We have a yard at least 20 years old. We have a sprinkler system, and well water with high count of iron. We water twice a week. We have noticed a ring around 1 oak tree about 12 inches from the tree out to the grass. The soil is sand and the grass is about 99% gone inside the ring. These are oak and red maple and Bradford pear trees. Also in front of the shrubs on the west side of the garage, the grass is NOT growing about 12 inches from the edge of the shrubs. The extension Agent told me, the following: 1. too much water. 2. not enough water. 3. too much shade. 4. compacted soil.

Texas is quite a distance from Illinois, yet your questions are similar to those here as well. My thoughts are too much shade. You have an irrigation system that has been in place many years and have to assume you manage this properly so I rule out too little, too much water for your trees. Up here these rings appear as the tree naturally grows taller and wider, limiting sunlight very near the base of the tree. there is also the heavy roots and crown area that just does not allow much of anything to grow. Western exposures are likely to dry out sooner and so here it could be a lack of moisture due to shrub roots and the sandy soil you mention and not sunlight. Compacted soils inhibit water from entering the soil profile. Sandy soils could become compacted, but not in a pattern as you describe. Tree and shrub roots can compete well against grass for soil moisture and I suspect that is part of the situation on the west side.