I've been vegetable gardening for a few years and want to re-do my raised beds. My questions is about the optimal soil depth for the new garden frames.
I want to buy or build three boxes of approximately the following dimensions: 3' wide, by 10' long, by 30" high. I want that kind of height so that I won't have to kneel to tend the garden but I'm also interested in the depth of the soil I put in.
I have read Edward C. Smith's Vegetable Gardener's Bible in which he describes the root depths of vegetables often going as deep as 3' so I feel that filling boxes 30' deep with an optimized mixture of loose, hummusy soil would be ideal. But I find Mr. Smith seems to give some conflicting answers as to how deep the loosened bed soil should be and I notice that most commercially available boxes or frames run between 6' to 20' of soil depth. Even the very tall boxes seem to have false bottoms to accommodate a soil depth of no more than 20". So I'm wondering if I should go to the trouble and expense of building a box that has 30" of soil? Will that be beneficial for my vegetables? Is that kind of depth pointless? Or even harmful? Am I, for instance, missing some issue of drainage or soil management?
Thanks for any advise you can share.
P.S. My garden is in a temperate, near coastal strip of Los Angeles (USDA Zone 10B, Sunset Zone 24B) and I want to continue growing a very wide variety of fruits and vegetables.