All of the above: You provided good drainage, good nutrients, and regular water. Of course, the grass is enthusiastic! The very best lawn I ever had was one that I installed in new construction. Because of drainage issues, the houses had been built on a 3' layer of rip-rap fill. Instead of letting the contractor bring in any old dirt he could find for topsoil, I laid down a double thickness of burlap, and covered it with a compacted foot of pure compost. After outlining the flower beds, I seeded the 12 x 20 remaining space with a good grass blend, and covered it with guaze for erosion control. Three weeks of daily hand-watering later, I had a lush, green carpet.
You can repeat the same effect on your entire lawn -- without the horrendous expense that this would be on a large scale [I can't imagine actually covering a large lawn with guaze]:
- Top the entire lawn with 4-6" of mixed compost and sand.
- In the spring, reseed the whole lawn.
- Water LIGHTLY daily until the new grass is established; make sure the lawn gets an inch of water a week thereafter. [Do let it go dormant in high summer.]
Chris Owens