DESERT CONDITIONS
How They Are "Different"
Visitors to the American Southwest often get their first look at the desert in winter. The weather is beautiful and inviting. Bright sunny days are the rule, and temperatures are comfortable day and night. During a "cold" winter, even the few nights of frost are not an inconvenience; the next morning the sun shines warm and friendly. Desert vegetation and yard plants are green. When it rains, the showers are gentle and kind. Winters in the desert are like summer "back home."
As those of us who live here could tell our winter visitors, it's not like that all through the year. The desert offers landscape gardeners some thorny challenges.
DESERT SOILS
Desert soils are short of organic matter. They are sandy and gritty and gravelly. With a sparse vegetation, no leaf litter is formed, and so organic material doesn't accumulate. It either burns up or is blown away before it dries into dust. Because of the low organic matter, desert soils are bright reds and yellows instead of dark browns and black.
Desert soils are low in nitrogen, the stuff that makes plants grow. Nitrogen usually comes from decaying organisms, both animal and plant, and is preserved by dampness. Deserts don't have the same abundance of animals or vegetation that wetter regions do, so nothing gets started. Any little beginning dries up and blows away.
Desert soils usually are low in phosphorus. Phosphorus comes mainly from rocks, but desert rocks are low in phosphates. The result: no beginning, no accumulation, no plant nutrients.
Desert soils contain potash in adequate amounts, largely because the rocks contained it; as they were broken by the heat, the nutrient became available and, because of the lack of rain, it wasn't washed away.
Desert soils contain a great deal of calcium, which can become an impediment to landscaping. First, a soil with too much calcium causes alkalinity, which makes plants-especially those from "back home," like azaleas, camellias, and gardenias-so uncomfortable they can't cope. Second, an excess of calcium in the soil interferes with the uptake of other plant nutrients, such as iron, copper, zinc, and manganese, even if they are present. In the midst of plenty, the plant starves for lack of the essential minor nutrients.
There's another handicap caused by calcium abundance that you learn to recognize early on-as soon as you dig a planting hole, in fact. Calcium dissolves in water; as the water dries, the calcium reforms as a solid. Stalactites and stalagmites are formed this way. In some soils, the reformed calcium looks like nodules of gravel. This doesn't matter too much, because roots can grow around the nodules and water can drain past them.
But sometimes it is a solid layer of a concrete-like, pinkish material three feet thick. That's serious because roots can't grow through it; they "bottom-out" on it. When abundant rains fall or when heavy irrigations are applied, the water can't drain away. Roots are drowned in this collected water, and the plant suffers. We call this stuff caliche, and it's a real nuisance. You have to physically get rid of caliche by digging it out. There's no way to remove it by adding chemicals.
Desert soils often contain sodium, and when they do the problems increase. First, sodium is a plant poison and, second, it destroys the structure of soils, making them greasy and impervious. Fortunately, sodium is water-soluble and can be washed out of the soil as long as there is good drainage.
Desert soils usually are sandy, which helps ensure good drainage. Once in a while, however, there are localized patches of heavy silt or clay. You usually find these in low-lying flatlands where flooded rivers poured their salt-laden waters. Nothing much grows in soils that have bad drainage. |
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