Pepper is an important vegetable crop in the United States with 44,800 acres planted annually (U.S. Department of Agriculture, 2016). Pepper crops are established using transplants since transplants are uniform in size, increase earliness, and decrease production costs associated with thinning of seedlings in the field (Biai et al., 2011; Schrader, 2000). In the transplant production phase, growing medium plays an important role in plant health. Most growers use a soilless mix as it reduces the risk of soilborne diseases caused by Fusarium, Phytophthora, Pythium, and Rhizoctonia and provides optimum conditions for seedling growth. Seed germination and transplant growing medium is critical to transplant production as it provides proper pH, aeration, and optimum water and nutrient retention for uniform seed germination and development of a healthy root system. Studies have shown that a high-quality transplant with a healthy root system has a positive effect on the vigor and productivity of the crop in the field (Leskovar and Stoffella, 1995).
Growers, whether they make their own growing medium or purchase it premixed, use medium that usually contains sphagnum peatmoss, vermiculite and/or perlite, and calcium carbonate. Sphagnum peatmoss often makes up greater than half the volume of the growing medium due to its ability to hold moisture while maintaining good pore space (Pill and Ridley, 1998; Schmilewski, 2008). Due to overwhelming use of sphagnum peatmoss in growing mixes, much research is now focused on finding environmentally friendly alternatives. Some alternatives to sphagnum peatmoss that have been investigated for their suitability in growing medium include coir, compost, bark, composted bark, and wood fiber (Schmilewski, 2008). Another alternative product that is being tested is biochar, but its potential impact on medium properties and seedling growth has not been tested and validated extensively.
Biochar is the term used for soil amendment made of charred organic matter and ash that remains after organic matter is thermally decomposed in a low-oxygen environment (Lehmann and Joseph, 2009). Biochar addition has been shown to impart beneficial chemical and physical attributes to mineral soils (Barrow, 2012; Laird, 2008). Benefits of adding biochar to soils include increased soil pH in low pH soils (Novak et al., 2009), increased retention of nutrients (Clough and Condron, 2010; Laird et al., 2010), and increased cation exchange capacity (Liang et al., 2006). Research on biochar as a component in soilless growing medium has been limited, although studies have focused on its suitability for nursery crops (Dumroese et al., 2011). Dumroese et al. (2011) evaluated pelletized biochar in combination with sphagnum peatmoss for production of forest seedlings. They found that amendment with 25% biochar pellets improved hydraulic conductivity and water retention at high matric potentials and beneficially increased medium pH. Studies have also focused on physical and chemical properties of biochar that contribute to retention of nutrients in soilless medium (Altland and Locke, 2012; Graber et al., 2010; Santiago and Santiago, 1989). Graber et al. (2010) reported positive effects of biochar on growth and productivity of pepper and tomato (Solanum lycopersicum) plants in a blend of coconut (Cocos nucifera) fiber and tuff (volcanic ash material). They reported that positive effects were mainly due to the presence of plant growth promoting and/or biocontrol agents present in biochar amended medium. In addition, they also cite the effect of low doses of biochar chemicals, many of which are phytotoxic or biocidal at high concentrations, to have stimulated plant growth at low doses (hormesis).
Studies have evaluated biochar to completely replace sphagnum peatmoss, use it for greenhouse production of mature plants, or transplant seedlings into biochar-amended medium. The influence of biochar in soilless medium on direct seeding of vegetable crops for transplant production has been studied less. This is important specially to understand effects biochar could have on seed germination. Germination assays to test the quality of biochar have been suggested as a means to evaluate biochar quality (Rogovska et al., 2012). In addition, the body of biochar research in soilless medium is far less complete with respect to identifying a suitable rate or concentration of biochar that could be used in vegetable transplant production. Also of particular interest is the transplant tray cell number, which is directly correlated with size of the transplant. Differences in transplant size resulting from differences in transplant tray cell number have been shown to affect stand establishment, earliness, yield, and quality (Nicola and Cantliffe, 1996). With the growing interest in replacing sphagnum peatmoss–based growing medium in greenhouse production systems, there is need to evaluate and assess use of hardwood-based biochar (described in materials and methods) in vegetable transplant production. The objectives of this study were to 1) determine the optimal amount of hardwood-based biochar that could be added to soilless peatmoss-based growing medium for pepper transplant production, 2) study the effect biochar on seed germination, plant growth, and growing medium chemical characteristics, and 3) determine if transplant tray cell number is an important factor when determining optimal biochar rates for transplant production.
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