Most people think that desert soil is almost all sand, but all sorts of soils can be found in the desert. The one thing that these soils almost always have in common is that they are dry and have little organic matter. Often they are quite alkaline and very seldom acidic. Desert soils are also frequently rocky and hard packed.
Soil types include sand, silt and clay. These are classified by the size of the particle. To determine the type of soil you have, take about a cup of your soil, put it in a quart jar, and fill the jar with water to about an inch above the level of the soil and shake well. Let it settle for 24 hours then look at the layers. The bottom layer will be the large sand particles that settle out first. On top of the sand will be silt and on top of that, clay. Organic matter will be floating on top. The best garden soil has some of each but is predominantly silt with a significant amount of organic matter. These soils are called loam soils. You probably do not have a good loam soil if you live in the desert.
You may also have Housing Development soil. When most Western US housing developments are built the soil is scraped off, removing all vegetation, fill dirt is brought in to cover low spots and arroyos and the land is reconfigured to the design of the civil engineers. In housing developments you could have anything from good topsoil to chunks of concrete buried under a layer of semi-fertile desert soil. If everything you plant in an area dies despite your best efforts to prepare the soil, water it and take care of it, dig down to find out what might be underneath.
You might also have a layer of caliche under the surface of the soil. Caliche is a layer of soil cemented together by crystalline salts - calcium carbonate, sodium nitrate or others - that is hard and impermeable to water and roots. It can also tie up iron and make it unavailable to plants. Most recommendations are to dig completely through the caliche layer in the entire root zone of the mature plant, but this is often impossible. When planting a tree or shrub dig through the caliche, or at least three feet deep, then dig smaller holes around the tree to allow water to drain through. If you are planting a lawn or groundcover on top of caliche provide at least 8 inches of good soil above the caliche. To dig through caliche, dig as large a hole as you can then fill it with water, After the water has soaked in, dig some more, add more water and keep this process up until you have a large enough hole.
Whatever type of soil you have, it can be improved by the addition of organic matter. Four inches of compost, dug into the top foot of soil, will help sandy soil retain moisture and keep clay soil from compacting and becoming pottery instead of soil. Two inches of compost will suffice if you have a good mixture that is mostly silt.
If you have time but not the equipment or energy to incorporate compost into the soil sheet composting can also work. Put a layer of newspaper several pages thick on the soil to be improved. Wet the newspaper and cover it with manure, half rotten hay or anything compostable that you can find in large quantities. You can put your kitchen scraps on the newspaper and cover them with straw, but put down only enough newspaper to be covered by the other material so extra paper does not blow away. Keep the pile moist enough that it does not blow away, add some worms to do the job of incorporating the organic matter into the soil. This process takes a season or two but it saves the work and expense of buying and incorporating large amounts of compost. After the compost decomposes you will have loose, fertile soil in which to plant.
Lynn Doxon started gardening when she was two years old. The had her own garden at the age of 8. She has a PhD in horticulture and worked for several years as an Extension Horticulture Specialist with the New Mexico State University Cooperative Extension Service.
She has an online course in organic gardening that is available at http://www.homegardenblog.net.
Her passion is that everyone, from the renter in the tiniest apartment to the homeowner with an acre of yard will grow some of their own food. She and her husband have the goal of developing an urban farm that provided the majority of their food.
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