Root substrates (substrates) are commonly used in the production of containerized greenhouse and nursery crops (Bunt, 1988; Nelson, 2003). Substrates are formulated from various inorganic and organic components such as peat, perlite, aged bark, rice hulls, various composts, and vermiculite to provide suitable physical and chemical properties as required by the specific crop and growing conditions (Bunt, 1988; Raviv and Lieth, 2008).
Among the most commonly evaluated and reported physical properties of substrates are bulk density, total pore space, air-filled pore space, and water-holding capacity. Bulk density refers to the weight of a given volume of substrate and is most commonly reported as grams of dry weight per 100 cm3 of substrate (Bunt, 1988; Wallach, 2008). Substrates with low bulk densities are usually preferred because they facilitate handling and reduce shipping costs (Bunt, 1983; Wallach, 2008). However, a relatively high bulk density is desired when plant stability is an issue such as occurs with tall or top-heavy crops. Total pore space represents the volume of the substrate not occupied by substrate solids. Most greenhouse root substrates have a total pore space of 60% to 85% (Arnold Bik, 1983; Boertje, 1984; Jenkins and Jarrell, 1989). Air-filled pore space is the volume of the substrate that drains after a saturating irrigation. Air-filled pore space is important because it provides for gas exchange between the root environment and the atmosphere outside of the container. An air-filled pore space of 10% to 20% has commonly been recommended for substrates to be used to produce greenhouse crops (Jenkins and Jarrell, 1989).
Water-holding capacity, often referred to as container capacity, represents the volume of water retained by a substrate immediately after a saturating irrigation and drainage and is expressed as a percentage of total volume or weight of the substrate. A water-holding capacity of 45% to 65% (v/v) has been commonly recommended for substrates for greenhouse crop production (Arnold Bik, 1983; Boertje, 1984; Jenkins and Jarrell, 1989).
The water-holding capacity of a substrate for greenhouse and nursery containers is further categorized as easily available, available, and unavailable water. The water held against tensions of 1 and 10 kPa is defined as available water (Bunt, 1988; De Boodt and Verdonck, 1972). The water held against tension between 1 and 5 kPa is defined as easily available water, and water held against tension higher than 10 kPa is defined as the unavailable water (Ingram et al., 1993).
These methods of describing substrate water content have significant limitations. The term availability and the specific pressures are not universally agreed on. The tensions of 1, 5, and 10 kPa are arbitrary and do not have physiological meaning across greenhouse and nursery crops. In fact, the limit of a tension of 10 kPa was suggested by De Boodt and De Waele (1968) because it strongly inhibited Ficus growth; thus, the definition of water availability is species-specific. There is no exact tension at which water is unavailable to plants in general (Buamscha et al., 2007).
Water-holding capacity itself is a time-specific measurement that is limited to the status of the substrate immediately after saturation and drainage. It does not provide information regarding how quickly water is lost from the substrate, the substrate water status over time, or the irrigation frequency required for a substrate under specific conditions. Furthermore, in a given container size, water-holding capacity is a function of the volume and size of pores in a substrate. Large pores drain after irrigation and provide air-filled pore space, whereas small pores retain water after irrigation. However, the terms large pores and small pores are relative. Evaporation has been shown to account for 60% of the applied water lost from the substrate in the first 3 weeks after planting (Argo and Biernbaum, 2005). The rate at which water is lost from a substrate resulting from evaporation depends on several factors. Beardsell et al. (1979) demonstrated that as the pore size decreased, the tension with which water was held, increased, and thus as the pore size decreased, the resistance to evaporation increased. Therefore, water-holding capacity only accounted for whether a pore was small enough to retain water at container capacity. It did not take into account that the water being retained occurred in pores of different sizes that held water at different tensions. These different populations of water would evaporate at different rates as a result of the different pore sizes. Therefore, two substrates could theoretically have the same water-holding capacity but lose water to evaporation at different rates and the two substrates would dry at different rates. As an example, Beardsell et al. (1979) reported that peat had a higher water-holding capacity than pine bark or sawdust but peat lost water as a result of evaporation more rapidly than these two other organic components. The authors concluded that this was the result of the wicking effect of the peat fibers increasing the rate of evaporation. In addition to the specific pore sizes, other factors such as substrate color and thermal characteristics can affect the rate of water loss resulting from evaporation.
The frequency of irrigation depends on both the water-holding capacity as well as the rate of drying of the substrate. Water-holding capacity, the rate of drying, and how frequently a substrate must be irrigated all relate to what greenhouse managers and substrate manufacturers have generally referred to as a substrate's wetness. Greenhouse managers and substrate manufacturers have generally described substrate wetness by referring to a substrate as being a “dry” substrate or a “wet” substrate. Substrate wetness is an important characteristic because it affects the amount of water required to grow a crop, the irrigation frequency, and fertilization frequency if a liquid fertilization program is being used. Although the term substrate wetness is not a specific term for substrates in the scientific literature, it is one in which the horticultural industry is particularly interested and a measure of substrate wetness that accounts for each of these three variables would be a valuable tool.
The objectives of this research were to 1) develop a method of measuring substrate wetness that included the variables of water-holding capacity, drying rate, and irrigation frequency; 2) to create a single numeric value that could be used to describe the wetness of a substrate and allow comparisons of wetness among substrates; and 3) to determine if the procedure provided reasonable or expected values and what level of variation might be inherent in such a procedure.
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Buamscha, M.G., Altland, J.E., Sullivan, D.M., Horneck, D.A. & Cassidy, J. 2007 Chemical and physical properties of douglas fir bark relevant to the production of container plants HortScience 42 1281 1286
Bunt, A.C. 1983 Physical properties of mixtures of peat and minerals of different particle size and bulk density for potting substrates Acta Hort. 150 143 153
De Boodt, M. & DeWaele, N. 1968 Study on the physical properties of artificial soils and the growth of ornamental plants Pedologie (Gent) 18 275 300
Evans, M.R. & Gachukia, M.M. 2007 Physical properties of sphagnum peat-based root substrates amended with perlite or parboiled fresh rice hulls HortTechnology 17 312 315
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