In the Garden:
Southwestern Deserts
February, 2010
Regional Report

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Golden barrel cacti drop almost no litter, thus there's no organic matter for surrounding soil.

Desert Soil Characteristics

New desert gardeners, especially those who gardened in other regions of the country where rich, dark earth is the norm, are often stymied (or stunned) when they attempt to push a spade into our hard soil for the first time. I hear lots of concerns from new gardeners or residents about desert soil's colorless appearance and lack of organic matter.

Not to worry. Desert soil is perfectly suited for growing desert plants, although it does have different characteristics from the dark earth found in other parts of the country. That black soil forms because their native plants drop large amounts of litter. Such organic matter continuously decomposes with the aid of plentiful moisture, eventually turning into dark, crumbly humus.

In comparison, the desert's native plants drop insignificant organic matter. Consider the leaf size of an ironwood tree versus a maple tree, or the build-up at the base of a cactus versus a pine tree. The desert's aridity also slows decomposition. Consequently, desert soil contains less than one-half of one percent organic matter: it just can't accumulate enough to become that rich, black, easy-to-dig soil you may be used to.

This lack of organic matter contributes to a lack of nitrogen in desert soils. Nitrogen is one of the most important elements required for plant growth. However, desert plants seldom suffer from nitrogen deficiency. They have either evolved to thrive without much nitrogen (cacti) or they make their own. Most of our desert plants are legumes (bean plants), which manufacture nitrogen in their root systems with the aid of soil bacteria.

Another characteristic of desert soil is high alkalinity, ranging from 8 to 8.5 on the pH scale. The pH level affects a plant's ability to absorb nutrients. Plants that are native to acidic soil conditions, such as gardenia, will struggle to obtain nutrients from desert soil. They often appear "chlorotic" or yellow. Native and desert-adapted plants readily absorb what they need.

To Amend or not to Amend?
If desert soil lacks organic matter and is highly alkaline, does it help to add amendments to planting holes to offset these conditions? No, say researchers. Although it was once common to amend backfill, they no longer recommend this practice. For a plant to develop, roots must spread outwards into the surrounding soil to provide a strong anchor. It is now known that roots tend to stay within heavily amended planting holes, basically becoming rootbound as if they were still in a pot. Also, it is virtually impossible to permanently and significantly alter soil characteristics. Slight reductions in pH may be obtained, but this is temporary and localized.

Exception to the Rule
Unlike landscape plantings, garden beds should be amended with organic matter. Annual vegetables and flowers complete their life cycle in one short growing season. Rich, well-drained soil is essential to fuel this burst of activity, and organic matter adds nutrients and improves drainage. Root systems of annual plants don't spread as widely or deeply as landscape plants. Because the root zone is relatively small and the plants are pulled out at the end of each growing season, it is practical to incorporate amendments. In my next report, I'll discuss how to improve garden soil.


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