Soils of the Southwestern US

It’s sometimes easy to take the soil beneath our feet for granted. Yet soil has always been with us—it is the foundation of our houses and roads, and from the soil comes our food, fiber, and paper. Soil is the interface between living earth and solid rock, between biology and geology. The engineer, the scientist, and the gardener may all look at the soil beneath them in different ways, but perhaps no one has a more integral relationship with soil than a farmer. The economic success of producing crops is intimately tied to the quality of the soil upon which those crops grow, and the most successful farmers are well versed in the science of their soil. Soils store and purify water, and they exchange gases with the atmosphere. They support agriculture and natural ecosystems and provide a grassy surface for our parks and fodder for our gardens. Everyone, everywhere, every day, depends upon the soil.

What is Soil?

Generally, soil refers to the top layer of earth—the loose surface of earth as distinguished from rock—where vegetation grows. The word is derived (through Old French) from the Latin solum, which means “floor” or “ground.” Soil is one of the most important resources we have—the most basic resource upon which all terrestrial life depends. The Southwest has a wide variety of soils, and each type of soil has a story to tell of its origin.

Soils form from the top down, and typically reach a depth of about one meter (3.3 feet) at their more developed stages, although some can reach much deeper. Soils are composed of a mixture of two key ingredients. The first is plant litter, such as dead grasses, leaves, and fallen debris. Worms, bacteria, and fungi do the job of breaking these down into nutritious organic matter that helps soil to nourish future plant growth. The second important component of soil is sediment derived from the weathering of rock that is then transported by wind, water, or gravity. Both of these components influence the texture (Figure 7.1) and consistency of the soil, as well as the minerals available for consumption by plants.

All soils might seem alike, but there can be vast differences in soil properties even within small areas! A single acre may contain several different soil types, each with its own assets and drawbacks. Some types of soil are clayey or prone to flooding, while others are stable enough to be used as a foundation for buildings. The most identifiable physical properties of soils are texture, structure, and color, which provide the basis for distinguishing soil horizons. Texture refers to the percentage of sand, silt, and clay that makes up the soil. Soil textures have specific names, as indicated in Figure 7.1.

Figure 7.1: Soil texture triangle.

Figure 7.1: Soil texture triangle.

Generally, the best agricultural soils are those with about equal amounts of clay, silt, and sand. A soil of that type is called a loam. Soils that are mostly sand do not hold water very well and dry quickly, while soils with too much clay may never dry out. Soil structure refers to the way the soil forms clumps, known as peds. Peds are identified by the shape of the soil clods, which take the form of balls, blocks, columns, and plates. These structures are easiest to see in recently plowed fields, where the soil is often granular and loose or lumpy. Soil color is its most obvious physical property. The color is influenced by mineral content, the amount of organic material, and the amount of water it routinely holds. The colors are identified by a standard soil color chart called the Munsell chart.

Five main variables affect the characteristics of soil worldwide. In the Southwest, all soils are the products of subtle differences among these five factors:

  1. Parent material is the original geologic material from which the soil formed. This can be bedrock, preexisting soils, or other materials such as alluvium, rock fragments, and windblown sediment.
  2. Climate strongly determines the temperature regime, amount of moisture, and type of biota that interact with the parent material. This affects the extent of chemical and physical weathering on the soil-forming material. For example, if a particular climate lacks precipitation, mechanical weathering from wind or ice fracturing will predominate. If, however, a climate has abundant precipitation, chemical erosion from water will be accelerated, resulting in substantial leaching.
  3. Topography, or landscape, of the area is related to the relative position of the soil on the landscape. This includes the presence or absence of hills and the slopes between high and low areas. As the slope increases, water can carry larger sediment sizes, allowing for large sediment loads during major precipitation events. Topography also influences natural drainage. Gravity moves water down slopes to depressions or streams and pulls free water downward through the soil. Soils on hills tend to be dry, and soils in depressions and valleys are often wet or saturated. Areas with steep slopes that are susceptible to frequent erosion typically have very young soils, as they do not have long to develop before the ingredients are rearranged and the clock is reset. Flatter, more arid areas may have more time to develop, but they have significantly less plant life and will produce a very different soil than will a wetter environment. Slope also frequently determines the types of vegetation covering a soil—for example, different slopes on the same hill might receive varying amounts of sunlight during the growing season—which in turn can cause the characteristics of the soils to diverge if differing forms of vegetation dominate opposite slopes.
  4. Biota or living organisms that live on or in the material affect soil development through their influence on the amount and distribution of organic matter in the soil. For example, plants contribute significantly to the formation of humus, and animals alter a soil’s characteristics by leaving behind decayed remains and wastes. Decomposers like bacteria and fungi help to free up the nutrients locked away in these remains and wastes, and these freed nutrients are then recycled and used by new life forms within the same soil. In fact, more than 90% of the nutrients used by a forest in a given year are derived from the decomposition of old organic matter fallen to the forest floor. Animal burrows also create spaces in the soil horizons that allow for deeper penetration of air and water, which, in turn, aid plant development by helping to dissolve mineral nutrients into a form that plants can absorb and process. For its part, organic matter impacts the water-holding capacity of the soil, the soil’s fertility, and root penetration.
  5. Time is required for soils to develop while the four elements mentioned above interact. Older soils have deeper and thicker subsoils than do younger soils, but only if other soil forming factors remain constant. In northeastern Colorado, for example, it takes approximately 500 years to generate a new 2.5 centimeters (1 inch) of topsoil beneath the prairie grass—but it only takes a few years for erosion and weathering to destroy the same amount of unprotected topsoil.

Several types of chemical reactions are important for soil development; of these, acid-base reactions are some of the most important and complex. When carbon dioxide (CO2) dissolves in water it forms weak carbonic acid. CO2 found in soil water can come from the atmosphere, where it dissolves in rainwater. Even more CO2 usually comes from the soil itself, where it is produced by respiring organisms. The amount of CO2 in soil gases can easily reach levels 10 times higher than the amount found in the atmosphere (over 4000 ppm in soil vs. 400 ppm in the atmosphere), making soil water potentially more acidic than rainwater. As this acidic water slowly reacts with fresh minerals, it buffers the soil’s pH and keeps it in a range (6–8) preferred by many organisms. Aciddriven weathering breaks down the soil’s primary igneous minerals, typically transforming them to silica-rich clays. As the soil’s primary minerals are depleted, it loses the ability to buffer acidity, and the pH of highly weathered soil can drop to around 4. These weathered soils tend to be rich in aluminum, iron, and titanium.

In highly weathered settings, soil loses most of its nutrients, and the store of nutrients that remains is mostly found in organic matter. In weathered soils, only the top 25 centimeters (10 inches) or so may be very biologically active, and rooting depths are very shallow. If this thin layer is lost to erosion, the underlying mineral soil may be infertile and incapable of rapid recovery.

Soil Orders

Just as rocks are classified into different types based on how they formed (igneous, metamorphic, or sedimentary), their mineral composition, and other characteristics, soils also have their own classification scheme. Soil develops in horizons, or layers, whose formation is dependent on the available ingredients, environmental conditions, and the time it takes to mature. Since the organic and chemical processes that form soils first impact the top of the soil column and then work their way downward, horizontal layers of soil with different characteristics are formed, resulting in divergent colors, textures, and compositions.

A vertical cross-section of all the horizons or layers of soil present in a given area is referred to as a soil profile. Some horizons are completely absent in certain profiles while others are common to most. Each horizon corresponds to a stage in the weathering of rock and decay of plant matter, and each is found at a specific position beneath the surface Figure 7.2). The O horizon at the top of the profile contains partially decayed plant material and transitions down to the A horizon, which contains mineral matter with a mix of humus and is commonly referred to as topsoil. Below the A horizon lies the B horizon or subsoil, which contains mineral material that has leached from above. The C horizon at the base of the soil profile contains partially altered parent material.

Soils can also be categorized by their location (northern vs. southern soils), the type of vegetation growing on them (forest soils vs. desert soils), their topographic position (hilltop soils vs. valley soils), or other distinguishing features. The system used to classify soils based on their properties is called soil taxonomy (Figure 7.3), and it was developed by the United States Department of Agriculture (USDA) with the help of soil scientists from across the country. It provides a convenient, uniform, and detailed classification of soils throughout the US (Figure 7.4), allowing for an easier understanding of how and why different regions have developed unique soils.

Figure 7.2: A typical soil profile shows the transition from the parent material (horizon C) to the highly developed or changed horizons (O through B). Not every soil profile will have all the horizons present.

Figure 7.2: A typical soil profile shows the transition from the parent material (horizon C) to the highly developed or changed horizons (O through B). Not every soil profile will have all the horizons present.

Figure 7.3: Soil taxonomy.

Figure 7.3: Soil taxonomy.

In soil taxonomy, all soils are arranged into one of 12 major units, or soil orders. These 12 orders are defined by diagnostic horizons, composition, soil structures, and other characteristics. Soil orders depend mainly on climate, parent material, and the organisms within the soil. These orders are further broken down into 64 suborders based on properties that influence soil development and plant growth, with the most important property being how wet the soil is throughout the year. The suborders are, in turn, separated into great groups (300+) and subgroups (2400+). Similar soils within a subgroup are grouped into even more selective families (7500+), and similar soils within families are grouped together into the most exclusive category of all: a series. There are more than 19,000 soil series described in the United States, with more being defined every year.

Figure 7.4: Dominant soil orders of the United States.

Figure 7.4: Dominant soil orders of the United States.

The 12 Soil Orders

Name Description Controlling Factors Percentage of global ice-free land surface Percentage of US ice-free land surface
Alfisols Highly fertile and productive agricultural soils in which clays often accumulate below the surface. Found in humid and subhumid climates. climate and organisms ˜10% ˜14%
Andisols Often formed in volcanic materials, these highly productive soils possess very high water- and nutrient-holding capabilities. Commonly found in cool areas with moderate to high levels of precipitation. parent material ˜1% ˜2%
Aridisols Soils formed in very dry (arid) climates. The lack of moisture restricts weathering and leaching, resulting in both the accumulation of salts and limited subsurface development. Commonly found in deserts. climate ˜12% ˜8%
Entisols Soils of relatively recent origin with little or no horizon development. Commonly found in areas where erosion or deposition rates outpace rates of soil development, such as floodplains, mountains, and badland areas. time and topography ˜16% ˜12%
Gelisols Weakly weathered soils formed in areas that contain permafrost within the soil profile. climate ˜9% ˜9%
Histosols Organic-rich soils found along lake coastal areas where poor drainage creates conditions of slow decomposition and peat (or muck) accumulates. topography ˜1% ˜2%
Inceptisols Soils that exhibit only moderate weathering and development. Often found on steep (relatively young) topography and overlying erosion-resistant bedrock. time and climate ˜17% ˜10%
Mollisols Agricultural soils made highly productive due to a very fertile, organic-rich surface layer. climate and organisms ˜7% ˜22%
Oxisols Very old, extremely leached and weathered soils with a subsurface accumulation of iron and aluminum oxides. Commonly found in humid, tropical environments. climate and time ˜8% ˜.02%
Spodosols Acidic soils in which aluminum and iron oxides accumulate below the surface. They typically form under pine vegetation and sandy parent material. parent material, climate, and organisms ˜4% ˜4%
Ultisols Soils with subsurface clay accumulations that possess low native fertility and are often red hued (due to the presence of iron oxides). Found in humid tropical and subtropical climates. climate, time, and organisms ˜8% ˜9%
Vertisols Clayey soils with high shrink/swell capacity. During dry periods, these soils shrink and develop wide cracks; during wet periods, they swell with moisture. parent material ˜2% ˜2%

Dominant Soils of the Southwest

Six soil orders are found in the Southwest, with the greatest diversity found within the Basin and Range.

Alfisols are partially leached soils with a high degree of fertility, in which clays often accumulate below the surface. They tend to develop in cooler, more forested environments, and commonly form a band separating humid areas from more arid areas. In the Southwest, they occur primarily at higher elevations (Figure 7.5).

Aridisols are very dry soils that form in arid environments, such as the Basin and Range region. Water content is very low or even nonexistent for most of the year, leading to limited leaching. The lack of leaching means that there is abundant calcium carbonate in these soils, making them quite alkaline. This soil type is unsuitable for plants that are not adapted to store water or to survive extreme drought. Because of the Southwest’s dry climate, Aridisols are the most abundant soil type there (Figure 7.6).

Entisols are soils of recent origin with poorly developed horizons, typically formed near floodplains and on recent erosional surfaces. These soils are found throughout the Southwest in association with river courses, rapid erosion, and wind-transported sediment (Figure 7.7).

Inceptisols are soils with poorly developed horizons that are associated with steep slopes and erosion-resistant parent material. Inceptisols are located in cool to very warm humid and subhumid regions. They are uncommon in the Southwest thanks to the area’s generally dry climate, and are concentrated at high elevations, especially in the Rocky Mountains (Figure 7.8).

Mollisols are the dominant soils of grasslands. Their thick, black A horizon makes these soils extremely productive and valuable to agriculture. These soils are extensive in the Great Plains, and are also found in bands along parts of the Colorado Plateau (Figure 7.9).

Vertisols are dark soils, rich in swelling clays. They dramatically change in volume depending on the moisture level of the soil, forming deeply cracked surfaces during dry periods and swelling again in the wet season to seal all the cracks. As a result, they are very difficult soils to build roads or other structures on. Vertisols are rare in the Southwest, and are present only in a few areas of Arizona and New Mexico (Figure 7.10).

Although these soil orders occur in other parts of the country, the Southwest’s arid climate allows for a secondary way of classifying soils—by the temperature zone in which they occur. Exposure to extreme temperatures means that many Southwestern soils are considered to be hyperthermic, and may not seem to have a lot in common with their equivalents in other regions. In addition, many of the area’s soils are thin because the requisite conditions for the formation of deep soils are not present.

Figure 7.5: Alfisols of the Southwest.

Figure 7.5: Alfisols of the Southwest.

Figure 7.6: Aridisols of the Southwest.

Figure 7.6: Aridisols of the Southwest.

Figure 7.7: Entisols of the Southwest.

Figure 7.7: Entisols of the Southwest.

Figure 7.8: Inceptisols of the Southwest.

Figure 7.8: Inceptisols of the Southwest.

Figure 7.9: Mollisols of the Southwest.

Figure 7.9: Mollisols of the Southwest.

Figure 7.10: Vertisols of the Southwest.

Figure 7.10: Vertisols of the Southwest.

Geology of the Southwest: Parent Material

The Southwest is home to a wide variety of parent materials—the minerals and organic matter from which its soils are derived (Figure 7.11). Mineral material determines a soil’s overall fertility and the vegetation it supports.

Weathered sedimentary rock is a ubiquitous parent material throughout many areas of the Southwest. The western half of Utah, for example, is part of a much larger hydrographic area known as the Great Basin, which contains sandstone, shale, and limestone that accumulated over millions of years. The eastern halves of Colorado and New Mexico are also home to an abundant supply of weathered sedimentary rock—sandstone, siltstone, limestone, and shale are among the most common bedrocks in this portion of the Southwest. Much of this rock was laid down during the Paleozoic, when shallow inland seas repeatedly flooded the landscape.

Figure 7.11: Physiographic and regolith map of the Southwest.

Figure 7.11: Physiographic and regolith map of the Southwest.

The majority of the Southwest contains extremely varied parent material generated during the tectonic events that led to the uplift of the Rocky Mountains. Orogenic events during the Mesozoic, eruptions of ash and lava from volcanic mountain ranges to the west, and inundation by the late Cretaceous Western Interior Seaway all contributed to the formation of the igneous, sedimentary, and metamorphic rock types found throughout the area. Soils in the Rocky Mountain and Colorado Plateau regions are derived largely from the erosion of these materials, while soils in the Basin and Range are mostly composed of gravel, sand, and other debris eroded from the desert and mountainous landscape.