By John Pohly, Colorado State University Cooperative Extension agent, Horticulture
Whatever Mother Nature's gifts to Colorado -- and there are many -- rich soil is not among them. We must make our own.
Rainfall, or in Colorado's case, lack of it, plays a major role in creating deep, rich layers of top soil. Eastern states, such as Iowa and Indiana, get more rainfall, resulting in more vegetation for organic matter. This leads to deep, dark, rich topsoil.
A thicker layer of vegetation also means less erosion, but with less vegetation in Colorado, wind erosion takes its toll on much of the top soil that does form here.
Front Range soils are largely heavy clay. To improve this type of soil and create better drainage, some people add sand. Research has shown, however, that you'll need to add anywhere from 50 percent to 80 percent, by volume, of sand to improve clay soil. Anything less will result in the formation of adobe.
A better way to improve a clay or sandy soil is to add organic matter. Organic material helps break up the soil for better air and water penetration; organics also will improve the water-holding capacity of sandy soil. Organic materials include peat moss, grass clippings, old leaves, compost, chopped straw or well-rotted manure.
To improve heavy clay soils, add l0 bushels, or one-half cubic yard, of organic matter per 1,000 square feet of ground. When adding organic matter, work it in as deep as the root system will extend. On lawn, this would be at least 6 inches.
Alkali or soluble salts are another problem. This situation occurs where the water table is near the soil surface. As water evaporates from the surface, capillary action brings mineral-laden ground water up to replace it. As this mineral-laden water evaporates, the mineral salts, which it contained, remain on the surface. Over time, white powdery mineral deposits will form on the soil surface. The best way to deal with this is to install drain tile to lower the water table. Then leach the soil with large amounts of water.
High pH or basic soils are found throughout Colorado with some acid soils found mainly in mountain areas. It is difficult to decrease the pH or make the soil more acid. You can add acid peat moss to try, or use acid-forming fertilizers such as ammonium sulfate or aluminum sulfate. Some advocate the use of sulfur for decreasing the pH value. Sulphur combines with the hydrogen ions in the soil to create a highly dilute sulfuric acid. This process, however, has not proven effective in practice.
Colorado gardeners have tried growing a variety of acid-soil loving plants, such as rhododendrons, heathers and cranberries, with little success. They usually blame the low success rates on the soil, but the problem usually is Colorado's low humidity and intense sun.
Whatever you decide to plant -- trees, shrubs or lawns -- improve the soil before the plants go into the ground. Afterwards is too late.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010