Septic Tanks, Leach Fields and Vegetable Gardens
Can you have a vegetable garden over a septic tank leach field? On some lots it seems like the majority of the lot is septic tank. Even if you have extra land, the septic tank takes up the best spot on the best soil.
If the septic tank leach field always worked there would be no problem with a garden over the top. Unfortunately, septic tank leach fields are not on the short list of perfect things. I guess most people have smelled a catastrophic septic tank failure. This is the kind of situation where you wouldn't want to harvest the garden, let alone eat anything out of it. The only thing you would lose in a situation like this would be one year's harvest. Even more hazardous would be a short term failure that introduced pathogens to the soil surface without creating an obvious smelly long term problem. In addition to being hazardous, this type of failure is more common. While human waste has been used as fertilizer for 1000's of years, it carries more risk than I think is necessary.
If at all possible, I would garden somewhere else.
If a leach field is the only place I could garden, I would stay away from root crops and leafy crops. Things that bear a fruit like greenbeans, squash and tomatoes would be the only crops I would grow. I would stake as many vegetables as I could, including things like cucumbers that I normally don't stake.
Washing and peeling would reduce pathogens. Of course high temperatures normally created while cooking will destroy pathogens also.
I don't know of any research based information on mulches in a situation like this. Some people think mulches would reduce the amount of pathogen that might splatter onto the plant. I lean toward the view, that mulches will reduce the amount of water evaporation creating a higher incidence of soil saturation. Saturated soil is not good in a leach field. I would avoid mulches.
Some people say stay away from raised beds for the same reason. I don't think a raised bed would cause a problem with reduced evaporation, but it isn't going to help solve any pathogenic problems either. The same cautions mentioned above would apply.
If you do decide to garden over a leach field be careful with your tillage. 40 years ago septic lines were 5 and 6 foot deep. I am not sure what the current depth is but I know it is a lot less. At 5 and 6 foot the nitrogen in the effluent was gradually sinking into the water supply. At 1 to 2 foot the nutrients have a chance to get recaptured by the plants growing on the surface. So the shallower depths are better for the environment but it does increase the possibility of destroying the lines during cultivation.
Plants that receive too much nitrogen from any source including leach fields will not be as nutritional as plants grown without excess nitrogen.
In conclusion, you can garden over a leach field but it is not the best situation.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.