Necessity for indicator plants to test contamination of new constituents in growing media
There is an increased demand to use other constituents in horticultural growing media. Often these products consist of recycled materials. Also organic fertilizers are based on remainings of agricultural materials. Besides knowledge on their chemical, biological and physical characteristics, it is important to know that these products do not contain harmful levels of pesticides. Some of these products, like rice hulls and bark, or ingredients of compost and organic fertilizers may have previously been in contact with pesticides. The use of pesticides contaminated constituents will affect growth of certain sensitive plants. By composting, residue levels sometimes are reduced. However, in compost made from materials that have been treated with persistent herbicides, like carboxylic acid herbicides, there may retain harmful levels of residues. Analytical screening is not sufficient, as the No Observable Effect Levels on sensitive plants are similar to or lower than the practical limits of detection. To determine the plant response of soil improvers and growing media the European Standard (EN 16086-1) test is used, with Chinese cabbage and barley as test species. As these species are not sufficiently sensitive for carboxylic acid herbicides, there is a need for extension of this test with indicator plants that show contamination of a constituent. A series of plants was tested with different concentrations of carboxylic acid contamination and symptoms were described. A risk assessment with regard to possible contact with pesticides is advised for growing media products of a recycled origin or a material with an agricultural background. In case of a risk on contamination with a pyridine carboxylic acid, it is recommended to expand the European Standard (EN 16086-1) with the indicator plant dwarf bean or (preferably) broad bean to test on sensitive residue levels.
Geuijen, W.H.C. and Verhagen, J.B.G.M. (2019). Necessity for indicator plants to test contamination of new constituents in growing media. Acta Hortic. 1266, 269-274
compost, growing media, herbicide contamination, indicator plants