My sod lawn will be 2 yrs old in August. My home is in a new housing development. I have a heavy clay soil - little top soil. A composite soil sample (3-4" depth) in April '10 had a pH 8.1. I subsequently had sulfur appllied (5 lbs/1,000 sqft) in April and Oct '10. The pH of a composite sample this April was 7.9. I subsequently had sulfur (and gypsum to loosen clay) appllied again at same rate as before. I aerated and overseeded Oct '10 and aerated again April '11. My root system is very shallow. Here's my problem. My lawn is now very thin, maybe 75% the density that existed last fall. Also, the grass is growing VERY slowly. Neighbor has mowed three times already, my lawn is almost ready for first mowing. Is the problem from too much sulfur (low pH) in the shallow root zone? Also, I understand from other posts that my best approach to the clay problem could be aerate and over-dress with organic matter or good top soil. Tough to guess how much material is required for over-dressing. If pulling 4" plugs, what would you guess (bulk vol.) is needed per 1,000 sqft? Thank you very much for your help.