Not all vegetables can fit inside the same size raised garden bed. Depending on their root depth, you may need a shallow, medium, or deep root bed
Building a raised bed deep enough for a viable vegetable garden can present some challenges. Although much root activity occupies the top 6 inches of soil, many vegetables need 2 feet of soil or more. Knowing the soil depth required for the vegetables you want to grow is essential. Soil preparation may include enriching or amending the soil under your raised bed to accommodate all the varieties of vegetables you want to plant in your garden.
Shallow Rooted Vegetables
Even shallow vegetable roots may extend to between 12 and 18 inches, according to Eartheasy. Leafy greens, radishes, bok choi, fennel and kohl crops -- broccoli, Brussels sprouts, cabbage and cauliflower -- are among shallow-rooted vegetables. This group also incluces onions, leeks and chives, corn and potatoes.
Medium-Rooted Vegetables
Medium-rooted vegetable plants have roots that grow 18 to 24 inches. This category includes beans, cantaloupe, cucumbers and summer squash. It also includes root vegetables with the exception of parsnips, peas, kale, chard, eggplant and peppers. Although most herbs are shallow-rooted, rosemary and sage send down medium-long roots.
Deeply-Rooted Vegetables
Vine crops including pumpkins, winter squash and watermelons are deep-rooted, with roots that extend 24 to 36 or more inches. These plants and their roots support long vine structures over long growing seasons. Heat-loving artichokes, okra, sweet potatoes and tomatoes are also deep rooted. Perennials asparagus and rhubarb are part of this group, as well.
Soil Height vs. Soil Depth
Accommodating most vegetables requires more effort than providing a 6- to 12-inch-high raised garden bed. No matter what your construction materials and method, building raised beds for a vegetable garden that are 2 to 3 feet high can require considerable expertise and money. A combination of prepared ground soil and raised-bed soil is a more economical, if still labor-intensive, solution.
Using Healthy Ground Soil
If soil under the bed is healthy, double-digging will ready it for supporting a raised bed. To double-dig soil, insert a shovel or spade the full length of its blade, usually 8 to 12 inches. Setting aside the first spadeful, dig down and turn over a second spadeful. Replace the first spadeful. Digging to the resulting depth of 16 to 24 inches provides deep-rooting vegetables in an 8-inch-high raised bed with enough space to sink roots for healthy growth. If you are planting only shallow-rooted vegetables, digging to a single spade depth may be adequate. Avoid chopping or pressing hard on turned soil, to preserve the air pockets created by turning and prevent forcing roots and diseases from the top soil farther into the ground.
Amending Poor Soil
The same double-digging technique distributes soil amendments like sand, compost, sulfur or limestone to the depth that deep-rooted vegetables require. Mounding any extra volume of soil created inside the walls of the raised bed will save on fill dirt for the bed. Ground and bed soils will blend adequately without further digging or turning.