Know when to water?

Soil texture determines how much and how often to water

By Judy Scott

Water infiltration is affected by soil porosity and texture, as illustrated in the Willamette Valley Soil Quality Card Guide.

In the summer, homeowners whose lawn and garden soil is sandy often lament that their gardens and lawn require more watering than those growing in finer soil.

Actually it's a myth that lawns and gardens in sandy soil need more water than other soils, said Don Horneck, agronomist at Oregon State University's Hermiston Agricultural Research and Extension Center. "The difference is not how much water the lawn or garden uses but how much and how often it needs to be applied in coarse-versus-fine soils.

"Grass is grass and garden is garden, and soil type does not have much to do with how much water is being used," Horneck said. "An extreme amount of water used by a lawn in summer would be half an inch per day, regardless of soil type."

Fine, textured (tiny particle) soils can hang on to more water longer than the coarser, larger-particle, better-drained soils in the Columbia Basin or central Oregon.

Soils with silt and clay, typical of many western Oregon soils, can store up to eight inches of water in the soil profile. But coarse sands in the Columbia Basin or central Oregon can store as little as two inches of water in the soil.

This means that the finer western Oregon clay/silt soil can store four times as much water in the soil as the Columbia Basin sandy soil. A gardener in the Willamette Valley might go eight days before needing to water again, while gardeners with sandy soil could go only two days.

The total water needed in each place might be the same but a different schedule is needed, Horneck said.

"You can get away with watering four inches every eight days with fine soil, because the soil can hang on to the water,” he said. “With sandy soil, you'd still use four inches, but you'd have to apply it one inch at a time, every other day over eight days."

It is rarely advantageous to water more than every other day because a coarse soil cannot store the water, Horneck said. Add any more water to a sandy soil, and the rest would leach down below the roots of the plant.

"The moral of the story is that you should water sandy soils with smaller quantities of water applied more frequently than with clay/silt soils," Horneck said. "You can water silt/clay soils less frequently than sandy soils without leaching water."

To learn more water-wise gardening, the OSU Extension Service offers several publications on-line or in print, including "Conserving Water in the Garden: Landscape and Lawn Care," EC 1531. Call 1-800-561-6719 to purchase a printed copy.