Oddly enough, dead grass over your septic tank is a good sign! It means your septic system is probably working the way it's supposed to. Brown grass appears during dry or warm weather and indicates that your grass isn't getting enough water. That's because the soil over your tank isn't as deep as the soil over the rest of your lawn. However, the worst thing you can do is to water the brown grass.
Brown grass over your septic tank is an aesthetic problem, but your lawn should recover in the fall. Providing the brown grass with additional water reduces your leach field's ability to absorb wastewater from your house and may even lead to the failure of your system. Even though the grass turns brown because there's not enough soil to support its root system, you shouldn't add topsoil over your tank, either.
Septic tanks send liquid waste from the tank through a series of drain lines positioned throughout the leach field. Microbes in the soil filter and clean the liquid waste before it reaches the ground water. These microbes need oxygen in order to do their work effectively. Compacted soil, or wet, soggy soil, has less oxygen in it. Adding additional soil reduces the amount of air available to the microorganisms that break down the wastes in your system.
If you have lush, green grass growing over your septic tank, it could mean that the hydraulic system isn't working properly. The trenches in your leach field are filling with liquid waste because the soil can't absorb any more water from your house. That wastewater is full of rich nutrients that give the grass over your septic system a good dose of fertilizer and turn it a rich shade of green. Other signs of trouble include slow-draining toilets, sewage odors, and sewage backing up into the house or surfacing on the leach field.
When planting over your septic system, remember not to dig too deep. Drain lines may be as close as 6 inches from the top of the soil. Wear gloves when working with soil over the septic system to reduce exposure to harmful organisms that may be present. Select plants that don't need much maintenance, watering or fertilizer. Ornamental grasses and herbaceous plants such as catmint, black-eyed Susans and verbena are good choices. Don't plant plants that are meant to be eaten over a septic system.