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. Soils are the principal medium of plant growth, and they provide habitat for a myriad of organisms—particularly decomposers. Soils store and purify water, and they exchange gasses with the atmosphere. Soils support agriculture and natural ecosystems, 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 the Earth as distinguished from rock—where vegetation grows. The word is derived (through Old French) from the Latin solum, which means “floor” or “ground.” It is the most basic resource upon which all terrestrial life depends, and soil is one of the most important resources we have. The West 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 the nutritious organic matter that helps soil to nourish future plant growth. The second important component of soil is the sediment derived from the weathering of rock that is then transported by wind, water, or gravity. Both of these components influence the texture (Figure 8.1) and consistency of the soil, as well as the minerals available for consumption by plants.

All soils may seem alike, but there can be vast differences in soil properties even within small areas. A single acre may have 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. The textures have specific names, as indicated in Figure 8.1.

Generally, the best agricultural soils are those with about equal amounts of clay, silt, and sand. A soil of that type would be called a loam. Soils that are mostly sand do not hold water very well and dry quickly. Soils with too much clay may never dry out.

Soil structure refers to the way the soil forms clumps. These clumps are known as peds. The 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:

1. Parent material is the original geologic material from which the soil formed. This can be bedrock, preexisting soils, or other materials such as till or loess.
2. Climate strongly determines the temperature regime, amount of moisture, and type of biota that interact with the parent material. This will affect the extent of chemical and physical weathering on the soil-forming material.
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. Topography 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. Other areas that are more arid and have a flatter topography, such as the deserts in the Basin and Range region of Nevada, 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 like the forests in the Cascade-Sierra region.
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 that had 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. 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. The effects of time can be seen when comparing soils on a glaciated area to either soils formed on recent flood plain deposits or soils in a non-glaciated area at the same latitude.

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 in soil water can come from the atmosphere, where it dissolves in rainwater. Even more CO₂ usually comes from the soil itself, where respiring organisms produce it. The amount of CO₂ in soil gases can easily reach levels ten 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. Acid-driven 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.

A second important type of weathering reaction is oxidation. In Hawai’i, for example, basalts mostly contain ferrous iron (Fe²⁺), which tends to give minerals a green to black color. As this iron reacts with oxygenated soil and water, the iron is converted to ferric iron (Fe³⁺), which generates strong red-orange colors. Ferric iron is not very soluble (as anyone knows who has ever tried to “wash off” rust), and it tends to accumulate in weathered soil profiles. The striking “red dirt” soils in Hawai’i are excellent examples of oxidation acting on ferrous iron-rich basalt (Figure 8.2).

In highly weathered settings, the mineral soil has lost most of its nutrients, and the store of nutrients that remains is mostly found in organic matter. In weathered soils, only the top 25 cm (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 to mature. More mature soils will develop a variety of horizons unique to their environmental conditions, creating 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 8.3).

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, and it was developed by the United States Department of Agriculture (USDA) with the help of soil scientists throughout the country. It provides a convenient, uniform, and detailed classification of soils throughout the country, allowing for an easier understanding of how and why different regions have developed unique soils.

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 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 smaller families (7500+), and the similar soils within families are grouped together into the smallest category of all: a series (Figure 8.4). There are more than 19,000 soil series described in the United States, with more being defined every year.

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 Contiguous Western States

Dominant Soils of Alaska

Andisols: These acidic soils are associated with volcanic ash and debris deposits. They are especially prevalent along the southern Aleutian Islands, where they support low vegetation, and in the southeastern panhandle, where they support forests.

Entisols: These are soils with poorly developed horizons of recent origin. In Alaska, they are common along the banks of the Yukon and other large rivers, but they are also the relatively productive agricultural soils in the Matanuska-Susitna area, Alaska’s “breadbasket.”

Gelisols: These are soils of cold climates that contain permafrost. Gelisols are found throughout Alaska and are by far the most common soils in the state. Decomposition of organic matter occurs at a very slow rate in these soils.

Histosols: These soils contain high concentrations of organic matter, due to their development in wetland environments with poor drainage and a slow rate of decomposition. They are associated with peat bogs and mucks in southern Alaska and the Alaska panhandle, where permafrost is less common.

Inceptisols: These are soils with poorly developed horizons, associated with both Alaska’s interior highlands and parts of the western coastal plains.

Mollisols: These are usually the dominant soils of grasslands, and, as such, are very uncommon in Alaska. They support forests with large proportions of spruce, birch, and aspen trees that are found sprinkled along the Pacific Coast.

Spodosols: These are acidic soils with an accumulation of iron and aluminum in the humus. These soils support cool, moist coniferous stands of forest and are associated with boreal forests. These are found primarily along Alaska’s southern coast.

Dominant Soils of Hawai’i

Andisols: The most abundant soil type in Hawai’i, these soils formed from volcanic ejecta such as ash and cinders—materials that weather to form clay minerals. These soils are extremely rich in organic material in their upper horizons, and have a large water-holding capacity. This makes them highly productive soils for agriculture, and they are easy to cultivate. They do lose some fertility in areas of higher rainfall (over 150 centimeters [60 inches] annually), as the heavy rains wash nutrients from the soil.

Aridisols: These desert soils are formed in areas where the rainfall is so low that vegetation is almost completely absent, such as on the leeward side of the islands. They are shallow with no developed horizons, and they become saturated with salts, as there is no rainfall to wash the salts away. With irrigation, however, they can become useful agricultural soils.

Entisols: These are poorly developed soils with a high mineral component and no real developed horizons. They occur on sands formed from coral reefs or on alluvium in drier areas.

Histosols: These soils develop from accumulations of organic materials on top of lava flows. They occur in cooler, moist conditions where anaerobic (low oxygen) environments are common. In areas of higher rainfall, they can become very acidic. The typical types of vegetation found on this soil order are Ohi’a trees and ferns.

Inceptisols: These young, poorly developed soils are found on active slopes or in river valleys where material is constantly being deposited. They are most common on the older islands, and also occur in river valleys on the older volcanoes of the younger islands.

Mollisols: In other parts of the globe these rich, dark-colored soils are found on grasslands. In Hawai’i, however, the Mollisols are reddish in color, due to their high iron content, and they are found on the coastal plains and gentle slopes up to around 300 meters (1500 feet) above sea level in drier areas (65 - 130 centimeters [25 - 50 inches] of rain or less annually). In the past, these areas were extensively used for sugarcane crops.

Oxisols: These are highly weathered soils that are very low in fertility and develop in hot tropical climates. They are found on low-elevation dry areas, or in some of the very wet highland areas. They are very common on the older islands of Kaua’i and O’ahu, but they are not found on the younger islands.

Spodosols: These soils form in forests in moist to wet areas on some of the uplands in Kaua’i and Moloka’i. They are not used for any form of agriculture.

Ultisols: These weathered soils are rich in the clay mineral kaolinite and form in warm humid climates with distinctive wet-dry seasons. They zone into Oxisols on some islands when rainfall decreases. They are acidic and yet, with the proper use of fertilizers, have become highly productive agricultural soils in Hawai’i.

Vertisols: These are very dark soils, rich in swelling clays. Their distinguishing feature is that they form deeply cracked surfaces during dry periods, but they swell again in the wet season, which seals all the cracks. As a result, they are very difficult soils to build roads or other structures on.