What would you do for this?

Another thread Reminded me of this: Our veg garden is atop the old lawn. The lawn is atop the ? Tons of sand which are our septic field. The septic field is unmapped because it got lost on the previous owners, so we can’t just dig up the lawn, we don’t know where the pieces of the septic field are. The lot is ringed with tall trees, my season is about 15 days shorter than my neighbors. The lawn is on the south side. The north side has a vernal pond, which is protected by law, a stone wall, ditto. The east side has trees and a hill and a stone wall. The west side is the front of the house, driveway and more vernal ponds. I can’t move the garden.

I have raised beds because I can’t grow a root crop without them. All the nutrients leach out in about 3 years. Mind you, when we got here, the actual soil atop the sand lookEd like a thin chocolate glaze. There’s actual soil there now, about 3” or so. I use leaf mulch, cut and drop the lawn, use a no mow lawn seed to fill in the lawn as needed, and raised beds. I raise beans and legumes. I rotate crops. I compost. Still about every 3rd year, my garden just fails. I have some small beds in the lawn I could convert into a mulch crop or fertilizer crop. Because of this, I buy compost. I make compost. I use compost. I put worms out there. But essentially, I am gardening on a granite platform covered in a sieve. The nutrients seep out and then go out on the granite ledge and away.

Advice? Ideas? My plans this year include swales and hugelculture, which I haven’t tried.

Jennie Little wrote:Another thread Reminded me of this: Our veg garden is atop the old lawn. The lawn is atop the ? Tons of sand which are our septic field. The septic field is unmapped because it got lost on the previous owners, so we can’t just dig up the lawn, we don’t know where the pieces of the septic field are. . I can’t move the garden.

I have raised beds because I can’t grow a root crop without them. All the nutrients leach out in about 3 years. Mind you, when we got here, the actual soil atop the sand lookEd like a thin chocolate glaze. There’s actual soil there now, about 3” or so.....

I know of someone else whose land is basically on sand with similar problems: water takes the  nutrients into the sand.

First off, I'd like to say you've made progress: 3" is still 3" rather than 1/2".

I'm certainly no expert but here's what I would experiment with:
-find a source of clay;
-dig a 5'x5' square down to the sand (carefully so you don't hit the septic system unawares);
-heap the black soil around the 5'x5' square as you dig down;
-make the 5'x5' square concave so if it could, it would hold water;
-drop 6" of clay on top of the clay;
-throw old hay on top of the clay;
-fence off the 5'x5' and run pigs in there for a week;
-lay down more old hay, grass, leaves after the pigs leave;
-put all the black soil back into the 5'x5' square;
-compost and add manure on top of this square;
-garden for 3 years and see what happens.

Compost.....compost....and more compost.  Keep doing that and you'll continue to improve what soil you have. It's my understanding that organic material tends to bind or hold nutrients, so there will be less leeching problems.  

Since the problem is leaching (you suspect, anyway), how can you slow or prevent that? What if you cover the beds all winter to keep snow off? What if you build hinged hoop covers for summer, that can be closed on rainy days (and great for cool spring and fall days too! I love them.)
Now you have close to zero uncontrolled water getting into the bed. You can selectively water with drip irrigation (even using collected rain water/snowmelt) so that 90% of the water gets used by the plants. This may take some experimentation to get the watering dialed in, but it’s a super cheap fix, and may extend your nutrient loss to 10 years, or beyond, if you are adding compost all the time. Now take it a step further, and dig out your beds and add a layer of logs in the bottom. Hugelkultur doesn’t need to be a tall mound, it works the same way regardless. The log layer holds moisture instead of letting it migrate down.
Choice B (more expense/work)- rebuild all the raised beds 6” off the ground, with fabricated aluminum base with 3” high sides (the base, not the bed). A drain on one end with a valve, and now you can collect the water that seeps through, nutrients and all, and re-water with it.

Most counties should have a detailed septic map. Mine is detailed but without obvious references . There should be metal in the tiles you can map with a metal detector. Also historical images from google earth can show where the limbs are

I second the idea of digging up the raised beds and adding logs to give you a good base to work from. I think with a hugel your beds will beat the three year limit.  

“ Most counties should have a detailed septic map. Mine is detailed but without obvious references . There should be metal in the tiles you can map with a metal detector. Also historical images from google earth can show where the limbs are”

Maybe. She’s in New England, an old part of the country. May be a system installed before codes and inspections, before google earth, or maybe more recently but not done in the typical legal fashion. Old systems used clay piping, which a metal detector won’t find, though it would find cast iron mains. It might also find the distribution box, if there is one, as they were often concrete with metal rebar rings in the lid. If it’s a pvc pipe system, then no metal except maybe clamps on fernco fittings. I grew up in New England in the time before google, and I can vouch that there were many rural septics installed by ‘some guy with a backhoe’ who knew how to do it, but never mapped it or got a permit.
She mentions “The septic field is unmapped because it got lost on the previous owners” so it’s possible that there is a map somewhere.
Regardless, it would probably take more work to find the piping and relocate the raised beds than to just mitigate the leaching.

When someone grow plants in a pot, they will have to add new soil, ever so often. If it is a open bottom container, sometimes it needs new soil less often. So a raised bed behaves just like a pot/container. But all hope isn't lost, we have a couple options:
Add minerals that have been leached (fertigation/worm tea/sea90/compost/rockdust/etc)
Unlock mineral with soil life, will break down the sand/rockdust/silt/etc into plant available minerals.
Create mineral some soil life will make nitrogen from the air.
Prevent leaching; bio-char just like activated charcoal will capture the mineral before it leaves. Any type of carbon is good over the long term, so sawdust/compost/woochip/husk/logs are all okay. each type does come with certain pros and cons though.

The other problem could be that the roots are going down into the drainfield and accumulating something into the raised beds that the plants don't like. Maybe the drainfield is creating and explosion of a particular soil that out-compete the vegetables. So maybe you can completely remove the drain field. I have seen some 4, 5, and 6 stage all in one septic tank:
1. Anaerobic/Settling Tank (maceration/grinder/solid separator before might be included)
2. Aerobic Tank with a mixer/airstone-bubbler
3. Clarification Tank (a filter wall some times separates this from the aerobic tank, some times it is even nested inside the aerobic tank to get 3side of filtration
4. Disinfectant Tank (chlorine tablets/UV filter), the outflow is bacteria free, crystal clear and ordorless. I could practically drink it, the EPA says it is safe for direct discharge
5. Nitrate Remover  (some systems have this before the Disinfectant Tank)
6. Phosphate Remover (some systems also have this before Disinfectant Tank)

“So a raised bed behaves just like a pot/container.”

My experience is otherwise. A typical container is less than 5 gallons, not really large enough to be a mini ecosystem. It will generally not support worms, for instance. I have never needed to add dirt to my raised beds, or change the dirt. I do not fertilize beyond watering with manure tea, and simply mulch heavily with shredded wood chips (shredded seems preferable to chipped). I randomly bury veggie scraps in small quantities all summer. My beds have a ton of worms and are very much living soil. When I plant things in containers I do add worms and manure but it’s rare that I dump one out a year later and find any worms at all. When I build raised beds I start with a layer of logs for the hugel effect, and it occurred to me to add a layer of partially decomposed sticks to the bottom of containers as an experiment, but this is my newest thing to try and I don’t have results yet.