Soils are highly variable in the arid West. They can be sandy and fast-draining, as they are in the Coachella Valley of California, or claylike and slow-draining, as they are in most of Colorado and Utah. In some areas there are soil pans or impenetrable layers to contend with. In most areas there is a scarcity of organic matter to work its way into and improve the soil.
The pH scale, numbered from 0 to 14, measures relative acidity or alkalinity. In areas of low rainfall, soils tend to be alkaline, meaning they are above pH 7.0, the neutral point. In areas of higher rainfall, soils are usually acidic, or below pH 7.0. The pH of a soil is important to plant growth because it affects the availability of nutrients. Some nutrients, such as iron, become tied up chemically as the soil becomes more alkaline. Problems with salinity (the amount of salt in the soil) also tend to increase with alkalinity.
Both acidity and alkalinity can be modified, but if you have a problem that cannot be corrected by any of the methods described below, consider having your soil analyzed. Most states in the West, with the exception of California, will conduct a soil test. Contact your state university or the cooperative extension office in your area for information.
Testing the Drainage
Most plants, especially drought-tolerant ones, require good drainage. If soils don’t drain well, the root zone becomes saturated with water, leaving no room for air. Plants can suffocate when that happens.
A simple way to test drainage is to dig a hole, fill it with water, and see how long it takes to drain. For more instructions, see Evaluating Drainage.
For fast-draining soils, the length of time it takes the water to drain is not as important as the holding capacity of the soil. Some loamy soils may drain rapidly but still hold water in the spaces between soil particles. Very sandy soils will drain quickly, sometimes in a matter of minutes, without retaining much water. Adding organic matter will improve the drainage of most soils.
Adding Organic Matter
In clay soils, organic matter such as composted leaves or ground bark opens up the tight pore spaces and improves aeration, water infiltration, and drainage. In sandy soils, organic matter creates a better mixture of large and small spaces between soil particles, holding air, moisture, and nutrients in the root zone of plants. The beneficial effects wear off, so replenish the supply of organic matter yearly.
Organic matter physically changes the soil structure. The volume of organic matter to add depends on the condition of the soil and the type of amendment. The more clayey or sandy the soil, the more amendment is needed. Generally, organic matter should make up about one quarter to one third of the amended soil. To improve the soil for a vegetable or flower garden, for example, add about 6 inches of amendment over the planting bed.
Thoroughly mix the soil and amendment together to a depth of 6 or 8 inches. (In very hard soils, mix it in as deep as you can.) Incorporate it well—there should be no streaks or pockets of amendment to affect root growth. Don’t work clay soil when it is wet or it will dry into bricklike clumps. Wait until the soil is barely moist and crumbly.
Gardeners in arid regions have to be concerned about the salt content of soil. A salty soil amendment can create a saline soil or add to soil salinity. Salinity is a common problem in alkaline soils. Manure, a frequently used amendment that is high in salts, should be added only in small quantities each year. Bagged steer manure is obtained from feedlots, where cattle are fed diets high in salts. It is very salty. Manures from stables or barns are somewhat salty.
Sawdust, bark chips, or other wood products should be well composted or nitrified (supplied with added nitrogen). Soil bacteria need nitrogen to fuel their decomposition of fresh wood. Because there isn’t enough nitrogen in the wood itself, the bacteria take it from the soil leaving little or none for plants, resulting in nitrogen draft or depletion. To be on the safe side, many gardeners add extra nitrogen fertilizer when they amend the soil. For more information about nitrogen fertilizers, see Nitrogen Draft.
Many types of soil amendments are available, such as garden compost, ground bark, pine needles, leaf mold, and a variety of locally produced materials. Coarse amendments work better than fine-textured ones in the arid West because they break down more slowly during long seasons of intense heat. Organic matter is depleted rapidly in very hot soils. Keep a cooling mulch on the surface to slow the rate of decomposition. Even so, you will have to add organic matter far more often than gardeners in cooler climates.
When Not to Amend
Sometimes it is better not to amend the soil. Recent research has shown that trees establish themselves and grow better when planted in unamended soil. If the soil in the planting hole is richer than the surrounding native soil, roots stay in the amended soil instead of growing out into the native soil. Planted in unamended soil, trees send out roots far beyond the original planting hole in search of nutrients.
The addition of organic matter can harm or even kill some native plants that thrive in unamended soil. Plants adapted to a particular soil—even if it is extremely rocky or claylike—will establish themselves and grow better if planted in similar soil. Consult a nursery specializing in native plants, a local native plant society, or the state university for detailed information on soil requirements for specific plants.
Soil Problems in the West
Lack of rainfall and the high mineral content of western soils create certain problems that must be corrected before most plants can grow successfully.
Caliche, a hard chalklike layer of concentrated soil minerals, is a kind of hardpan common in the desert Southwest. It is an impenetrable layer beneath the soil surface that impedes drainage and root growth. It also increases the soil pH, often tying up iron in the soil and making it unavailable to plants.
If there is caliche where you want to grow deep-rooted plants, you must provide water drainage to the subsoil. This can be done by making a chimney—a narrow channel down through the pan to porous soil. Sometimes a heavy iron bar will do the job; in other cases a jackhammer is needed. For more information, see Hardpan.
Arid regions often have alkaline soils because they contain lime or sodium in quantities high enough to increase the pH to more than 7.0. In areas with high rainfall, the lime is leached out, leaving the soil more acid. Gardens irrigated by alkaline water are also likely to have alkaline soil.
In arid climates, adapted plants are not bothered by alkalinity until the pH exceeds 8.2—and then nutrients such as iron become tied up and unavailable to plants. (In areas with lots of rainfall, a pH as low as 7.5 can cause alkalinity problems.) Soil sulfur or an ammonium fertilizer will reduce the alkalinity. For more information, see Alkaline Soils.
Salinity—an accumulation of soluble salts—is a common problem in arid regions. It is particularly troublesome in valleys where salts collect in low-lying areas and there isn’t enough rain to wash them down into the soil. Sometimes white salt deposits can be seen on the soil surface. The problem is worse in tight-textured, claylike soils because of their poor drainage.
A high salt content in the local water supply can create or add to a problem of soil salinity. Soils in regions irrigated by the Colorado River are more saline because of the high salt content of the water. Salts can also build up in a garden from repeated use of fertilizers and manure.
Salts are absorbed by plants; in severe cases they prevent seed germination, retard growth, and burn the foliage. The best solution is to leach the soil—apply water slowly for several hours until the salts are washed down past the root zone of the plant. Do this once or twice a year. For more information, see Saline Soils.
Alkali or Sodic Soils
Alkali soils are the result of low rainfall and poor drainage. They consist of clay soil with enough sodium bonded to the clay particles to destroy the structure. The lack of structure makes the soil behave like potter’s clay, and become almost impervious to water. For more information, see Sodic Soils.