Each time you water, apply enough water to wet the soil to the bottom of the root zone. Water is measured in inches. If a tin can is placed on a lawn when the sprinklers are turned on, the soil has received an inch of water when the water in the container is an inch deep. If a watering basin is 3 inches deep, 3 inches of water is applied to the soil when the basin is filled with water. Because sandy soils do not hold as much water as clay soils, it takes less water to wet them to the same depth. To wet 1 foot of a sandy loam that has dried out to the point where average plants need water (just barely moist), apply about 1 inch of water. To wet 1 foot of a loam or clay loam that is just barely moist, apply about 1½ inches of water. Soils vary greatly in their ability to hold water, so these figures are only guidelines. To see how much water your soil needs to wet it a foot deep, measure the amount of water you apply at an irrigation. The next day, after the wetting front has moved down as far as it will go (clay soils may take 2 or 3 days), dig a hole to see where the wetting front is. You will be able to calculate from this how much water you need to apply to wet the soil to the bottom of the root zone. Then, by measuring the time it takes your sprinkler system to apply an inch of water, you can calculate how long you need to leave the system on to apply enough water.
Related Links
About Watering
How Water Moves in Soil
Overwatering
When to Water
