St. Augustine won't spread

[olamorrep]

For the last several years, my lawn continues to get "thinner". It is now at the point where the dirt is very visible between the St. Augustine grass plants. In the past, the turf was so thick that it was "spongy" when you walked on it.

Two and one-half years ago the grass all but disappeared from the side and back yards. I had those areas resoded, but they are now getting "thin" again.

I am in southwest Houston close to Sugar Land. The soil is clay. The lawn has been in since 1975.

In the sping I fertilize with 13-13-13 and in the fall with 12-24-12.

The grass is mowed when it needs it, and the mower is set to the highest setting, i.e., approximately 3 1/2 inches. The grass usually gets watered after mowing.

Some of the lawn gets mostly morning sun, some mostly afternoon sun and some is under very large trees. That doesn't seem to make any difference.

I had a nematode problem last year, but a couple applications of the stuff recommended by the garden center got the grass growing where the lawn had gotten bare.

We have gotten a lot of rain this year, but the grass appears to be getting "thinner". It just grows higher, but will not spread out like it used to.

In addition, when it is fertilized, it doesn't get the dark green color that it used to get after fertilizing. It gets darker but not as dark as before.

Any suggestions on how I may proceed to identifiy and/or solve the problem(s) would be appreciated.

Thanks.

Frank