is kimmsr

1 app of synthetic wouldn't cause your worm population to collapse.

I struggled much longer than I needed to because I didn't "listen" to my lawn. The weeds you have will give you clues, which can be confirmed via a soil test. Do not skip this step or you will never master organic lawn care.

There are probably 3 main issues (maybe 4).

#1 - Dandelions indicate low calcium. Calcium is required to release nutrients from the soil. Dandelions also show up as natures way of aerating a compact lawn. Grass cannot penetrate through compaction, but Dandelion roots can! They are trying fix a problem

#2 - Clover is OK, but if it's taking over it can be annoying. I still don't have a 100% solution for clover as I still have more than I would like, but... adding high nitrogen feed to areas with too much clover does seem to allow the grass to out competed the clover.

#3 - Fall overseed program. Don't cheap out on seed. Fall is your best chance to create a thick lawn that will smoother out weeds. Fescue & other non-spreading grasses cannot just "fill in" thin spots, you need to add seed, no other way.

#4 - Fall aeration. As mentioned above, you may have compact soil. Aeration fixes that and allow you to pump organic matter and feed down into the open cores at the same time, allowing oxygen to get down into the soil profile.

I cannot say with certain that Corn Gluten Meal works as a pre-emergent. I still get weeds, but even if it doesn't, it's almost 10% nitrogen, about as high as you can get organically, so it makes sense to apply this every spring. Again, I have gone cheap on this application, because the stuff is expensive... that's why I can't make a solid conclusion on it. I bet if you applied at a heavy rate it would work well.. but I've also read it takes 3 years to reach high % efficiency, and the timing is critical as well.

My soil test from 2 years ago showed 14.7% organic matter.

14.7 percent organic matter two years ago could be 2 percent today depending on how active the Soil Food Web is, or was. Organic material needs to be added to soil all the time since it is the only part of the soil equation that is digestible by the Soil Food Web, and "disappears' as they digest that OM and convert it to plant food.

kimmq is kimmsr

That soil test sound suspect to me. 14% organic matter would indicate that you have a an extremely high level, maybe three times normal high OM soil. What was the prior use of the property? Maybe you tested soil that was under the rabbit pen or had been used for a worm farm.

I also don't believe that a high organic matter lawn soil will have organic matter decline from 14% to a 2% level while growing grass for two years. The heavily farmed and tilled midwestern soils have documented declines in organic matter going from 6% to 2% after 100 years of the practice. I'd sure have to see thorough documentation of tests that support statement to find it believable. Show me the evidence you have to support that, please.