In last week’s conversation we looked at synthetic auxins, a group of persistent herbicides that can have devastating impacts on broad-leafed plants like beans and peas, tomatoes and eggplants and pumpkins and cucumbers, causing stunting, curled leaf tips and in some cases death.
Synthetic auxins are coming into our food systems in commercial composts, garden soil blends and ingredients that might go into our home composts.
Lab testing for synthetic auxin residue is prohibitively expensive for most commercial operators, let alone backyard growers, but there is a pretty simple test we can do if we want to feel more confident spreading an unknown garden product across our precious soils, and it is as simple as raising seedlings!
A ‘bioassay’ is a test to see if plants (or other life-forms) are adversely affected by some kind of condition, be it soil, water or air etc. In this case we want to test if the compost we have made, or a commercial product we have bought, has any residual synthetic auxin in it.
Our bioassay would go something like this: We take reliable seeds from a species known to be sensitive to contamination (broad beans are a good choice for synthetic auxins and can be propagated year-round) and plant a few seeds (5 – 10) in a sample of soil that we know to be uncontaminated, this is the ‘control’ in our experiment.
At the same time, we also plant some of the same seeds into the material we are wanting to test, and plant both samples into a similar sized pot or tray. If the test material is not really a good growing medium on its own, it might need to be cut with other material, but we’ll need to be sure this additional material is clean and uncontaminated too, so chose something you have grown successfully in previously.
We then clearly label our two samples, water them and put them in a sunny spot and look after them like we would any other seedlings. When, or if, the seeds start to emerge, keep notes of the differences in growth patterns between the two samples.
This test will likely take two to three weeks, as the second set of leaves need to emerged before we draw any conclusions, as herbicides don’t always impact the first set.
If we are testing for synthetic auxins, we will be looking for our test seedling to show stunted growth, curling leaf edges or just failure to sprout. To be sure, we check against the growth in our control plants, if they are thriving, and our test plants are struggling, we can be pretty sure the problem is in the ‘suspect’ material. If both sets of plants are doing fine, we can feel more comfortable using the tested material on our precious garden soil.
Given the number of contaminations we are seeing in the region recently I think the simple bioassay is a test worth having in our ‘tool kit’.
– Joel Meadows works with *Yes In My Back Yard, (YIMBY), a community-scale composting initiative in Castlemaine and surrounds. Send questions or comments to hello@yimbycompost.com, or to book in for a compost workshop!
