The risk for N immobilization in organic renewable materials is an important criterion for selecting materials for growing media and for optimizing the fertilizer application in blends with these materials. Shorter methods are preferred for practical reasons while some effects related to N immobilization can only be assessed in the long-term and the risk may change over time. More than 50 different batches of materials were collected in 2024 from different processing plants or growing media producers. These materials were clustered in categories based on the type and processing of the material, i.e., wood-based biochars, wood fibers, straw-like fibers, green plant fibers, composts, products based on manure or digestate, bark, peat and coir products. Risk for N immobilization of these materials was assessed with five different protocols. The protocols differ in length (5 to 128 days), temperature (15-36°C), amount of N that is supplied per L of growing medium (70-560 mg N/L substrate) and type of N fertilizer (pure chemicals with NO3-N or NH4-N, mineral or organic fertilizers). The specific conditions of each protocol allow to provide complementary information on the N immobilization process. The outcome of the protocols was compared, taking the effect of the category of the material into account. Practical aspects are discussed for each of the protocols in relation to specific categories of materials. This research allows to advice on the best protocol to assess the risk for N immobilization for different categories of renewable organic materials for horticultural substrates.

Mushrooms (Agaricus bisporus) are an economically important crop worldwide. Current production methods utilise peat, which forms the casing layer. Peat functions both as a water reservoir and to induce formation of fruiting bodies. Due to endeavours of many countries to reduce peat use, the industry is under pressure to find effective alternatives. An ideal candidate material would have characteristics similar to peat, such as water holding capacity and porosity. Despite previous efforts assessing the performance of novel casing media, suitability of peat-alternatives remains largely uncertain.

This research develops and optimises an experimental method for culturing A. bisporus in microcosms and to utilise these in evaluating novel characterisation methods, including X-ray computed tomography (CT). Cylindrical containers of 500 ml, following a watering and incubation regime based on industry practices, were determined to provide the best trade-off between commercially representative mycelial growth and scan resolution, based on growth variability between replicates.

X-ray CT was proven to be effective in demonstrating the physical characterisation of the internal structures of casing materials. High-resolution measurements of air-filled porosity and pore surface area showed clear distinction between peat and alternative materials. In particular, porosity of bark-based casing materials was found to increase significantly (P < 0.05) between starting the experiment (Day 0) and the first flush of mushroom growth (Day 23). Additionally, binary images were used to assess casing colonisation. Furthermore, ongoing research is being conducted to determine if it is possible to visualise and segment hyphae with X-ray CT by adapting methods pioneered to analyse roots. This could allow unprecedented understanding of fungal growth response to different casing materials.

Water repellency is one of the most issues for some growing media (GM), limiting their ability to rewet and therfore to properly and efficiently supply water and nutrients to the plant root system during irrigation.

A new cutting-edge technology, based on direct contact angle measurements of water droplets deposited on the surface of single particles, has been implemented to directly measure the changes in wettability for various growing media constituents according to their water content.

Two samples (white peat and wood fiber) previously equilibrated at 66%, 50%, 40%, 33%, and 15% moisture content by mass (w/w) were selected as reference raw materials for this study.

For each modality, 40 drops of water were deposited on 40 different particles selected as randomly as possible to obtain a representative measurement of the contact angle.

The results showed that wood fiber progressively changed from a very hydrophilic (66% w/w, 50% w/w) to a slightly hydrophilic (15% w/w) character during drying, while white peat became slightly hydrophilic from 50% w/w.

Also, thanks to a high precision in the contact angle measurement, a large heterogeneity in wettability has been observed due to the diversity of particles. More than the mean contact angle, the wettability of a material can also be characterized by its distribution, extent or the median of the values measured on a set of particles. 

An detailed description of particle morphology and the analysis of its relationships with physical and hydraulic properties may help GM manufacturers to optimize the selection of raw materials, and of particle size fractions, used as growing media constituents. To this end, our recent works have thus allowed to describe relationships between particle mean particle length and width and static physical properties (i.e. water holding capacity, WHC, and air filled porosity, AFP), using dynamic image analysis (DIA) and Hyprop systems, respectively.

To continue these investigations, similar analyzes were carried out on different particle size fractions of some raw materials (here, a white peat) with additional measurements of saturated hydraulic conductivity. Analyzing these results has allowed to assess pore tortuosity and air diffusivity for each of the particle size fractions.

As for static physical properties, flow properties are also related to particle morphology. Logarithmic relationships were observed between peat particle morphology (length and width) and both saturated hydraulic conductivity (Ks) and air diffusivity (DO2). A plateau (i.e. maximum values) for Ks and DO2 was reached for a mean particles length bigger than approximatively 2 mm, as already observed by Durand et al. (2023) for static physical properties (WHC and AFP).

This work also confirms that mean particle length is a relevant parameter for predicting water and air flow properties.