Teacher's Guide - Desert Soils

Deserts cover 1/5 of the worlds land. Over 1 billion people try to make a living out of the desert soils. These soils are dry, getting less than 10 inches of rain a year, and often are not very high in organic matter. Deserts can be very hot, or very cold. Even though it is dry, it does not mean that there is nothing living. After a rainfall, the entire desert blooms. These soils can support agriculture, but often need to be irrigated with water brought in from elsewhere. This can cause salinization. Semi-arid regions of the world are also becoming less agriculturally productive, and turning into deserts, which is caused by human activities.

This lesson is appropriate for grades 4+.

Desert Soils

Lesson Objectives:

1) What are the characteristics of desert soils?

Desert soils are usually light colored
Desert soils have B horizons. Often the B horizons have accumulations of calcium carbonate (caliche), gypsum, and/or salts.
Desert soils have sparse vegetative cover that varies with temperature and elevation
Desert soils are highly variable: They may be deep, shallow, salty or covered with desert pavement, or have crusts or cement-like horizons near the surface.
Deserts cover 20 to 33% of the Earth’s land surface, and can be found in the tropics, at the poles, and in between.
Desert soils (Aridisols) occupy about 12% of the Earth’s ice-free land surface, and 8% of the United States, all in the western states: Texas, New Mexico, Colorado, Wyoming, Montana, Arizona, Utah, Nevada, Idaho, California, Oregon, and Washington.

2) Which ClORPT factors are most important in desert soil formation?

*Climate – Desert soils form in arid climates, where evaporation is more than five times the annual precipitation from rain and snow. Deserts can be hot (like the Sahara or the Mohave) or cold (like Antarctica or at high elevations) or in between (like in Wyoming, Idaho and Oregon. Soil formation is usually limited to near the surface because water is not moving through the soil. Desert soils have B horizons.
Organisms – The organisms vary with climate, but all are adapted to dry conditions. Desert soils support sparse vegetation, which varies with temperature (latitude and/or elevation), and may include cacti, shrubs (sagebrush, creosote bush, saltbush), grasses and/or wildflowers. Few microorganisms live in these dry soils, but algae, bacteria, mosses and lichens can form biological crusts on the surface. Ants, termites, reptiles and rodents live in these soils.
Relief – Desert soils may be nearly level or very steep.
Parent material – Desert soils commonly form in windblown sediments (eolian or loess), and sediments from rivers (past or present) or ancient lakes
Time – Desert soils may be very young or really old (hundreds of thousands of years)

3) Since desert soils are dry, and support little vegetation, can desert soils be used for agriculture?

Range
- Some desert soils support shrubs that goats and sheep enjoy for browse (eating).
- Desert soils may support some grasses, especially after a rain that animals can graze.
- Some desert soils are used by ranchers. It may take 50 to 75 acres (20 to 30 hectares) to feed one cow or a few goats or sheep.
Crops
- People have grown crops using irrigation in temperate and tropical deserts for millennia (before 3000 BC).
- Irrigation water comes from rivers or aquifers (underground areas of porous rock or sand and gravel that hold lots of water)
- California’s Central Valley desert soils produce more than 250 types of fruits and vegetables.
- Irrigation in most desert regions causes a buildup of salt in the soil, a process called salinization.
- Some plants can tolerate more salt than others, but salt in the soil affects plant growth and yield. When the salt levels are too high, plants die.

4) How does salinization work?

All water contains dissolved salts. When the irrigation water is applied to the soil, salts are added with it.
In humid regions, there is enough precipitation to leach (wash) the salts below the plant root zone. In arid regions, the salts stay in the root zone.
When irrigation water evaporates from the soil surface, the salts stay on the soil surface in solid form, often forming a salt crust.

5) Identify the importance of desert soils

Desert soils, or Aridisols, occupy the third-largest land area on the Earth
Desert soils often are used for wildlife and recreation
Desert soils can be used for crop production, if irrigation is available

6) Locate on a map deserts are found in North America and/or the world.

Glossary of Terms:

In current glossary:

Organic matter
organisms
salinization
parent material
erosion
ClORPT
runoff
sediment
slope
topsoil
productive
relief
Groundwater
Horizon

Other Glossary Words:

Irrigation- the process where water is diverted from other places to provide water to crops, so that crops have enough to grow.
Desert Blooms- the sudden blooming experienced in the desert after rainstorms when all of the dormant seeds spring to life
Biological Surface Crust – on the surface of the desert, organisms like lichens, algae, and bacteria form a protective shield, keeping the desert soil in place.
Physical Surface Crust - when raindrops strike the ground very hard it breaks the soil apart, and generates a crust that is platy, protecting the soil underneath.
Desert pavement – most sands, silts and clays have eroded by wind or water, leaving gravel and cobbles on the surface
Eolian – Parent materials deposited by wind with more than 5% sand
Loess – Parent materials deposited by wind with less than 5% sand

Activities or Information:

1) Information

2) Plans:

Oasis in the Desert-New York Times: in this lesson, students explore the challenges of building a thriving human civilization in a desert environment.
Surface Crust in the Sonoran Desert- University of Arizona For those with access to a desert:
Crisis in the Horn of Africa- New York Times (Grades 9-12)

Test Questions:

1) Why would people want to grow crops in the desert, and how would they do it?

2) Where are deserts located in the United States?

3) Do desert soils have a lot of organic matter? Why or Why not?

4) What is salinization? Why is it a problem?

Teacher's Guide - Desert Soils

Know Soil, Know Life