Chances are, if you live in a desert environment, the soil in your desert garden is going to be highly alkaline. If that's the case, you'll need to amend the soil in some parts of your garden.
The first thing you'll want to do is test your soil. This will help you determine which plants are suitable for your site, and what amendments, if any, you will need to make to the soil. There are numerous soil-testing kits available, both at your local nursery and through mail order.
If you would prefer not to do your own testing, there is another, foolproof method of getting it done. For a small fee, your Cooperative Extension Service can have your soil tested, and will issue a full report. It will usually take a couple of weeks to receive the results so do it as soon as possible. (Fall and winter are the best times, but you can do it any time.)
You will need to do a little soil collecting first. Using a bucket, wheelbarrow, or any other collecting basin, dig about a shovelful of soil from each of the locations you plan to plant. Then, mix all the soil together. Put about two pounds of the soil mixture in a plastic bag (a quart sealable bag will do nicely) and deliver it to your county agent office.
The report will indicate any chemical, organic and mineral deficiencies (and/or accumulations)in your soil. This makes it easy to add these items.
However, in a desert garden, you do not want to amend all the soil. Only the soil in the "oasis" and possibly a portion of the soil in your "transition" zone will need to be amended.
Let's start with the "oasis" portion of your garden (See Designing Desert Gardens). That's usually the portion nearest the house, and most used areas (patio, entryways). Adding compost to this area will help the soil hold water. Spread about four inches of compost to your "oasis" area and till it in. While you're at it, also till in any other components your soil test indicates (nitrogen fertilizer, for instance).
I would do the same thing in your "transition" zone (see Transition Desert Gardens).
In the "arid" zone of your desert garden, do not add any compost or nutrients, unless your soil is highly acidic. (Arid Zones in The Garden). You do, however, need to till or double dig to loosen up the soil prior to planting. The reason you don't want to add anything to this area is you're going to be planting hardy, desert plants. They need to build their own root systems without added nutrients. If you add nutrients, chances are the roots will only grow where the nutrients are (plants are like children...they go to where the candy is), and they will become root bound and eventually die. This is "tough love for plants."
Here's an important note to remember: Some wild seeds, including a great many undesirable weeds, will lie in the soil for 50 years or more without germinating (see Field Bindweed). Tilling will encourage them to germinate. Thus, don't till anywhere you don't want to plant. Mulch and weeding will discourage unwanted plants in your cultivated areas.
Next article: Appropriate use of lawns (turf) in a desert garden.
