Soil: They Don't Live in the Desert

Soil: the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles. (Oxford Dictionaries, online version)

Typical Mojave  (or Mohave) Desert scene

            It is obvious the authors who wrote this popular definition of soil did not live in the Inland Empire, Great Basin, Mojave (or Mohave), Sonoran or Chihuahuan deserts. Is it any wonder that people moving into desert environments from less agri-hostile locations have so much difficulty growing things? The importance of soils in desert production should not be underestimated.

The further a plant is from its native environment, the more time, energy and money is needed for it to perform satisfactorily. When we design landscapes with plants less suitable to desert environments, we can be certain man-hours and resources must be increased resulting in higher costs and increasing budgets.

Selecting the right plant, planting it in the right location, in the right soil and managing that soil correctly is paramount to managing limited resources efficiently.

When I was studying at the University I was taught that the average soil for plant production should have test results like these: pH of 6.5-7.5, organic matter content of 3 to 5%, maximum salinity less than 4.0 mmhos/cm (dS/m) and a porosity (collective air spaces) of approximately 50%. As an inexperienced student, I assumed soils outside of these ranges would extensively damage or even kill plants.

Typical raw Mojave Desert soil, low in organic matter, pH 8.2 and higher and never has been cropped or developed.

Then I moved into the Mojave Desert and became aware that plants could live outside of these ranges. Valid soil tests from desert soils under development, but already growing honey mesquite, rabbit brush, desert sage, creosote bush, Opuntia, Cholla, Agave and other desert species, routinely provided a pH of 8.2 and above, organic matter content that was not measurable, salinity exceeding 100 dS/m and toxic salt levels like boron over 50 ppm. I knew I wasn’t in plant Hell, but thought I could see it from here.

Studying and working in the arid and desert West and finally the deserts and arid regions of the Middle East, Central Asia and northern and southern Africa I have learned that nearly any desert soil is manageable. Management decisions change when the plant palette changes.

Orchard management in Ethiopia

My personal definition of soil has changed. Soil is a mixture of substances, either natural or man-made, which will grow plants if properly managed. Irrigated desert soils must be managed if they are to grow plants. Soils do not need to be black or dark brown and they may not have organic matter in them but they will still grow plants.

Years ago friends of mine at the University of Sonora in Hermosillo taught me a valuable lesson about amending desert soils. I began production trials of two selections of edible Opuntia from Sonora, Mexico. I planted pads the same distances they were planted at the field station in Hermosillo. They were planted in unamended, raw Mojave Desert soil at UNLV’s Center for Urban Horticulture and Water Conservation in North Las Vegas, Nevada.

Nopal cactus from Hermosillo, Mexico, (USON) planted without organics (left( and with organics (right) after two years (left) and one year (right).

One year later faculty from the University of Sonora came to visit the plots and see the progress that was made. The Opuntia were all growing but I was disappointed in their progress and production of nopales (edible immature pads) and tunas (cactus fruit).

They asked me if I had amended the soil with composted manure before I planted the pads. I had not. I assumed that because these were desert plants they would perform better in a desert soil that was not amended. I was wrong.

We replanted a second area with fresh pads of the same varieties, this time amending the soil with composted manure. The results were remarkable. The second planting of Opuntia caught up to the older planting early in the first growing season and exceeded it soon afterwards. This was my first lesson: desert plants might tolerate unamended desert soils but they may not thrive in them.

The desert provides a unique environment for the evolution of soils and plants that can grow in those soils. High temperatures, low humidity, intense sunlight and lack of rainfall contribute to desert soil’s unique appearance, structure, biology and chemistry. Once water is added to this mix, everything changes and it can change rapidly.

This does not mean desert soils should not be improved. They can and should be improved if we are to grow a wide range of plants. But it is important to understand that amendments produced in desert environments may have different qualities from amendments produced in wetter, cooler climates.

Desert soil improved with compost for vegetable production. It will be deeply incorporated and the  amended soil shaped into beds for production.

Two major problems which can occur in soil amendments produced in desert environments are high levels of salts and difficulty in wetting (hydrophobic) the amendment due to a very low moisture content. These are frequent criticisms of amendments and composts produced in desert climates.

Salinity problems arise from a high total salt content in the amendment or from specific compounds which contain elements that can damage plants. Specific elements that can damage plants, and should be reported in laboratory tests by suppliers, include boron, sodium and chlorides. With some monitoring by amendment producers, levels of these specific elements that can be damaging to plants can be lowered or minimized.

Salt damage to Meyer lemon.

Total salinity of amendments produced in desert climates may be a problem if the range in salinity acceptable for soil amendments is 3 - 4 dS/m and poor quality water, such as Colorado River water, is used to leach excessive salts. It takes more water to reduce salts in desert environments than in wetter climates. More water usually translates to higher costs of production.

Remember that the salinity of Colorado River water in the Lower Basin approaches 1.2 dS/m. In other words, for every 300,000 gallons of water used for leaching, about 1 ton of salt is delivered to the compost. These salts must also be leached, reducing the leaching effectiveness of the applied water.

Colorado River

Compounding this problem is very high evaporation, exceeding 4/10 of an inch of water per day, during summer months. Evaporation of this water leaves behind salts, adding to the salt load which must be lowered. This increased salt load also requires more water for leaching.

Where does this water used for leaching go? In the case of Las Vegas and other desert communities along the River, these plant nutrients are fed into the Colorado.

Soil amendments which contain quite a bit of course plant fibers can become excessively dry in desert environments. Once dry, these amendments can be difficult to wet without a lot of effort or wetting agents. Hydrophobic amendments like these can also increase the difficulty in leaching salts.

Poor root development in sweet corn growing in a raised bed due to hydrophobic soil forming from incorporated compost.

Establishing acceptable physical and chemical criteria for soil amendments is important for the landscape and nursery industry. Even more important is to educate consumers how to interpret tests for soil amendments when applying them to specific landscapes. In this way soils and soil amendments can be better managed. “Me too” standards is not a sustainable answer to across-the-board recommendations for all landscapes.