Thinning grass and weed-infested lawns can indicate soil compaction. Compacted soil prevents air and nutrients from getting to the roots of the turf grass, which causes it to die back. To evaluate your soil's compaction problem, push a screwdriver or pencil into the turf. This will be very difficult to do if you have compacted soil. If you notice a problem, know that it's time to aerate your lawn. You can do the job with a lawn aerator, available for rent or purchase at most home centers.