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Jay Young, Charles Heuser and E.J. Holcomb

The use of spent mushroom compost (SMC) as a media amendment for containerized greenhouse and nursery crop production is a promising alternative to disposal of this by-product of mushroom production. Fresh SMC is the compost that is removed from the mushroom house and used without further weathering. The objectives of this study include first, identification of key factors involved its successful use and second demonstration of the effective use of SMC by nurserymen. The plant material used includes both bedding plants and woody perennial species. Results demonstrate that the key factor in the use of SMC for plant production is high soluble salts. Leaching can reduce the high soluble salts. In addition, special consideration should be given to the reduction in potted media volume over time due to composting that continues after the material is removed from mushroom production. SMC as the sole growing media was not as effective as when SMC was amended with a commercial nursery growing mix. Several species were grown in 0%, 25%, 50%, 75%, and 100% mixtures of SMC and a commercial nursery mix. All species grew well in 50% SMC/50% nursery mix.

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John R. Young, E. Jay Holcomb and Charles W. Heuser

Though high electrical conductivity (EC) levels are commonly held to be the primary limiting factor for using spent mushroom compost (SMC) as a growing substrate, EC can be reduced by leaching. This allowed SMC to be successfully used for growing plants. Leaching reduced EC of the substrate solution from as high of 30 dS·m-1 (mmhos·cm-1) to 2 to 3 dS·m-1, a level acceptable for growing plants. The initial EC and container capacity determined the number of leachings and total volume of water required to lower EC of SMC substrates to acceptable levels. As the concentration of SMC was increased, a higher number of leachings or larger volume of water were required to adequately reduce EC levels. In trials spanning 2.5 years, SMC was effectively used as a substrate in the production of marigold (Tagetes patula) `Yellow Girl'. Benefits to plant growth from SMC incorporation included a slow release of nutrients as the SMC decomposed and a good air-filled pore space/water-holding capacity when amended with a commercial nursery mix. From these trials, it is recommended that SMC be incorporated at rates of 25% to 50%. It is not recommended that SMC be used in concentrations over 50% because the EC may be too difficult to manage and the high levels of air-filled pore space of SMC. Though season may affect the initial EC level of SMC, such variation is minimized by leaching while differences in plant response are more likely to be attributed to environmental conditions. No differences in plant growth were observed among SMC sources.