Ill.: Park Seed
I receive a lot of email from readers putting in raised bed gardens, but who seem doubtful that you really need to fill the whole thing from the base to the summit with top-quality soil. They don’t seem to realize just how deep vegetable roots can go. I have beds 2 feet (60 cm) high filled with the best soil I could buy locally and I find that scarcely enough, at least, not when you want productive vegetables rather than a wimpy crop.
Some people even want to put a layer of gravel in the bottom. Gravel! Can you believe that? Or fill it with so-called “black earth” (actually black peat): inexpensive, for sure, but just about the worst soil amendment on the market. Yikes!
I think it might help to see just how deep some vegetable roots grow, as the chart below shows. Maybe you can skimp when it comes to herbs (most are shallow-rooted and not particularly needy of rich soil), but when it comes to vegetable gardening, you always want the best soil you can get and as deep as possible.
Soil Depth Requirements for Common Garden Vegetables
12?–18? (30–45 cm)
18?–24? (45–60 cm)
24?–36?+ (60–90 cm+)
|Bok Choy||Bean, pole||Bean, lima|
|Chinese cabbage||Eggplant (aubergine)||Rhubarb|
|Cilantro (coriander)||Melon||Sweet potato|
You still want to fill your raised bed with third-quality soil? Or black earth? Or gravel? Go ahead, but don’t say I didn’t warn you!