ne of the concerns that I have when growing our own food is the high (and increasing) use of glyphosate (Roundup®) and other herbicides around us. I do wonder when I buy soil, sand, woodchips, wheat straw, sugar cane mulch or other ‘amendments’ for use in my garden (as one must occasionally do in suburbia) whether there is any contamination of these things. Am I setting myself up to fail by buying these materials from the local landscape supplier or, dare I say it, Bunnings? All because of undeclared or possibly even unknown herbicide contamination?
Also, when you are moving onto a new (particularly suburban) property, has the previous owner been a liberal applier of herbicides? Even if it is just around the fences or perhaps outsides of the buildings, how would you know?
Well, it turns out that there is a cheap, easy and simple technique to find out if the materials you are buying or your soil is contaminated, and you probably have the gear to do it already in your shed.
The technique is called ‘bioassay’ which is defined as “……. an analytical method to determine the concentration or potency of a substance by its effect on living animals or plants (in vivo), or on living cells or tissues (in vitro).” At least according to Wikipedia.
Techniques
The basic idea is to pick a plant which is sensitive to the particular material (in this case herbicide) you want to test for and expose its seeds to the potentially contaminated soil or other materials in a controlled manner, while also setting up a blank to reduce the likelihood of spurious results. In this case there are two basic techniques. One is used when the seeds can be planted directly into the material to be tested (eg soil, potting mix or compost), and the other technique is used when the material to be tested does not generally have seeds directly planted into it (eg wood chips, gravel, straw, hay or sugarcane mulch).
Planting technique
Take a representative sample of the soil or material to be analysed and fill a punnet with some of it.
- Take potting mix or other planting material known to be free of herbicide and place into another punnet for use as a control.
- Sow both punnets with appropriate viable seeds from the same source and place in a well lit area.
- Water all punnets with the same water source, as appropriate to keep the seeds damp.
- Observe seedlings as they emerge and note any differences, provide water as required.
- Continue the observations until the four leaf stage, preferably two or three weeks at least.
- Note any differences between the test and control seedlings.
Extraction technique
Take samples of the material to be analysed, soak in a container 1:1 with tap or tank water by volume (roughly no need to be too pedantic) soak for several hours.
- Fill two punnets with damp growing medium such as seed raising mix and sow both with appropriate viable seed.
- Pour off the water from the material being extracted, strain to remove solids if required.
- Water one (marked) punnet with extracted solution and the other with tap or tank water, from the same source as the water used for extracting the material to be tested as a control.
- Observe seedlings as they emerge and note any differences, providing water as required.
- Continue the observations until the four leaf stage, preferably two or three weeks at least.
- Note any final differences between the test and control seedlings.
The canary in the mine
The above step by step process for both techniques gives you a bare bones idea of how the analysis works. One issue is to work out which seed to use as the indicator species. I have seen it recommended that any fast growing seed can be used. However, depending on the species’ sensitivity to the particular herbicide being tested for, there may be a false negative where you think everything is OK but your veggies still suffer.
Fortunately, from the reading I have done, there appears to be a readily available vegetable which is rated as either ‘severely sensitive’ or ‘extremely sensitive’ to a number of commonly used herbicides: the tomato! The cucumber is up there as well, but the tomato seems the most popular. Herbicides that tomatoes are listed as being sensitive to include – glyphosate, Paraquat, sulfonylureas and imidazolinones ( eg metsulfuron, sulfosulfuron, imazapyr, imazapic), atrazine, and synthetic auxins like Dicamba, 2,4-D, aminopyralid, clopyralid and picloram.
What to look for
In general terms, symptoms of residual herbicide damage include: stunting, yellowing of plants generally, or particularly of new leaves, and/or twisted and malformed new shoots. The main thing is to identify any differences between the test plants and the control.
If there are no observable differences between the test and control punnets, then you will be right to go with the material you have tested.
My example
I wanted to get some gravel from my daughters’ place (which they were going to get rid of anyway) to go into a frog hotel. My concern was that, because they had only been there a comparatively short time and I could see the gravel would be an obvious place to use a herbicide to keep grass out of, it could be contaminated with glyphosate or similar by the previous owners. Thus it could cause harm to my prospective frog tenants.
Day Zero
I picked a bucket of gravel from various areas and brought it home, then soaked 3 litres of the gravel in 3 litres of tank water for two hours. I then decanted the gravel water into a bottle. I filled a couple of punnets with my own seed raising mix and sowed tomato seeds (‘Moneymaker’ variety for those interested) and set the punnets up side by side in the greenhouse in separate containers. I labelled each punnet and proceeded to keep them watered with either tank water or gravel water for two weeks. The tomato plants treated with gravel water have reached the four leaf stage well and truly without any evidence of yellowing or stunting. If anything, they seem a bit happier than the control seedlings!
Day 21
While I will leave them to grow and continue the test, I have concluded that there is unlikely to be any glyphosate risk to froggies wishing to check into my hotel.
Much later
But, What if?
A ‘no contamination’ result is good, but what happens if your test shows up that there is herbicide contamination of whatever you are testing?
One action could be to not use the material which you have bought in, hopefully you only got a small amount so that won’t be a problem. Or send it back to the supplier, or leave it out in the weather for 6 months or a year for the herbicide to break down or leach out. Hopefully it is not a really persistent herbicide like aminopyralid, clopyralid or picloram which can even survive commercial composting. If in doubt, conduct another bioassay after a reasonable time period, just to be sure.
What if you have found that your soil is contaminated with a herbicide? In most probability it will be glyphosate (Roundup®) because of its popularity. What can you do rather than dig up all your topsoil, take it away to be dumped somewhere, then pay to replace it?
Well, now that you asked….
When I was reviewing articles on the net to help with my knowledge of the wonderful glyphosate and what it does, I came across a very well researched and detailed post on the ‘Deep Green Permaculture’ website (link below). Readers digest version – raising pH and providing calcium or magnesium ions (by say, adding garden lime, calcium carbonate) to bind it will considerably reduce the effectiveness of glyphosate. As usual, the whole story is somewhat more complicated. The whole article is well worth a read for the background information about what glyphosate is and what it does alone.
