Soil Penetration Testing

Overview

Few soil characteristics affect plant growth as profoundly as soil drainage. Standing water prevents air from reaching the roots, and, often, rot sets in. Some plants, of course, do quite well in these conditions, but you'll have to search them out, and your palette of shrubs and flowers will be more limited. Rather than guess at the severity of drainage problems, do a simple test to measure the speed at which water seeps through your soil.

Method

A soil drainage test for gardening doesn't need to be as rigorous as a percolation test for installing a septic system, but the basic method is the same. You dig a hole, fill it with water, then see how quickly the water drains out. For most plants, digging a hole a foot deep is sufficient, but if you suspect poor drainage at a deeper level and wish to plant deep-rooted shrubs and trees, dig down to the problem layer. Fill the hole with water, then measure how far the water level drops over the next hour.

Testing Clayey Soil

You must take a few special precautions when testing a soil high in clay. First, after you dig the hole, use a knife to scarify the edges, breaking up any compacted soil that might hinder drainage. Next, fill the hole with water and leave it overnight. This allows any clay particles to absorb the water, swelling as they would during a prolonged rain and increasing the drainage time. The next day, do your penetration test.

Evaluating The Results

If your water level drops 2 to 4 inches in the hour, you have good drainage; 4 to 6 inches is excellent drainage; and over 6 inches is excessive drainage. If you are growing moisture-loving plants, add water holding organic matter if the water level drops over 2 inches. Plant drought-tolerant shrubs and flowers if it's over 6 inches, unless you're willing to add a considerable amount of moisture-holding material. On the other hand, if the water level drops an inch or less, you have poor drainage. Repeat the test in other areas of your yard to see how widespread the problem is.

Causes of Poor Drainage

If areas of your yard fail to drain well, the next step is identifying the cause of the problem. Sometimes, especially around new houses, heavy equipment has compacted the soil, eliminating the pore spaces in the soil that allow water to drain through it. If so, you can work at breaking up the compacted soil without worrying about long-term problems. If your soil is high in clay, the flat, platelike clay minerals may be packed so closely together that water fails to penetrate easily. Clay is easily compacted, especially when wet, and this may be contributing to the problem.

Improving Clay Soil

With a clayey soil, you can improve drainage by creating raised beds or mounds; but the best long-term solution is to add lots and lots of organic matter, compost, manure, leaves, straw or whatever is handiest in quantity. As organic matter decays, it produces humus, a sticky substance that binds the clay minerals into small particles that act like sand, increasing the pore space.

Keywords: testing soil drainage, water penetration testing, water drainage percolation

About this Author

Over the past 30 years, Mara Grey has sold plants in nurseries, designed gardens and volunteered as a Master Gardener. She is the author of "The Lazy Gardener" and "The Complete Idiot's Guide to Flower Gardening" and has a Bachelor of Science in botany.