Biochar is a term that refers to a black carbon-rich material that is produced from organic matter at temperatures lower than 700 °C in an oxygen-limited atmosphere (Lehmann and Joseph, 2009) and is generally considered to be similar to charcoal. There are different processes used to make biochar, including pyrolysis, gasification, and torrefaction, and these processes may differ in temperature, residency time, and oxygen availability. Although different processes have their advantages and disadvantages, researchers worldwide are working on optimizing and evaluating biomass conversion processes to improve quality and performance of biomass-based production of fuels, chemicals, and biochar. Specific and unique properties of each biochar product depended on the properties of the original feedstock material (Altland, 2014) and method of production employed (Spokas et al., 2012). It has been reported that the higher the temperature, the smaller and more porous the resulting biochar particles became (Kloss et al., 2012). These smaller particles tended to have proportionally more surface area (Shackley et al., 2013) which had benefits such as an increased CEC.
Due to its high carbon concentration, biochar has the potential to be used in a number of applications including soil conditioning, as activated carbon or in chemical manufacturing. The application of biochar to soils contributed to the sequestration of carbon from the atmosphere, since carbon captured from the environment by the biomass was shown to be retained in the soil (Manya, 2012). Wood contained around 50% carbon that increased to 70% to 80% once it was processed to biochar. This carbon could be stored from the atmosphere when applied to the soil (Winsley, 2007). In addition, the utilization of biochar improved the quality of the soil because of its sorption qualities that helped to retain nutrients and nitrogen (Ippolito et al., 2012).
The components of horticultural substrates used in commercial greenhouse and nursery operations can be a major production cost, as most customary components are commonly shipped from outside the United States or manufactured substrates are transported significant distances from the production facility to the end user. Interest in using biochar for horticultural purposes has increased substantially in recent years due to its potential as a low-cost substrate component. In particular, biochar products have been shown to be a potential replacement for perlite in greenhouse substrates (Northup, 2013), because it is lightweight, porous, and thought to have potential cost-saving benefits over perlite. Other reported benefits of using biochar products in substrates include the potential of increasing CEC (Nemati et al., 2015) and rhizosphere biology (Graber et al., 2010).
Numerous researchers have evaluated the use of biochar and products similar to biochar as substrate components and have reported mixed results. Coal cinders were evaluated as components in substrates to enhance the physical and chemical properties of a pine bark nursery substrate (Neal and Wagner, 1983) and again as a component in the growth of azalea [Rhododendron obtusum (Wagner and Neal, 1984)]. Coal cinders were found to contain high concentrations of heavy metals, which limited their use in substrates even though plant growth trials proved to be successful with up to 50% (by volume) cinder incorporation. Regulski (1984) reported the use of a gasifier residue as an amendment in a pine bark substrate to have reduced shrinkage, compared with the pine bark control, and provided increased easily available water and water buffering capacity for the duration of a 9-month crop. Holcomb and Walker (1995) reported the growth of chrysanthemum (Chrysanthemum indicum) and poinsettia (Euphorbia pulcherrima) in coal gasification slag amended substrates to be equal to plants grown in a peat:perlite control at up to 50% amendment (Bi et al., 2009; Evans et al., 2011). Researchers who conducted a container experiment using natural field soil amended with rice hull biochar demonstrated increased plant growth for lettuce (Lactuca sativa) and chinese cabbage (Brassica chinensis) when compared with plants grown in unamended soil (Carter et al., 2013). Pepper (Capsicum annuum) grown in coconut (Cocos nucifera) coir fiber amended with wood-derived biochar was shown to have increased plant growth and yields as compared with those grown in coconut coir fiber alone (Graber et al., 2010). Those authors speculated that the improved plant growth was a result of the biochar stimulating the beneficial plant growth promoting rhizobacteria populations or due to hormesis (positive plant growth response to low doses of phytotoxic or biocidal chemicals) caused by the biochar. Red oak (Quercus rubra) biochar added to peat or peat-vermiculite substrates resulted in an increased shoot biomass of hybrid poplar (Populus sp.) cuttings as a result of increased nutrient concentrations and availability due to the high CEC and initial nutrient content of the biochar (Headlee et al., 2014). In contrast, Northup (2013) reported that the incorporation of hardwood-produced biochar into the substrate resulted in either no effect or decreased growth of pepper, tomato (Solanum lycopersicum), cucumber (Cucumis sativus), marigold (Tagetes patula), and petunia (Petunia ×atkinsiana).
There are conflicting reports on the potential for biochar to be used as a soilless substrate component. This could be due to the wide range of biomass materials used to produce biochar, which may alter the properties of the final product. Any biochar products that are to be used in horticultural substrates must have chemical properties within acceptable ranges for such use. Therefore, the objective of this study was to evaluate the chemical properties of feedstocks common to southeast United States and their resulting biochar products and determine if the chemical properties were within suitable ranges for growers to use the biochar products as root substrate components.
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