Answer: In warm summer areas it can be done, but it takes several months to sterilize the soil. You would need to water the area so it is damp down about three feet, then cover it with clear plastic, well weighted to keep it from blowing away. Leave it there for the season. At the end of the season, the heat should have killed off most weed seeds near the surface as well as many other forms of life including some pests and diseases.
In addition to killing off the "bad bugs", however, this treatment also kills off the "good bugs" as well as takes the area out of production. For these reasons it is a somewhat extreme method of last resort.
Sometimes gardeners will try covering an area with black plastic early in the season in order to try to warm the soil faster. When the cover is removed, the warming can cause weed seeds to germinate early, when they can be destroyed before you plant. Then a normal planting would follow. If you do this, avoid digging prior to planting because this will bring more weed seeds to the surface where they will in turn germinate.
In most cases, weeds can be controlled through early season shallow cultivation combined with a year round mulch, and disease problems can be avoided through careful rotation of plantings, good sanitation in the garden, selecting disease resistant plants, and by adding copious amounts of organic matter on an ongoing basis to keep the soil healthy.
If you are seeing some very serious disease problems, you may want to consult with your county extension about steps you can take. Sometimes, unfortunately, it gets to a point where you are better off relocating the garden and starting afresh.
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