Safety of (new) raw materials

26 february 2021

The first three of five webinars for RHP certified companies took place in February. The theme of the first webinar was safety, an important point to look at in raw materials used for growing media.

A safe raw material, potting soil or growing media does not pose a threat to the culture. Or the grower, the environment or the consumer. It is good to continuously monitor safety of a raw material or additive. Think of plant diseases, weeds, human pathogens and plant toxic substances (from a previous process), but also after a possible sanitation process or an adjustment in the properties. And with biostimulants that growers want to have mixed in, you cannot just assume: "If it doesn't help, it won't harm."

Phytosanitary risks

Module 210 of the RHP product certification scheme (see: My RHP) describes the inventory of phytosanitary risks. We need to recognize and, if possible, eliminate risks in new and already known raw materials. A raw material can also have a risk that is too high, making it unsuitable for usage under the RHP quality mark. 


At certain risk levels, sanitation of the raw material is necessary, e.g. by composting, steaming or irradiation. The Bacillus globigii test can be used to assess whether a sanitation process is sufficiently effective. For compost there is a hygiene model based on temperature measurements, but RHP will also use the “Bacillus test” to be able to monitor the process better.


Weeds can carry diseases or overgrow the crop. Removing them takes time and money. Weeds in raw materials must be prevented as much as possible in the entire process, to prevent culture damage as a result. For the RHP quality mark, the weed test is carried out as a random check on the entire process from working cleanly during substrate production, from extraction of raw materials to delivery of the growing medium to the grower. Weeds are also a good indicator for other phytosanitary risks. If weeds are found in a product, this indicates that the preceding process was not sufficiently safe. The consequence may also be that the product carries more risks, which cannot be properly tested in a laboratory. In this case, process control is essential.

Plant toxic substances and visible contaminations

For example glass, plastic, stone and metal can be seen in a raw material. There are methods for identifying these contaminants. In case of secondary products from a primary process from another industry, additives may end up in the raw material. Think of polyurethane foam to which mattress manufacturers used to add dyes that are toxic for plants. Contamination of a raw material can also occur during the process, storage and transport. And then there are contaminants that are not directly harmful to plants, but for which legal requirements apply, such as heavy metals and PFAS in The Netherlands.

Residues of pesticides

If we start working with secondary raw materials from agriculture, horticulture and the food industry, residues of pesticides may come along. Herbicides are particularly harmful to plants. This will be an important new theme from now on. It weighs in the assessment of whether a raw material is suitable for making a safe substrate. And the retail sector also sets requirements in this area.

Plant tolerance

With a growth test RHP can, among other things, test the safety of a raw material on a few indicator crops. It is the ultimate test to see if a plant grows well on a raw material. Toxicity that does not show up in laboratory research, will become visible in a growth test. Recently, RHP has also been able to properly test coarser raw materials by extracting them. The growth test can now be performed on all kinds of raw materials and fractions. A safe dosage can also be considered, whether or not in combination with other new raw materials.

In short, safety is the foundation of a good growing medium and it deserves continuous attention. Next month, we will highlight pH and nutrition.

For RHP-certified companies: you can download the hand-outs of the presentations of the webinars at My RHP.

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