The effects of composting period and mineral amendments on a 50:50 blend of pine and hardwood bark

This research was designed to refine a composting process for a 50:50 blend of pine and hardwood bark to be used as a container growing medium. The first experiment studied the effects of composting period and mineral amendments on the preparation of blended pine and hardwood bark media, and the succeeding experiments studied plant growth in the media prepared in the first experiment.

Equal volumes of pine bark and hardwood bark were composted in partitioned windrows. Daily heat production during composting was monitored and used to determine targeted endpoint temperatures. Targeted endpoint temperatures were 50°C and 40°C. Mineral amendments tested were S, KNO₃, and MgSO₄. For comparison with composted media, four media currently being used in the nursery industry were also tested. Physical and chemical properties of all media were examined.

Composting to 50°C took only six weeks while nine weeks were required to reach 40°C. Shorter composting to 50°C resulted in 7% less shrinkage and a greater percent air capacity. Composting to 40°C resulted in a greater amount of small particles, percent total pore space, and percent water holding capacity. Mineral amendments had little or no effect on physical properties of composted media. The pH and electrical conductivity of composted media were not influenced by endpoint temperature or mineral amendments. Physical and chemical properties of composted media were intermediate of comparison media.

Rhododendron cv. 'Red Ruffles', Photinia x fraseri, and Juniperus conferta cv. 'Blue Pacific' were grown in the previously described media. Photinia and juniper grew equally well in media composted to 50°C and 40°C. Azaleas grew best in media composted to 40°C. Mineral amendments had little or no effect on plant growth. Plants grew as well or better in composted media than in comparison media. Since six weeks of composting to a 50°C endpoint and elimination of S, KNO₃, and MgSO₄ frequently produced better growth than other media tested, it appeared to be an excellent medium for container nursery production.