Gardeners can't depend on Mother Nature to do all the watering, all the time. That's why watering your garden wisely (and conserving moisture once it gets in the soil) is a fundamental skill that your crops will thank you for cultivating.
What Type Is Your Soil?
How well your garden soil retains the water it receives depends on what type of soil you have. To keep things simple, we'll classify soils into three general types: sand, loam, and clay.
Sandy soils have lots of air spaces between the particles. This allows for good drainage—sometimes too good. Water moves through sandy soil fast, and the soil tends to dry out rapidly.
Clay soils are just the opposite. They have almost no air spaces between particles and drain very poorly. Clay absorbs water slowly but once wet, holds lots of water (often too much).
Loam is the middle ground between sand and clay. It absorbs water well and dries out at a nice moderate rate.
But no matter what type of soil you have, the key to keeping water in your garden is compost, compost, compost. Compost helps improve any soil by acting like a moisture-retaining wick. Every shovelful of that rich organic matter you add to the garden boosts your soil's ability to hold water.
Keep in mind that you'll have to apply more—not less—water to a garden high in organic matter to wet the soil to the depth you want. But because the soil holds more water, your garden may be able to go longer between waterings.