Step-by-step Guide to Conducting a Bio-assay

A quick and easy way to determine if soil is contaminated is to conduct a bioassay. In general, a bioassay is any test that uses a living organism to test for activity of a substance. For our purposes, we are going to use the seed of herbicide-sensitive plants to test for herbicide activity. This can be done during the winter in a warm greenhouse. To conduct a bioassay, follow these steps:


1. Collect enough soil from the field in question to fill a quart-size container. Also collect samples from a field with similar soil characteristics known to be free of herbicides.


2. Conduct the experiment in a greenhouse. Make sure seedlings receive full sun conditions. Many herbicides require high light levels to function, and their effects could be masked under low light conditions.


3. Sow the containers with radish seeds to test for broadleaf-active herbicides and oat seeds to test for grass-active herbicides. Cover the seed with a little soil, just as they would normally be grown in the field.


4. Water the containers to maintain soil moisture. Do not over-water.


5. After seedlings germinate, thin them so there are just 2 or 3 plants per pot. Observe the seedlings for several weeks, comparing the potentially contaminated soil to the ‘control’ soil. Some herbicide injury takes longer to manifest than others, so be patient. Look for differences in plant size, foliar spotting, foliar wrinkling, twisted stems, and anything else that looks out of the ordinary. When finished, shake the soil off the pots to observe root development. Again, observe control pots to know what is ‘normal’.