Woods End Laboratories

Healthy Soil

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Herbicide Bioassay

Herbicide Bioassay

Bioassay for herbicide injury of 5 different composts, followed by a rank-order statistical damage code.

Woods End’s 10-day BIOASSAY detects trace levels of bioactive auxinic herbicides in soil and compost. The test uses Sugar Beets (Beta vulgaris) and Red Clover (Trifolium pretense) against a negative and positive control to detect down to 1ppb (1ng/g) of bioactive auxinic herbicide. Chemicals belonging in the class of auxins are 2,4-D, Dicamba, Clopyralid, Picloram and Aminopyralid.

Our Test Procedure requires seeds to be germinated in Standard Soil (CEN control soil) or equivalent to which are added 1 – 10- 20 -50% v/v test material (compost or soil with suspected contaminant). After 10 days growth under grow-lux light, the plants are ranked by a scale from #1-6 as determined from previous calibration assays. Results are converted to injury-index potential by PIRM software.

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THE RESIDUE ISSUE: Residues of pesticides used in lawn, garden and farm occur frequently in organic residues.  The occasion that these residues become troublesome when recycling via composting or soil application is rare. However, when present in sufficient quantity these may exercise a plant-negative “phytotoxic” influence.

HERBICIDES FOCUS: A wide category of herbicides are used in agriculture and horticulture. The types most likely to influence plants after composting are those known to have carryover potential in crop rotations. One such group is called “auxins” because they mimic natural plant hormones, are systemic, and slow to biodegrade. Farmers have learned how to manage post-application damage by choosing which crops follow in a rotation. However, this context is lacking from composting which is processed and sold into any and all markets.  An example would be applying compost to legumes (such as beans) which are sensitive to auxinic-herbicides, but application to monocots like corn would have no harmful effect.

VEGETABLES AFFECTED: Many common vegetables are partly sensitive to herbicides for which no label guidelines exist, posing a unique challenge for manufacturers and end-use. Auxins for example are not labeled for tomatoes and there is no tolerance limit. However, auxins present in compost will affect tomato growth sometimes leading to epinasty (deformation). While affected plants often outgrow the damage, some form of testing and characterization may be warranted.

LAB ANALYSIS: Laboratory testing using “GC-MS” (Gas Chromatography/Mass Spectrometry) is commonly used to pinpoint and “quantify” chemicals present at low levels in compost. Unfortunately the results of GC-MS do not necessarily relate to potential plant damage.  The selectivity of the lab test apparatus in relation to molecular composition of the target chemicals means that chemicals can be overlooked although present.

PLANTS AS BIOASSAY: The alternative offered by Woods End Lab’s is to employ plants as bioassays. Woods End has worked carefully over two decades to develop effective plant herbicide detection methods that are relevant to plant growth. The bioassay technique helps to distinguish the potential damage categorized into plant sensitivity groups. The test can predict the probable level of auxinic herbicide and to some extent it can distinguish which herbicides are present, since clopyralid, as an example, has a different plant-target profile than does picloram or aminopyralid, auxinc herbicides encountered in farming.

Plant Injury Risk Management: The final result of Woods End herbicide bioassay test is “PIRM” – Plant Injury Risk Management. With this tool the likelihood of passing on risk to customers is largely obviated.

Contact us:  if you have concerns about chemical compost residues: we’ll help you manage it.


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