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.

This article identifies criteria for assessing the physical properties (water retention characteristics, wettability and physical stability) of growing media which influence the availability of air and water to plant roots. The various materials that are currently in use are assessed for these properties. The analysis of physical properties indicates that weakly decomposed (H1–H5, generally referred to as white) Sphagnum peat is still indispensable for soil-less horticulture. Whilst a number of materials can be used as peat additives, especially to improve aeration, no alternative products with equivalent physical properties are available at present.

The precise measurement of the water retention and shrink/swell properties of growing media or soil over time is important for the effective management of irrigation and fertilization. A new apparatus was developed for simultaneously and continuously measuring the water retention and shrink/swell properties of growing media during several drying/wetting cycles with varying intensities (0↔–5 kPa, 0↔–10 kPa, 0↔–32 kPa). The measurements on slightly decomposed Sphagnum peat showed encouraging results. Regardless of the intensity of drying, water retention and shrink/swell properties are mainly modified after the first drying process, resulting in degradation of density and water retention, whereas these properties are unaffected by the other cycles, even if hysteresis phenomena are always shown to take place. Variations in drying intensity reveal different physical behaviors with an inflection point observed (i.e., a change in physical behavior) for the shrink/swell and water retention curves for the highest intensity tested (from –20 kPa).

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 role of clay incorporation on the physical properties and wettability of peat-growing media was assessed from water retention curves and from contact angle and water drop penetration time measurements, respectively. Two peat substrates presenting different degrees of decomposition (weakly and highly decomposed Sphagnum peat) were used and mixed with clay in the form of powder with a peat:clay ratio of 90:10 (by vol.). Results indicated relatively little change in water retention resulting from clay incorporation in the peat-growing media tested. On the other hand, they showed a significant improvement of the wettability of both substrates in the driest conditions when peat-growing media presented a pronounced hydrophobic character, whereas the wettability of materials was not changed in the wettest conditions when they were largely hydrophilic. The influence of the degree of peat decomposition was also observed with somewhat higher wettability for the weakly decomposed peat:clay mixture than for the highly decomposed peat:clay mixture. Because the hydrophobic character of highly decomposed peat is more pronounced and appears at a higher water content than weakly decomposed peat, the effect of clay addition also appears at higher water contents for highly decomposed peat. Incorporating clay into peat-growing media should be considered for its ability to improve the wettability of growing media with a hydrophobic character (i.e., to improve the ability of the growing media to be rewetted) rather than only its ability to influence the water retention characteristics of the growing media.

ISHS International Symposium on Growing Media and Soilless Cultivation IMPORTANCE OF PEAT PARTICLE SIZE EFFECTS ON SPATIAL ROOT DISTRIBUTION AND CHANGES ON AERATION PROPERTIES