Successful potting, whether small-scale on the balcony or large-scale in horticultural germ and seedling production, requires potting substrates of consistently high quality. In developing a new framework for the neutral and independent quality testing of plant and potting soil, the experts at DLG have supplemented the basic legal requirements (e.g. the German Fertiliser Ordinance DüMV) with other physical, chemical and biological criteria of potting substrates that are relevant to users.
The physical properties tested are the volume weight of the packaged goods as well as dry matter and particle size distribution. Oversized particles are a particular issue because potting soils are to partly made of compost. These oversized particles are not sufficiently broken down and as such may upset the homogeneity of the substrate. Here, an upper limit of five percent of the volume applies to grain sizes above 20mm in diameter.
In terms of chemistry, the key soluble macronutrients and micronutrients are analysed in the test and the potting soil is checked for compliance with the permissible limits for heavy metals and pollutants. Further parameters tested at the lab are salinity and pH levels.
Finally, the plant and potting soil must also be free of major human pathogens such as salmonella or mould and may contain only a maximum of three germinating weed seeds or sprouting plant parts per litre of substrate.
However, the most important test is for plant compatibility: when the germination test identifies substances harmful to plants, this results in an automatic fail.
By signing up for DLG quality seal testing, manufacturers commit themselves to the ongoing, impartial monitoring of the quality of their product by DLG. For this purpose, fully trained staff turn up unannounced at the manufacturer’s premises or at retail outlets to collect samples for analysis several times per year.