We live on very slight sloping 3/4 acre block. It has established lawn of approx 9 years. We have a septic system in front lawn. This appears to be working correctly and is serviced yearly. The front half has quite thick grass but the ground has become very soft to walk on to the point of leaving indents from feet and the rideon mower. The back yard which is the highest point has always been hard with much less thicker grass. It is also soft now to the point it indents also and is covered in small mud mounds from worms. We have used a rideon without a catcher and recently started using a new mower with mulcher. The front has gotten much softer over the years. Any suggestions on why our yard is getting softer.