Peat has been the most important growing media component for decades. However, due to the ecological impacts of peat extraction, such as greenhouse gas emissions and biodiversity loss, there is a strong incentive to reduce and eventually eliminate its use. If bans are pronounced, tools are needed to assess if substrates are indeed peat-free, but no such method is available.

To address this, we developed a novel method utilizing peat-specific testate amoeba (TA) species. TA are unicellular microorganisms living in aquatic and terrestrial habitats that produce a shell which is characteristic for each species and allows identification. They are commonly used as bioindicators of present and past environmental conditions in peatlands.

The method is a decision support system based on deep learning to detect peat specific TA within extensive collections of microscopy images from commercial growing media. We identified a shortlist of species exclusive to peatlands. Samples of peat were analysed, and present TA were photographed. These images were then used to train the machine learning system.  The species Archerella flavum was found in all peat samples, and Amphitrema wrightianum was also frequent. Both species were absent in non-peat samples.

The proof of concept for this method has been successfully established, enabling the detection of peat in a sample. Our next step is to improve the robustness of the method by expanding the dataset of images and developing a quantification tool to measure the proportion of peat in samples. Additionally, we plan to include samples of cultivated Sphagnum moss to determine if the TA they harbour are different from natural peat.

We would be keen to collaborate with horticultural growing media producers and regulators.

Growing media can change in quality when packed and stored after production. Growing media in palletized bags or big-bales sometimes do show problems when applied. Components decomposed due to biological activity which, especially under anaerobe conditions, can lead by example to loss of nitrogen, bad smell or plant toxic compounds. Constituents like wood fibre, bark, rice hulls are susceptible to breakdown and green waste compost can bring in microbiome which can stimulate this breakdown. Whereas aerobe breakdown often besides loss off nitrogen does not lead to problems for the plant besides nitrogen deficiency, anaerobe breakdown sometimes does. Growing media based on various components were incubated in air tight glass vessels. By means of the The FluoMini Pro - optical O2/T sensor based on the principle of fluorescence intensity. A sensitive coating is placed on the inside of the vessel and by means of UV the fluorescence intensity is measured from the outside of the vessel. During a period of 12-16 weeks the oxygen level was measured in a high frequency. For measurement of pH, EC, macro and micro nutrients, samples were taken each 2-3 weeks. Initial results do show a major influence of the composition of growing media on availability of oxygen, as well as a sharp decline of nitrogen. Eventually a plant response test will be carried out at the end of the incubation period to observe the effect of changing nutrition as well as possible plant toxicity.

The use of renewable constituents in horticultural growing media is increasing. These originate amongst others from agriculture or wood industry, where pesticides are used for crop protection or wood preservation, respectively. To get insight into the occurrence of residues of these pesticides, approximately 350 samples of various renewable constituents were analysed on presence of residues.  A quarter of the samples showed to be contaminated with herbicides, fungicides and/or insecticides. Glyphosate and clopyralid were the most commonly found herbicides. Totally 65 different pesticides or their metabolites were detected. Mostly, these residues were related to the origin of the constituent: e.g. residues of pesticides used during previous cultivation or preservation.

Residues might be taken up by plants when grown on growing media containing residues. Presence of herbicide residues (e.g. pyridine carboxylic acids and glyphosate) can result in growth problems. Uptake of residues was tested in plant response tests. Based on the composition of the growing medium, the plant exposure to residue was calculated. Contaminated samples were selected to prepare growing media, and plant response tests were performed according to the European Standard (EN 16086-1) to investigate inheritance of residues to plants (Chinese cabbage, broad bean or tomato). For herbicides, such as glyphosate and clopyralid, the focus was also put on their toxicity levels for plants.

Unlike for food, there are no maximum residue levels for growing media. Additionally, residue levels are normally expressed on weight base. This research shows that expression on volume base, preferably at laboratory compacted bulk density according to EN 13040, is more applicable on growing media.