Loblolly pine bark is the primary component of nursery container substrates in the eastern United States, but a shortage of the widely used organic material is prompting researchers to investigate new materials as potential alternatives. A recent study by James E. Altland and Charles Krause of the USDA Agricultural Research Service at the Ohio Agricultural Research and Development Center was designed to determine if ground switchgrass can be used as an alternative substrate for short production-cycle woody crops.
Altland and Krause conducted two experiments with 'Paprika' rose (Rosa L. 'ChewMayTime') potted in 15-centimeter containers. The experiments revealed that switchgrass processed to an appropriate particle size and amended with typical nursery materials can provide a suitable substrate for short-production-cycle woody crops.
Containerized nursery and greenhouse crops are grown almost exclusively in soilless substrates. Substrates for outdoor nursery crops are primarily composed of softwood bark amended with peatmoss, sand, pumice, perlite, compost, and/or other materials. Softwood barks used by the nursery industry are regional and highly dependent on local inventories; most nurseries on the East Coast, Midwest, and in the southern United States use loblolly pine bark, while nurseries on the West Coast use douglas fir bark. A decrease in forest products output, coupled with increased use of bark as a fuel at paper and lumber mills, has caused a decline in the amount of pine bark available for horticultural use. Increasing demand for wood-based ethanol over the next 20 years will cause even greater competition for pine bark and other woody biomasses.
"Research on the use of pine wood materials as alternatives to pine bark is proving successful in the southeastern United States, where pine plantations, paper mills, and lumber mills are abundant," stated Altland. He explained that widespread use of these materials would be difficult to adopt in the northeastern and midwestern United States where there are fewer forestry operations and softwood forest plantations. A potential answer to the pine bark shortage is switchgrass, a native grass grown throughout the U.S. for its biofuel potential. Grown in the upper Midwest where farmland is abundant, switchgrass offers high yields and shows potential as a replacement for pine bark for use in the nursery industry.
The researchers' objective was to document the suitability of locally grown switchgrass as the primary component in container nursery substrates with a short production-cycle woody crop. In the first experiment, substrates were composed of coarse-milled switchgrass amended with 0%, 30%, or 50% peatmoss and fertilized with 100, 250, or 400 mg/liter nitrogen from ammonium nitrate. In the second experiment, substrates were composed of coarse-milled or fine-milled switchgrass amended with 0% or 30% peatmoss and fertilized with the same nitrogen rates as in the first experiment. According to the research team, coarse switchgrass alone had high air space and low container capacity, while fine switchgrass had "physical properties more consistent with what is considered normal for nursery container substrates."
Tissue analysis of crops used in the experiments revealed that roses grown in switchgrass substrate for 7 to 9 weeks had low to moderate levels of calcium and iron, but all other nutrients were within acceptable ranges. Despite varying substrate physical properties and pH levels, all roses at the conclusion of the experiment were of high quality. The researchers observed that roses have been reported to grow over a wide range of pH and are considered better suited to well-drained substrates. "Thus," they noted, "it is not surprising that roses in this study grew well."
Future experiments are planned to evaluate amendments to lower and stabilize pH, moderate physical properties, supplement additional calcium and iron, and document substrate conduciveness to root pathogens. A wider range of plant materials with more specific requirements for pH and substrate water content will also be used in future research.
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