Peats are by far the most widely used components of horticultural substrates [1
], mainly due to their physical properties. Their sources of supply are very important worldwide, with a not very prohibitive cost. However, its use is increasingly being discussed because peat is a fossil and non-renewable resource, and peatlands have a major environmental function of storing large quantities of carbon [2
]. Although the use of peat by horticultural activities accounts for only 30% of its consumption and only 0.02% of the peatland area worldwide (800 km2
for a total of about 4 million km2
), new European incentives are aimed at limiting the exploitation of wetlands and rehabilitation of peatlands after extraction [3
Thus, substrate companies are seeking to reduce peat content in their recipes by incorporating more renewable organic materials with a lower carbon footprint, while maintaining or even improving the agronomic qualities of substrates. These mixes usually aim to increase the air-filled porosity (AFP) of peat-based growing media using peat substitutes with coarser porosity. More recently, emerging works are looking for organic alternatives to synthetic wetting agent used for reducing the risks of peat hydrophobicity occurring during its drying, by promoting mixes with both complementary and more hydrophilic peat alternatives.
Hydrophobicity is a major potential risk for many organic substrates, especially peat-based substrates [5
]. Several sources can lead to a degradation of wettability and then water capture and water retention properties induced by the acquisition of hydrophobic properties. Michel et al. [6
] showed a lower root development in some different substrates managed with a too restrictive irrigation. They also measured a large decrease in the ability to capture and to retain water in peat:bark mixes after open-air storage for few months. The change from hydrophilic to hydrophobic character during drying of peats and barks has been demonstrated from contact angle measurements [7
]. This decrease in wettability was also observed by Fields et al. [8
] and Michel et al. [6
], on the basis of hydration curves for a large majority of substrates depending on their moisture content. In most cases, the drier a substrate, the more it exhibits hydrophobicity, leading to increase difficulty to rewet and to recover its initial retention properties. Considering data in the literature obtained from both contact angle measurements and hydration efficiency tests, a classification of the wettability of materials was established by Michel [5
]: black peat < bark < white peat < wood-based products < coir.
The market for wood fiber as a substrate component has been expanding worldwide for about ten years, although its industrial development started in the 1970’s [10
]. Wood fibers can be distinguished according to the tree species used, mainly conifers (Pinus
), due to their lower phytotoxic molecule content compared to hardwood species [11
], and the defibration (twin-disc refiner, extruder, hammer-mill) processes. This interest in for wood fiber is due to its wide availability around the world, its renewability and reduced carbon footprint compared to peat or other materials, as well as its low production cost. Literature on the physical properties of some wood fibers reports low water holding capacity around 0.25–0.35 v
, air-filled porosity varying from 0.5 to 0.7 v
, and water buffering capacity close to 0, therefore wood fiber is mainly used in mixes for its aeration properties [12
]. Additionally, Jackson [14
] reported a higher root growth due to an increase in air content induced by wood fiber addition in peat-based substrates.
The objective of our work was to study the rehydration properties of wood fiber and its influence on mixes with peat in different proportions. This work also aimed to quantify the influence of the initial water content (i.e., the intensity of desiccation) on the rewetting properties of raw materials and mixes.