Detecting persistent herbicides in compost

The main use of green compost is in agriculture and because application rates are relatively low it is unlikely to cause problems related to herbicide residues.  Green compost is also a valued product used in sustainable growing media.  This work is targetted at growing media manufacturers and explains how they can screen composts to avoid possible contamination from persistent herbicides.

Clopyralid and aminopyralid are herbicides that retard the growth of some plants by mimicking natural plant hormones (auxins).  They have been licensed internationally to control annual and perennial broadleaf weeds.  When the manufacturers’ guidelines are followed, residues should not be present in materials destined for composting, and the resulting composts will present no risks.  However, herbicide residues have been identified in composting feedstocks and compost based products as a result of herbicide users not following the guidelines.

WRAP carried out research to develop a robust test that growing media manufacturers can use to identify green composts that contain herbicide residues.  That test is already in use and recommended by the Compost Certification Scheme for compost destined for use in growing media.


The test is a bioassay that uses field beans that are grown in the compost under analysis.  Thorough testing has found that field bean is very sensitive to herbicide residues and a reliable species for use in this test.  The research found that herbicide residues are quite common in green composts, but because the field bean test is so sensitive, it can be used to identify unsuitable composts before they are used in sensitive situations.