Do you ever wonder how “clean” your soil is? When you plant a new vegetable garden, do you consider everything that has been done in that area? Have the previous owners sprayed weed killer? Did your neighbors fertilize their lawns which drain into your yard when it rains? Do you have a fence or another structure adjacent to your garden that contains pressure treated wood? We had a fence installed around our backyard last year in order to keep our then future toddler contained in a safe area. This was decided before my crusade to convert the lawn to food gardens. The fence is made of cedar panels but with pressure treated wood posts in concrete footers.
We’ve heard a number of times about not planting vegetable gardens in planter boxes made of pressure treated wood. The caution is because pressure treating involves forcing chemicals into the wood, via pressure, in order to prevent decay from rot and bugs. Those chemicals can slowly leach out of the wood and into the soil which can be sucked up by your edible plants and then consumed by you. Sounds yummy, right?
This was not a concern for us at the time because our existing garden boxes were all raised, not very close to the new fence, and we weren’t planning to put anything new along it. Fast forward to this year and that is exactly the area I have been eyeing up to put my next gardens. I also want to try ground level beds instead of raised beds for a more natural feeling landscape instead of raised boxes everywhere.
Copper Contaminants
Since chemistry was one of my weaker science topics in school, I began researching the chemicals and how far they can travel in the soil but did not find any consistent information that applied to our situation. The only way to know what was really happening with the chemicals near those fence posts was to do soil testing. The fence has been installed for 7 months now which should (I hope) be enough time for chemicals to start leaching. The pressure treated wood posts each have a tag on the end which gives the information about what they were treated with. The photo of our fence post shows that it was treated with MCA-C or Micronized Copper Azole.
I found a soil testing company that had an office in my area called Waypoint Analytical, LLC. Their testing prices seemed reasonable ($12.50 per sample) so I followed their collection procedures and sent the samples off in the mail. The samples were collected from two locations in our yard: one was a mixture of soil taken at each fence post and the other in the middle of the yard at various locations. The test in the middle of the yard was to be a “control” test to compare to those along the fence.
Results
The results came in within a day of being received at the testing center. Copper is a necessary element in soil and seems to be at a normal level if found in the range of 2-100 ppm. The results showed the fence tests were at 3.8 ppm and in the middle of the yard were at 1.9 ppm. These numbers are at the far bottom end of the range so our worries have been put to rest.
However, the standard testing package included testing for other elements and it found that our phosphorus, iron, and magnesium were slightly elevated above the optimum levels. The phosphorus level really jumped out at me because it is typically found in fertilizer. We haven’t used any since we’ve lived here but who knows what was done before we purchased the house 3 years ago. A few searches on the internet later (because everything on the internet is true, right?) revealed that optimum levels should be between 25 and 50 ppm and ours is just above at 70 ppm. It seems that the only way to reduce phosphorus is to wait it out and don’t apply any more fertilizer. That won’t be a problem.
As for the high iron and magnesium, there are many natural reasons that the iron in our soil could be elevated. Most information suggested that using nitrogen fixing plants will reduce the iron levels. Can you guess what convenient-for-us plant fixes nitrogen? White clover. Looks like I will get started on planting that sooner rather than later. The high magnesium levels can be caused by the clayey soil all around our yard. Apparently magnesium toxicity is pretty hard to achieve in soil and since our levels are not that high, there’s not much we can or plan to do about it.
In the end, we don’t know if the chemicals will continue to leach over time or if we’ve reached the maximum level of concentration and won’t see any further increase. I may continue to test every year or so just to keep tabs on what is going on in our soil. The worse case is that we’ll just have to turn those areas into a raised bed condition to stay away from the “dirty” dirt.
Even if you don’t have a pressure treated wood fence in your yard, the results of the soil test can tell you a lot about the area that you are planting in and if you need to make any adjustments to your garden. Do you plan to do some soil testing now or have you previously and received some interesting results? Tell me about it below.