Evaluating Drainage

Gardening instructions often tell you plants need “good drainage”, but how do you know if your drainage is “good”? It might be very good on a sloping lawn, but bad in the low spot below the lawn. Drainage problems are usually in a few low spots on your property. Here are several simple tests.

Observe puddles after a rain or sprinkling. Most puddles should disappear within a few hours after the rain stops. If they persist for days, there is a drainage problem.

Another sign of poor drainage is a white or tan crust on the soil surface during dry weather. The crust is composed of soil salts that have been deposited as water migrated up to the surface, then evaporated. This migration to the surface only occurs when the water can’t drain downward. (A similar ring of salt crust is sometimes found around the wet patch under a drip emitter as water migrates away from the emitter, then evaporates.)

A more thorough test can be made if you dig a test hole and watch it drain. Dig a hole 2 feet deep by any width (you can dig a narrow hole with a posthole digger) and fill it with water. Allow it to drain completely so that the surrounding soil is saturated. Then fill the hole again and use a ruler to measure the rate the water level drops. For most plants, 1/2 inch per hour (or a foot a day) is very good drainage. Half that rate, or 6 inches a day, is adequate. If it’s as slow as 3 inches a day, you have a drainage problem.

Poor drainage can be caused by different problems. Make these tests to see what is impeding drainage on your soil.

High Water Table

Test for a high water table by digging a hole. The hole will fill with water to the level of the water table. A water table within two or three feet of the surface can cause the soil above it to be soggy, and keep tree roots from growing deep enough to anchor the tree securely. For more information, see High Water Tables.

Compaction

Foot traffic compacts the top couple of inches of soil. If the soil puddles, but a hole drains quickly, the problem is at the soil surface. For instructions on relieving compaction, see Compacted Soil.

Plowpans and Hardpans

These layers of compacted or rocklike soil are usually from 6 inches to a couple of feet below the surface. Hardpan feels like rock when a shovel strikes it, but it’s possible to break through it with a digging bar and determination. Plowpan is a compacted layer about 6 inches under the surface. Probe for it at the sides of a hole. For more information, see Hardpan.

Shallow Soil and Layered Soil

Shallow soil occurs when bedrock is within a foot or so of the surface. Layered soil can be seen in the sides of the hole you dig, as a sharp transition from one soil texture to another, as when sandy soil overlays clay soil or vise versa. Both of these problems can be observed just by digging. For more information, see Shallow Soils and Layered Soils.

Sodic Soil

Sodic soil is soil with high levels of sodium. Sodium destroys clay structure chemically, making it drain poorly. Test for sodic soil with the same hole in which you tested for drainage. Spread about a cup of gypsum in the bottom of the hole and mix it into the soil, then fill the hole with water again. Wait a day, fill it with water again, and see if the drainage is improved. If your problem is excess sodium, the gypsum will cure it, improving the drainage. For more information, see Sodic Soils.

Heavy Soil

If none of the above tests reveal specific problems, your poor drainage is probably caused by heavy, poorly structured soil. For information about working with heavy soil, see Heavy Soils.