I am doing container gardening — rather large plantings that will incorporate small trees and shrubs — and therefore want a potting mix that will last longer than the usual for smaller containers, and will provide some nutrients. I just read about soil-based potting mix, but there is no further info in my text. Can you describe this, and tell me if it commercially available, or do gardeners mix up their own recipe?
There are a variety of opinions about soil-based potting mix. Taylor’s Guide to Container Gardening (edited by Roger Holmes, Houghton Mifflin, 1995) provides a recipe for a “real soil” mix combining equal parts garden loam, compost or peat moss, and coarse sand. The sand should be as coarse as possible, and should not be able to pass through a window screen. According to the guide, “the success of any mix using soil depends on the soil’s quality.” For large pots and planters, the mix should be equal parts coarse, medium, and fine materials (from Landscaping with Container Plants, by Jim Wilson, Houghton Mifflin, 1990), for example:
Coarse material: small nuggets of pine or fir bark
Medium material: pulverized pine or fir bark
Fine material: moistened sphagnum peat moss
Extension.org has information on organic potting mixes. ATTRA Sustainable Agriculture Program has additional information
Mother Earth News published an article by Barbara Pleasant in the December 2008/January 2009 issue entitled Make Your Own Potting Soil which should be helpful. The recipe includes pasteurized compost or soil.