Wettability of Organic Growing Media Used in Horticulture: A Review

We review changes in the wettability of organic materials used in horticulture in relation to their water content and impacts on their physical properties. Beyond horticultural growing media, this approach could be generalized to understand the physical behavior of water-repellent soils and determine the effects of organic matter on some physical characteristics (e.g., soil structure, runoff risks). Hydrophobicity of organic matter is the primary factor affecting soil physical properties such as soil structural stability after drying. In horticultural production, the measurement of wettability is even more relevant due to the large use and high content of organic materials in growing media and to the large and rapid variations in water content in the media (due to the limited volumes of pots or containers), both of which can quickly create hydrophobic conditions. This review discusses the methods used for measuring the wettability of growing media, the evolution of wettability vs. water content, ways for limiting hydrophobicity, and finally the relationships between growing media wettability and physical properties. In addition to the indirect estimation of hydrophobicity based on physical properties such as water holding capacity, contact angles obtained from capillary rise and hydration efficiency tests are the most common methods describing the changes from hydrophilicity to hydrophobicity of organic growing media in relation to water content. Wetting agents and clay have been shown to reduce the water repellency of growing media under the driest conditions. A general trend of increasing water repellency during desiccation from highly decomposed peat > bark > weakly decomposed peat > wood products > coco fiber has been documented. As for water retention, hysteresis phenomena in contact angles have been shown during drying–wetting cycles. The influence of hydrophobicity on water retention characteristics has also been modeled, providing evidence that hydrophobicity is a function of the growing medium pore size domain. The reciprocal effects of physical and biological roles of roots on the wettability and physical properties of growing media should be investigated in future research.