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My lawn is half green and half brown. I can't figure it out.
The blades that are green look great. But there is a 50/50 mixture of
dead brown grass that is still rooted to the ground.

Last year I mistakenly thought this was thatch, so I bought a
dethatcher. But then when I learned how to check thatch by looking at
a cross section of the soil and grass, I found I had no thatch
problem.

Here's what it looked like last year:
http://www.mbsoundworks.com/tempstuff/grass1.jpg
http://www.mbsoundworks.com/tempstuff/grass2.jpg

The pictures were taken before the drought, so it wasn't a water
problem. You can see what it looks like compared to my neighbor's
yard!

Can anyone help me?

I'm in northwestern Illinois.
As the lawns are waking up, I can already see that it's going to look
the same as it did last year.

I plug-aerated in the Fall, and put down winterizer.
I've put down my Scotts pre-emergent/fertilizer already.

I'm willing to do the work, but I'm at a loss as to what the problem
is!