How to Conduct a Bioassay Test

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Conducting a bioassay test is a way to determine if composted horse manure contains persistent herbicides that could be detrimental to plants. It’s not that complicated, but there are a few steps to follow to ensure reliable results.

Once contaminated horse manure has been added to your garden beds, it may create problems that take years to overcome. I learned this the hard way, and I’d like you to learn from my mistake.


Joe with tomato seedlings

A bioassay test is not complicated, and no special equipment is required.


Why Perform a Bioassay Test

While composted horse manure can be a great nitrogen-rich amendment for garden soil, the wrong horse manure can be devastating to plants. The problem starts when a horse is fed hay that’s been treated with persistent broadleaf herbicides. Those herbicides don’t break down in the horse’s digestive tract and may not break down after one season in a compost pile either. If that composted manure is added to a garden, the herbicides can kill vegetables and other plants. Not all plants are as susceptible to herbicides but some like beans and tomatoes are especially sensitive.

According to the U.S. Composting Council, persistent herbicides can remain in manure for three months to three years, if not longer, before breaking down into inert compounds. In my garden, contaminated manure caused problems for four years.

Maybe the manure you plan to use came from horses that were never fed hay treated with persistent herbicides. Or maybe the manure decomposed long enough that the herbicides have become inert. Either way, you won’t know whether the manure is risky until you’ve planted something in it.


Tomato seedling damaged

This tomato seedling was planted in contaminated soil and shows signs of foliage deformation.

Start with a Clean Control

To test whether composted manure is contaminated or not, you’ll need to mix it with soil and plant something in it. But what if that soil was contaminated somehow? That’s where the “control” comes in. 

A bioassay test starts with soil from a reliable source that you know is free of persistent herbicides.

When I need soil for control, I use Soil3, a clean and trusted source that is a humus compost created in a high heat composting process using grass clippings, wheat straw and cow manure. I’ve used this product for years and I know I can trust it. You can use any bagged soil products or you can get bulk soil from a trusted vendor.


Tags in pots of soil

Start with the control group and label it, then move it out of the way before potting the experimental group to avoid any cross-contamination.


Control Group & Experimental Group

The control group will contain just your trusted soil. The experimental group will contain that trusted soil plus some of the composted manure. It’s important to have at least three in each group.

Start with the control group, labeling each pot (including the date of planting) so you don’t confuse the samples for ones in the experimental group later on. Plant each pot with seedlings of the same variety that are approximately the same size. Move the potted seedlings out of the way before moving on to the experimental group to avoid any risk of cross-contamination.

The only variable in the experimental group should be that the pots are filled with the composted horse manure mixed with trusted soil, blended at a ratio of 50-50. The pots and the seedlings should be exactly the same as the ones used in the control group.

I use tomato seedlings in my bioassay tests. You can conduct a bioassay test using seed, but it will take longer to get results. I prefer to use seedlings because as soon as the roots begin to grow out, the plants will begin to show signs of problems with that soil. If you do decide to use seeds, beans are a good choice because they germinate and grow quickly.


This video shows a side-by-side comparison of my tomato plant control group and those planted in the questionable compost.


Identifying Problems Caused by Contaminated Soil

If the 50-50 mix is free of herbicides, seedlings will love it. The manure provides lots of fertility that the plants will enjoy. But if there is an herbicide present, you’ll know it soon.

Plants affected by contaminated soil will look stunted or will have leaves that look deformed or really small. Also look for new growth and coloration and if the main stem has split. A side-by-side comparison of the control group and the experimental group will reveal whether the plants look like healthy plants should. 

With seedlings, you’ll see signs of problems within two weeks at most. With seeds, you have to wait for germination, sprouting and root development — which can take three or four weeks.

If the manure was contaminated, you have sacrificed a few seedlings to perform this test. That’s a drag, but believe me that it is better than contaminating your garden beds.


Beans in suspected killer compost

Bean seeds or seedlings are good candidates for a bioassay test. These beans show signs of herbicide damage.


Links & Resources

029-My Five Biggest Gardening Mistakes of All Time (and What I Learned From Them)

joegardener Blog: Backyard Composting: A Simple Recipe for Making Great Compost

joegardener Blog: The Complete Guide to Home Composting 

joegardener Online Gardening Academy: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.

joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.

joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges.

joegardenerTV YouTube

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joegardener Facebook

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Growing a Greener World®

U.S. Composting Council: Persistent Herbicide Incidence Reporting Form

U.S. Composting Council: Persistent Herbicide Information

Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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