Vermiculite is a naturally occurring sheet silicate mineral, formed by hydrothermal alteration of minerals such as biotite and phlogopite (Brindley and Brown, 1980). Flakes of unprocessed vermiculite are mica-like and contain interlayer water within their structure. When the flakes are heated rapidly, above 870 °C, the water flashes into steam and the flakes expand into accordion-like particles. This process is called exfoliation, and the resulting material is lightweight, fire resistant with excellent insulating and absorption properties (Potter, 2000). Vermiculite has been widely used as a horticulture substrate component because of its lightweight and its capacity to hold water and mineral nutrients (Nelson, 1998). Most commonly, vermiculite has been blended with peat as a component to increase substrate water-holding capacity (Boodley and Sheldrake, 1977; Stamps and Evans, 1999). Additionally, vermiculite has been commonly used as a top coating material because it is sterile, holds significant water in its layers, and seedlings can easily emerge through it upon germination (Dana and Lerner, 2001). Walker et al. (1984) reported that watering was less frequent when a top coating was used in seed germination. A vermiculite top coat was also shown to prevent surface packing of the substrate, reduce the incidence of tipping over of seedlings, and increase the seedling rating as compared with no top coating (Walker, 1984).
Cork oak (Quercus suber), native to southwestern Europe and northwestern Africa, is a forest tree species grown in the Mediterranean basin countries, where it is exploited for cork production (Manzanera et al., 1993). Cork is produced by the cork cambium in the outer bark of the cork oak, and it is considered a natural and renewable raw product (Silva et al., 2005). Cork consists essentially of suberin, lignin, and cellulose (Kolattukudy, 1978) and also contains a small amount of fatty acids, terpenes, long chain aliphatic compounds, and saccharides (Pereira, 1988). These compounds give cork unique properties such as high elasticity and low permeability to liquids. Because of these physical properties, cork has a wide range of traditional applications, such as in green buildings and handicrafts, but cork stoppers for wine bottles remains the primary use and the highest value market (Silva et al., 2005). Waste cork from the cork-based products industry, low-quality cork, and virgin cork (rough and irregular cork bark coming from the cork tree the first time it is harvested) are all used to produce cork granulates (Gil, 2009). This granulated cork material, discarded and unsuitable for cork products production, can be used in a variety of applications and different task such as in industrial components, various fillers, insulators, or as compounds to be mixed with other materials.
Limited research has been reported regarding granulated cork in substrates. Bazzocchi and Giorgioni (1987) reported that nephthytis (Syngonium podophyllum) grown in cork oak bark had slower growth than when grown in a peat-based substrate. In this case, the lower growth rate was attributed to nutritional and water deficiencies. Aguado et al. (1993) reported that geranium plants grown in substrates with cork had lower dry shoot weights than plants grown in peat-based substrates without cork. The reduced growth rate was attributed to a possible phytotoxic effect of phenolic compounds in the cork. However, other researchers have reported that cork oak bark was a suitable substrate for growing plants. Carmona et al. (2011) reported that particle density, bulk density, effective porosity, container capacity, and aeration capacity of cork was within the requirements for nursery substrates as recommended by Carmona and Abad (2008). Additionally, Ortega et al. (1996) reported a study in which they examined the growth of different species in a phytotoxicity bioassay of cork bark substrates and found that radicle growth of chinese cabbage (Brassica pekinensis), lettuce (Lactuca sativa), pepper (Capsicum annuum), and radish (Raphanus sativus) plants was higher in water-washed cork than in vermiculite. The authors speculated that the hot water wash technique used to wash the cork before use removed phytotoxic compounds.
Despite the benefits reported for using vermiculite as a top coating in seed germination, the relatively high costs of vermiculite as a result of mining, transportation, and heating, reported environmental and health concerns (Bandli and Gunter, 2006), and the desire to use byproducts in substrates have resulted in an interest in alternatives to vermiculite in horticulture substrates. The objective of this research was to compare the efficacy of vermiculite and granulated cork as a top coating for seed germination and seedling production.
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