I have received a number of questions concerning using sand on clay soils. In most cases, this is not recommended.

Q. My yard is a combination of imported topsoil over coral or red clay over coral. I have heard that a good soil mix has clay, compost and sand. Should I mix in some "washed" sand to improve my red clay soil? — R.M, 'Ewa Beach

A. An ideal soil, called loam, contains a balanced mixture of sand, silt and clay, with an additional 5 percent compost. However, we do not have loamy soils in Hawai'i. They are either very sandy or heavy clay. Mixing sand into a clay soil will not improve drainage or compaction unless you mix an equal amount of sand and clay to a depth of at least 12 inches. Compost is the best amendment to improve tillage and drainage of a clay soil. Till in three or four inches of compost to a depth of eight to 10 inches.

The underlying coral is common in the 'Ewa Beach area and can present an additional problem. The coral is very alkaline and will eventually interfere with the absorption of some of the mineral nutrients by the roots of many landscape plants and turf grasses. The result is usually seen as yellowing of the leaves. Mixing in several inches of compost along with powdered sulfur and ammonium sulfate during the initial soil preparation before planting will help acidify the soil and delay the onset of these problems. Unfortunately, the roots will eventually grow deeper into the coral layer and the yellowing will develop. There is no absolute solution to this condition. Plants that have a high salt tolerance can usually tolerate higher alkalinity and are best used in these coral areas.

Q. I have low spots in my backyard. Is it OK to fill in these spots with sand, as I have seen on golf courses? — L.T., Kalama Valley

A. Sand should not be used to fill in low spots or to level out the lawn unless you already have a very sandy soil. They use sand on golf courses because golf greens are constructed mostly of sand, with a few inches of sandy soil on top.

Layering sand over a clay soil will cause uneven water distribution, and the sandy areas will dry out faster. This leads to dry spots. The sand also holds less nutrients, causing the grass in that area to thin and wear easily.

It's best to use a good topsoil to fill in low spots and even up the lawn, preferably a soil that is similar in texture to what you already have. Gradually build up spots that will require more than an inch or two of fill to avoid suffocating the underlying lawn. Allow enough time for the grass to grow back up through the fill before adding more. If you don't want to wait that long, dig out the turf in the low area, level out the area with soil and re-plant the turf.

Q. How often should I water my lawn? — S.K., Kailua

A. This will depend on whether you have a sandy or clay soil. Each watering session should wet the soil to a depth of at least 12 inches. Clay soils hold much more water than sandy ones and require a longer watering session to soak to that depth. The general rule is to water clay soils less often, say once every three days, and give longer watering times. It will take about one gallon of water per square foot of lawn to wet a clay soil to a depth of 12 inches. A run time of about 15 to 20 minutes per zone should be enough for an average sprinkler system. This will usually be enough water to last three days.

Since sandy soils hold less water, they will require more frequent watering — possibly every day in very hot, dry weather — but the run times will be shorter. Five to seven minutes per zone should be enough for one or two days. Let the weather be your guide, and water only when necessary.

Jay Deputy is an education specialist in landscape horticulture and turf at UH-Manoa and state administrator for the Certified Landscape Technician Program. Lawn care questions? Reach him at deputy@hawaii.edu.