In most areas of North America a rule of thumb is that vegetables need about an inch of rainfall per week. If nature doesn't provide, then it's up to the home gardener to fill the gap. A 15 x 25 foot plot needs about 225 gallons of water per week. A plot double that size needs double the volume of water, and so on. If it rains a bit, you can reduce your watering a bit But be careful. Although it's better to water too much than too little, try to do neither. A deluge may result in insufficient oxygen for the plant and interfere with root growth and function.
When you saturate the soil, you add water until it reaches field capacity—that is, the point at which the air spaces in the soil can hold no more water. Ideally, you should keep your garden soil somewhere between this condition and the point at which moisture is so scarce that plant roots can no longer take water from the soil.
How often you water is as important as how much. If you sprinkle for a brief period each day, you will only wet the upper few inches of the soil. Light watering like this should be avoided because it causes roots to grow too close to the surface, adding the risk that a sudden hot spell will severely damage plants. The soil should be wet to a depth of 6 to 12 inches each time you water. Then it need not be watered again until the top few inches begin to dry out You can take a trowel to dig in and check. If it rains, don't water until this trowel test shows that watering is needed. For many gardens, a good soaking once a week is just right
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