Is my garden poisonous?

July 19, 2012 4:01 PM

Hey Dr. Hodgins: my garden soil is pretty like a rainbow! How do I find out what's making it that way, and whether it's safe to grow veggies in?

My family moved into a new house 3 years ago primarily for the possibility of hobby farming the large acreage. I immediately set about clearing and turning about 1/3 of an acre of it into an organic garden. I even harbor dreams of opening a small farmer's market stand in a few years.

Problem is, whenever it rains there are rainbow slicks across every puddle down there (and it is only *just* above the water table, so there are a LOT of puddles) and I'm growing less and less certain of whether the "organic" veggies I grow might actually be infused with toxic chemicals and hazardous to our health.

I finally got around to looking up soil tests, but it seems that most soil testing places won't test for chemicals, pesticides, gasoline, oil, etc. (I don't even know if those are what the problem is!) I looked up "environmental testing" on Angie's List and it seems most places only test for mold and asbestos.

I really want to find out WHAT is causing the rainbow slicks, and whether it is hazardous to eat vegetables grown in that soil.

Where do I go and who do I call? Extra points for concrete recommendations in the Baltimore/DC metro area or someplace I could ship a sample to.

posted by GardenGal to Home & Garden (6 answers total)

I'd get in touch with the University of Maryland Extension Service's Home and Garden Information Center (link to soil test page). Looks like you can request a soil test bag.
posted by MonkeyToes at 4:09 PM on July 19, 2012 [1 favorite]

Best answer: I can't recommend any testing company in particular for you, but I can at least reassure you that there are natural substances that produce rainbow effects on water surfaces (trees, quite commonly - lignin can look a lot like oil contamination). If by clear, you mean cleared of trees, this is the most likely culprit (and nothing to worry about).

Otherwise, any site history that you know or can find out will guide you in what and how to test. If petroleum is an issue, there are certainly tests that can be done (look for companies that remove old oil tanks or remediate old under ground fuel storage tanks). Testing for chemicals in general is not going to be possible (everything is chemicals and it would be very expensive to test for a laundry list), unless you have something in particular to go on.

Bear in mind, also, that there are standards for the length of time a farm has to transition (using no pesticides) until it can be declared organic. So even if there was long ago pesticide use, it very likely isn't an issue now from an organic certification (or your peace of mind) standpoint. Pesticides do generally break down over time.
posted by ssg at 4:26 PM on July 19, 2012

Just scoop up some of the puddle and test that, there are lots of places that do that kind of work or you can send it to a lab. Could be groundwater contaminant, local source (like an old heating oil tank buried under your garden plot) some kind of natural hydrocarbon seep, sheen due to decomposing vegetation (lignin) or caused by iron bacteria.

One quick test to differentiate between oil and bacteria is to break up the sheen. If it immediately comes back together you're probably looking at hydrocarbons. If it fragments, its likely just iron bacteria. This only helps with the oil/ bacteria question tho so still get it tested.
posted by fshgrl at 4:31 PM on July 19, 2012 [1 favorite]

Best answer: Yeah, before you start testing things at random, I'd start with some research. Your state environmental agency should have some kind of search function for things like LUSTs or remediation sites. USEPA has one too, but you'd probably know if there was something like a superfund site nearby. Typically anything outside of a 1/2 mile to 1 mile radius you don't have to worry about. But if you've got a gas station down on the corner with a leaky tank, there's a possibility you could be impacted.

If you don't find any problem sites nearby, and can't smell any petroleum odors, I'd tend to think you're ok. But if you really want to test your soil, you're right, there's no test for gasoline. If petroleum contamination is suspected, you'd probably start with testing for BTEX (benzene, toluene, ethylbenzene, xylenes). There's a whole laundry list of other contaminants you could conceivably test for, but it's going to start to get expensive petty quick, and I don't think pesticides or metals would be causing your rainbows anyway. I don't know of any particular labs in your area, but places like TestAmerica can ship you a cooler of sampling supplies and you put your dirt in a jar and ship it back to the lab.
posted by gueneverey at 8:13 PM on July 19, 2012

Response by poster: Good to know it might be a natural source, thanks guys. Yes, by "clear" I meant "cut down trees". And we do bring most of our garden leaves there to decompose, so hopefully it's lignin.

Thanks for the links gueneverey, I will definitely check them out! Yeah as far as I know there's no heating tank or anything there, it'd been just woods for a looooong time, but the previous bordering neighbor had notably done some illegal dumping there (unbeknownst to our previous owner). I discovered a nice bit just full of construction debris when I went to till. I wouldn't put it past him to have dumped a bunch of old oil or something too.

So, plan: 1) check for petroleum products. 2) If not, don't worry about it too much. 3) Win!
posted by GardenGal at 11:56 AM on July 20, 2012

Talk to the Extension office. They may be able to help you find an expert. I'd think that frequent turning of soil would help get rid of hydrocarbons through air & sun exposure. Fertilizer is made from oil, so it may be manageable. Test for lead.
posted by theora55 at 11:12 AM on July 21, 2012