Just a note in addition to the above, this University of Georgia Extension article on container growing notes that "One of the best organic matter amendments to use is ground pine bark having a quarter to 3/8 inch particle size. Nurseries usually have this size bark on hand for their potting needs. Either fresh or composed pine bark is acceptable. Compost hardwood barks before use." They suggest if using field soil in containers, 50:50 soil:organic matter works well.
http://extension.uga.edu/publi...
As this article on evaluating container substrates says, pine bark is the predominant substrate in the nursery industry:
https://www.extension.purdue.e...
