Peat substrates are well known to become hydrophobic during desiccation, thus degrading their water retention properties. Synthetic wetting agents are commonly incorporated to limit the risk of hydrophobicity, but substrates companies are searching for more sustainable alternatives. To that end, the effect of wood fiber addition in peat-based mixes was measured using contact angles and hydration curves. The study was carried out on two raw materials (white milled peat and wood fiber) and binary mixes. The results showed a shift from hydrophilic to more hydrophobic character with a decrease in the ability to rewet of peat-based substrates in relation to the intensity of drying, whereas wood fiber remained hydrophilic. Increasing wood fiber content in peat-based mixes improved the rehydration efficiency, but with a lower intensity of that measured with synthetic wetting agent addition. Our results highlighted the hydrophilic nature of wood fiber and demonstrated an additional benefit of wood fiber use in peat-based growing media.

The physical properties of growing media are dependent on the morphological characteristics of the particles composing them. Thus, their characteristics can be more precisely altered for specific purposes by a better morphological design of materials to optimize the use of raw materials and increase water efficiency. There are many references on the relationship between basic particle size and physical properties, but the arrangement of the particles and the resulting physical properties are also affected by the shape of the particles. Growing media have seldom been characterized by shape criteria and, therefore, their influence remains unknown. A dynamic image analyzer, the QicPic device, was used to assess particle shape and size for a wide diversity of growing media constituents. As well as Feret_MAX and Chord_MIN diameters describing individual particle length and width, respectively, individual particle shape was analyzed in terms of several descriptors (aspect ratio, circularity, roundness, and convexity). A classification was established to discern different particle shapes and all materials were described accordingly. Correlations between particle morphology descriptors were reported, showing that the greater the particle length, the smaller the width/length ratio, circularity, roundness, and convexity. Circularity, roundness, particle length, and its associated relative span were identified as the most relevant parameters describing materials’ morphology. This work shows a large diversity in particle morphology of growing media constituents, which were categorized into four classes of materials. Three classes were mainly described according to their particle shapes, with a decreasing elongation and an increasing circularity, roundness, and convexity: (1) fine and coarse wood and coir fibers; (2) all Sphagnum white peats, milled or sod; and (3) black peats, sedge peat, coir pith, fresh and composted pine bark, green waste compost, and perlite. A fourth class was represented by coir medium (mixing pith and fibers) and was above all characterized by high diversity in particle length. These findings extend the characterization of the materials for a more thorough evaluation of the links between particle morphology and physical properties.

The use and exploration of more sustainable materials for growing media, in response to the search for alternatives to peat, are expanding. Many locally available organic resources are currently under study for this purpose. However, their biological stability remains a major limitation. Understanding the effect of biodegradation on the physical properties of these materials, in order to assess their long-term suitability for ensuring water supply and root system respiration, remains largely unexplored. The objectives of this research were to evaluate the biological stability of various organic materials over a 6-week biodegradation process; to investigate how biodegradation alters key physical properties; and to determine whether these alterations stem from changes in particle size distribution. Initial and post-biodegradation physical properties and particle size distribution were analyzed. Biodegradation was measured via respirometry. The protocol employed aerobic conditions, fertilizer addition, and a temperature of 35°C to reach a substantial biodegradation rate. As a result, the methodology employed led to biodegradation rates comparable to, or exceeding, those reported in the literature. No significant changes were observed in particle size distribution after biodegradation, probably due to overall low biodegradation rates coupled with heterogeneous particle sizes. No significant changes were observed in total porosity or available water either. The significant changes observed were a decrease in air-filled porosity, resulting from a general increase in water retention properties at suctions of −10 cm and above. These observations therefore suggest the hypothesis of a change in the integrity of the particle surface, with the formation of fine pores, rather than fragmentation leading to changes in particle size and organization. These observations highlight the importance of assessing changes in physical properties during relatively long biodegradation processes for potentially biologically unstable raw materials, in order to adapt agronomic practices to the changing properties of the materials.

Growing media constituents have heterogeneous particle size and shape, and their physical properties are strongly related to them. Particle size distribution is usually analyzed through sieving pro...