Growing vegetables in raised beds is highly recommended when there is limited space, or if your soil does not drain well or is stony. But to be successful, the selected site needs at least eight to 10 hours of full sun if your intent is to grow tomatoes, peppers, onions, garlic, eggplants, squash, cucumbers and snap beans. With less than eight hours of direct sun, you will be limited to growing lettuce, spinach, Swiss chard, cabbage, broccoli and kale.
Raised beds give you the advantages of good drainage, additional growing space and easier management because you have less bending. There are disadvantages, too: having to irrigate more frequently, higher summer soil temperature, colder fall and spring soil temperatures and problems using power equipment such as tillers.
A common mistake in establishing raised beds is using commercial potting blends, which are engineered for growing plants in pots and small pans. These shallow containers allow water to accumulate at the bottom, in a perched water table, within reach of the roots. Because commercial potting blends are rich in organic matter and porous materials, they have a high air-filled pore space, which does not make for good water retention. This means having to irrigate and fertilize frequently to obtain a desirable crop. The combination of higher growing media temperatures and low water-holding capacity demands frequent irrigations. More frequent irrigations result in a greater loss of nutrients as water moves through the soil.
You’ll do better by manufacturing your own soil by purchasing subsoil containing 50 to 60 percent sand. Do not purchase topsoil, for it will be full of weed seeds and live roots of perennial weeds. Subsoil, the layer beneath, the topsoil, is relatively free of weed seeds and roots. Blend the subsoil with one-third by volume of compost if your raised beds are a foot or less in height. If the raised beds are deeper than a foot, fill to within three inches of the top edge and cover the soil with a two-inch-thick layer of compost. Spade or rototill the compost into the upper six inches of soil.
Regardless of which method you use to fill the raised bed, allow at least two weeks before taking soil samples for testing. Since most subsoils tend to be acidic, most likely it will be necessary to add limestone, but without soil test results, the exact amount cannot be estimated.
If your interest is in organic gardening, top-dress each year with a one-inch-thick-layer of compost prior to spading or tilling in the spring. Compost has a mineralization rate between 10 and 12 percent, which is essential to maintain a proper nutrient level for the garden to be productive. The mineralization rate is highly dependent upon soil temperature, and raised beds have a higher soil temperature than gardens.
For conventional gardeners, follow the recommendations on the fertilizer bag. Commercial fertilizers tend to be acidic, so your soil should be tested every three to four years. Apply additional compost at least once every three to four years when using conventional gardening practices. Organic particles in compost deteriorate with time, and you are seeking to maintain a three to five percent level of organic matter for carefree gardening.
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