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.
BlokC.de KreijC.BaasR.WeaverG.2008Analytical methods used in soilless cultivation p. 245–289. In: Raviv M. and J.H. Lieth (eds.). Soilless culture. Elsevier London UK
BuntA.C.1988Media and mixes for container-grown plants. 2nd Ed. Unwin Hyman Ltd. London UK
DekkerL.W.RitsemaC.J.1994How water moves in a water repellent sandy soil—1. Potential and actual water repellencyWater Resour. Res.3025072517
DekkerL.W.RitsemaC.J.OostindieK.2000aWettability and wetting rate of sphagnum peat and turf on dune sand effected by surfactant treatments p. 566–574. In: Rochefort L. and J.-Y. Daigle (eds.). Proc. 11th Int. Peat Cong. 6–12 Aug. 2000 Quebec Canada
de JongeL.W.JacobsenO.H.MoldrupP.1999Soil water repellency: Effects of water content, temperature, and particle sizeSoil Sci. Soc. Amer. J.63437442
EllerbrockR.H.GerkeH.H.BachmannJ.GoebelM.O.2005Composition of organic matter fractions for explaining wettability of three forest soilsSoil Sci. Soc. Amer. J.695766
FontenoW.C.FieldsJ.S.JacksonB.E.2013A pragmatic approach to wettability and hydration of horticultural substratesActa Hort.1013139146
FontenoW.C.HardenC.T.1995North Carolina State University horticultural substrates lab manual. North Carolina State University Raleigh NC
GautamR.AshwathN.2012Hydrophobicity of 43 potting media: Its implications for raising seedlings in revegetation programsJ. Hyrol430–431111117
HannaH.Y.2005Properly recycled perlite saves money, does not reduce greenhouse tomato yield, and can be re-used for many yearsHortTechnology15342345
JacksonB.E.WrightR.D.BarnesM.C.2010Methods of constructing a pine tree substrate from various wood particle sizes, organic amendments, and sand for desired physical propertiesHortScience45103112
LeteyJ.1969Measurement of contact angle water drop penetration time and critical surface tension p. 43–47. In: DeBano L.F. and J. Letey (eds.). Proc. Symp. Water-repellent Soils
MichelJ.C.RiviereL.M.Bellon-FontaineM.N.2001Measurement of wettability of organic materials in relation to water content by the capillary rise methodEuropean J. Soil. Sci.52459467
NelsonP.V.2012Greenhouse operation and management. 7th Ed. Pearson Upper Saddle River NJ
RichardsD.LaneM.BeardsellD.V.1986The influence of particle-size distribution in pinebark:sand:brown coal potting mixes on water supply, aeration, and plant growthSci. Hort.29114