The soil without any treatment. The grass is thin. You want a better turf... thus you fertilize. But the only thing available to you are primary nutrition... nitrogen, phosphorus, potash... solid nutrients... thus... They release their food at the surface encouraging, even forcing the grass roots which are lazy to come to the surface to feed. Natural deep rooting is reversed, this is also true with quick release liquid fertilizer that is not organic and does not incorporate itself within the soil structure. Mat and thatch forms... roots continue to form at the surface. Further surface applications of fertilizers, especially slow-release ones, continue the formation of mat and thatch at the surface which forms a barrier to proper circulation within the soil of air-nutrients-water. You need more watering because it evaporates at the surface. Continued mat and thatch development forms an ideal environment for fungus and insects to breed and cause damage. Soil hardens up without proper circulation and the presence of air-water and the grass roots themselves.