Wettability of a material was defined by Letey et al. (1962) as the ability of a liquid to spread over a material’s surface. In substrates, proper wettability ensures a more even distribution of water (and nutrients) throughout the root environment. Appropriate wettability also improves water-holding capacity, which has been shown to increase plant growth (Plaut et al., 1973). Horticultural substrates often have wettability issues resulting from the nature and high volume of organic matter (OM) components in them. These components, primarily composed of sphagnum peatmoss and pine bark, can become hydrophobic, thus reducing wettability (Dekker et al., 2000a; Michel et al., 2001). The molecules of OM contain many organic acid functional groups on their exterior surfaces, like carboxylic acids and phenolic acids, among others. These acidic functional groups tend to repel water from the particle surfaces when in a balanced state with hydrogen cations bound to oxygen anions (Ellerbrock et al., 2005). As substrates dry, hydrophobicity can intensify, complicating the wetting and rewetting process during plant production (Valat et al., 1991). Thus, many organic substrates can develop hydrophobicity issues that hinder water efficiency (Beardsell and Nichols, 1982).
There are several factors that can influence a substrate’s wettability, including, but not limited to, MC (de Jonge et al., 1999; Michel et al., 2001), substrate pH (Gautam and Ashwath, 2012), hydrophobicity of the substrate (Fonteno et al., 2013), and preferential flow (Dekker and Ritsema, 1994). Measurement of substrate wettability has been difficult to assess with the most common method in the literature being the measurement of contact angles (Michel, 2009). Another method for measuring substrate wettability described by Letey (1969) and re-evaluated by Dekker and Ritsema (2000b) is known as the water drop penetration time (WDPT) test. To test WDPT, a drop of water is placed on the surface of a substrate and the time it takes for the drop of water to completely penetrate the substrate is measured. This method is less expensive to perform; however, results can vary as a result of the subjective nature of this test. A more recent method described by Fonteno et al. (2013) for determining the wettability of a substrate is known as the hydration efficiency test. In this method, known quantities of water are passed through a substrate and effluents are collected to determine the quantity of water sorbed by the substrate.
Wetting agents (WA) are chemicals (dry or liquid form) that increase the wettability of substrates by enabling substrates to be more uniformly wet during/after irrigation events. Wetting agents are used to change the properties of water by allowing the individual water molecules to break some of their hydrogen bonds and spread out more evenly over the surface of a substrate. Wetting agents, like all surfactants, are chemically composed of two parts, a hydrophilic hydrocarbon tail and a hydrophobic lipid head. The hydrophobic end will adhere to the surface of the substrate particle leaving the hydrophilic end exposed. The water molecules will then bind to the hydrophilic end and spread out across the surface of the particle. This reduces surface energy of the solid particle and promotes a more uniform distribution of water over the surface. Wetting agents are commonly used in many substrates to achieve proper hydration with fewer irrigation events after potting.
Hydration efficiency was defined in this study as the ability of a material to capture and retain water in the fewest number of hydration events (water applications). The objectives of this study were: 1) to characterize the wettability of traditional substrate components and compare them with two newer pine tree substrate components; and 2) to determine hydration efficiency of these components.
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