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Soil Formation

Soil Formation

Washington Soil Atlas

Soil forms layers or horizons, roughly parallel to the earth’s surface, in response to five soil forming factors. The whole soil, from the surface to its lowest depths, develops naturally as a result of these five factors. The five factors are: 1) parent material, 2) relief or topography, 3) organisms (including humans), 4) climate, and 5) time. If a single parent material is exposed to different climates then a different soil individual will form. If any one of the five factors is changed but the remaining four factors remain the same, a new soil will form. This process is called “soil genesis”.

It is thought that roughly 95 percent of the world’s soils have been moved or transported to their present location. Only 5 percent of the world’s soils are “residual soils” or soils that formed in place from the existing parent material. However, there are other differences in soils that are determined by whether a soil is formed in mineral material (sand, silt, and clay) or whether a soil formed in organic material (plant and/or animal matter that is in various stages of decay and decomposition). Soils that formed in dominantly organic material are classified as Histosols according to Soil Taxonomy. Parent material contributes both chemical and physical properties to an individual soil.

5 Soil Formation Factors

Parent Material
Parent Material

Geologic events have provided Washington with a wide variety of parent materials. In more recent times, many of us are familiar with blowing dust (loess) during wind storms and volcanic ash from the eruption of Mount St. Helens in 1980. These are two of the parent materials that contribute to Washington State’s soils. Probably the most common parent material which covers almost all of the land surfaces in Washington is volcanic ash from numerous eruptions of the Cascade Mountains in the western United States. The volcanic ash is very common either by itself on the surface (volcanic ash mantles) or mixed with other material in surface horizons. Volcanic ash has very specific properties such as very light weight, capacity to hold large amounts of water, and susceptibility to compaction by vehicles when it is wet. These are some factors that influence soil genesis in Washington State.

There are also many rock formations in Washington so there are also many kinds of rocks in Washington soils. Examples are granite, schist, limestone, basalt, tuff, and many others. Glaciers from the highest mountains in Washington and from north of Washington in Canada have ground up many kinds of rocks and pushed glacial till along the earth’s surface and commonly deposited it far from the source, creating yet another kind of soil parent material.

Water has transported and deposited large quantities of material in all parts of the state which has created another kind of soil parent material. Slower flowing water carries particles such as sand, silt, clay, and tiny rocks downstream until the water slows enough that the materials drop out of the water and form sediments that become soil parent material. Fast-moving water carries gravel, cobbles and stones from a wide variety of rock formations downstream and deposits them in various locations to form another kind of parent material. Material that has been transported by water is called alluvium.

Swampy areas or old lakes collect dead and/or dying plants and animals to form peat which is also a soil parent material. Peat deposits, protected from decay by being saturated with water, form the organic soils (Histosols). They usually also contain small amounts of mineral matter like sand, silt, and clay. These low-lying areas commonly collected volcanic ash when it was deposited from the air and when it was washed off adjoining uplands. A few soils form in thick deposits of volcanic ash in lower landscape positions.

It is thought that about 70 to 75 percent of the earth’s crust is made up of sedimentary rocks and the remaining 25 to 30 percent is made up of igneous rocks and glacial materials. Coarse-grained igneous rocks such as granite weather to sandy types of materials; thus soils that formed from these kinds of rocks have a sandy texture. Fine- grained sedimentary rocks such as siltstone and shale weather to finer textured materials; thus soils that formed from these types of rock are more clayey.

All of the different kinds of parent material have been subjected, in varying degrees, to the other four soil forming factors. All five of the soil forming factors are acting at the same time at different rates of speed and with different degrees of efficiency.


Very few Washington landscapes are flat. Most of them exhibit some relief or topography related to the type of landform that they occupy. A landscape location 1) has an elevation either above or below another part of the landscape, 2) has a distinct shape (convex, concave or linear), 3) faces a specific compass direction, and 4) is only one component of the landscape. These factors influence drainage, runoff, deposition, and erosion as well as the collection of solar energy.

The most common flat landscapes are those in wide valleys along rivers. The soils in these areas range from poorly drained to moderately well drained and have very limited runoff. During floods, soil material commonly is deposited in these landscape positions and erosion generally is not a concern. Erosion from surrounding slopes also results in additional depositions of soil material on the valley bottoms. These landscapes are often cooler because of cold air drainage from higher surrounding areas. Livestock often over-use these areas which results in compaction of the surface soil.

South-facing slopes (south aspects) are warmer and dry out faster because they receive more solar heat than north-facing slopes (north aspects). This affects soil genesis because the warmer temperatures speed up most chemical reactions and increases the evaporation of water from the soil profiles. The drier nature of south aspects result in production of different natural plant communities than those on the more moist north aspects. The shape of the topography also contributes to how the slope disperses water. Concave-shaped slopes tend to concentrate water which causes more erosion and runoff. Convex-shaped slopes tend to disperse water more uniformly. Concave positions in flatter landscapes tend to collect water and these soils are more poorly drained and may have a water table near the surface.


Washington’s climate, like its topography, varies greatly from place to place. In fact, soil climate changes quite rapidly in very short distances. Annual precipitation varies from about 7 inches in parts of the Columbia Basin to more than 300 inches in the Olympic Rainforest. Some areas in Washington receive very little snowfall and other areas receive many feet of snow in winter. Accumulation of snow in winter and melting of snow during the spring and summer provides runoff water in areas where precipitation is low. Great differences in temperature and in the number of frost free days also occur across Washington.

Temperature changes with increases and decreases in elevation and it also changes with aspect. Moisture and temperature differences are also evident in soil genesis. Climate directly and indirectly effects soil formation. Less development occurs in drier areas because as water quickly moves into and through a soil it increases the rate of weathering of soil materials. For example, soluble materials such as organic matter, clay, and calcium carbonate and other salts are moved downward in a soil profile and sometimes out of a soil profile if enough water is available. In general terms, the depth at which soluble material occurs in a soil profile indicates the amount of water that the soil individual receives. Thus a record of the average annual precipitation and average annual soil temperature on each site is important.

The amount of moisture within a soil profile also impacts the soil pH. Soil pH is a determining factor in the kinds of plants that can grow on a soil. It also affects the availability of other nutrients that plants need to grow.

If a soil is wet, soil characteristics illustrate that fact. Soils that have a water table that moves upward and downward during different times of the year contain mottles (rusty spots) or what soil scientists call “redoximorphic features” that are similar to spots that form on a shovel that is left out in the rain. These yellowish and orange “rusty spots” help soil scientists determine where a water table occurs in a soil. Most of these wet soils in Washington are in depressions or along rivers and streams. Soil wetness is a concern for many uses unless the soil can be drained. When a soil is “too wet” there is also very little if any room for air in the soil profile. Many kinds of plants and animals are not able to live on or in these soils because of the lack of soil air. However, there are plants that have adapted to life on wet soils and these plants help a soil scientist identify wet soils.


Both living plants and animals (including humans) affect natural soil formation. The kinds of plants that grow on a soil impact the kind of plant residue that form and is incorporated into the soil. Surface soil horizons are most affected by the kinds of plants that grow on a site. Needles, twigs, leaves, stems, and roots of plants are incorporated into the soil and broken down by the different kinds of organisms that live in the soil. In Washington, soils that have darker surface horizons generally have more organic matter than do those that have a lighter- colored surface horizon. Different kinds of plants produce materials that are acidic and others produce material that is alkaline. This too changes the kind of soil that forms on a site.

Micro-organisms in the soils in Washington vary according to the kinds of plants that grow on the site, and the kinds of plants are impacted by precipitation, temperature, and soil pH. Micro-organisms are present in all soils, and they occur in extremely large numbers. One teaspoon of soil contains many millions of tiny, microscopic microorganisms. These organisms assist in the breakdown of soil parent material, organic matter, and other weathering products contained in the soil. They also produce substances that help plants absorb nutrients and water from the soil.

Other organisms such as worms that burrow into the soil create little channels that assist in the movement of water and air into and through soil. Burrowing animals such as voles, moles, and ground squirrels mix the soil as they dig homes which also helps to move water and air into the soil profile.

In Washington, it is obvious that dry soils support certain natural plants and wetter soils support different natural plants. For example, soils in the Columbia Basin (central Washington) are some of the driest in Washington. Some of these soils only receive about 7 to 10 inches of precipitation annually. Grasses such as bluebunch wheatgrass and Idaho fescue and Wyoming big sagebrush can grow successfully in dry areas. Few trees grow in these areas except along rivers and streams.

As soils receive more precipitation, a certain pattern of plants occur. In dry forest areas, ponderosa pine trees grow along with specific natural understory plants. The next wetter forest zone is the Douglas-fir zone and it too has a certain group of natural understory plants that grow with it. Western red cedar, Sitka spruce, noble fir and other tree species grow along with their specific natural understory plants in the wetter and colder areas in Washington. Climate plays a huge role in soil formation in Washington and it also contributes to the several thousand kinds of soil that are identified, mapped, and correlated inside Washington States’ boundaries.


Time is the last of the five soil forming factors to consider. However, this does not mean that it is not important. Washington landscapes, and the soils developing on them, are products of dynamic on-going soil-forming processes. Time is just as important as each of the other soil forming factors. Time, in the way most humans think of it for soil forming processes and soil landscape development is relatively long. In geologic time, many of the soil-forming processes and landscapes that result from weathering are relatively temporary. Geologically speaking, landscapes are continually building and degrading throughout time. It takes time for all things to happen even though some things are now measured in nanoseconds.

Young soils are usually easy to recognize because they have little or weak soil horizon development and the horizons commonly are indistinct. The soil parent material and the intensity of weathering have not yet produced highly visible evidence such as clay or carbonate movement and deposition which form subsoil horizons. Normally, soil scientists think of soil development in terms of soil age. Older soils have more and stronger horizon development than do younger soils. Young soils are weakly developed and have indistinct soil horizons while older (mature) soils are strongly developed and have well defined soil horizons.

One of the first processes to occur during soil formation is the movement of organic matter into the surface of a soil giving it a characteristic dark color. An often asked question is, “How long does it take to form an inch of topsoil?” This question has many different answers but most soil scientists agree that it takes at least 100 years and it varies depending on climate, vegetation, and other factors.

In a wet, hot climate soil horizons will form fairly quickly compared to those in cold, dry environments. Therefore, soils in cold, dry climates develop rather slowly in comparison. It is not just the amount of time that determines the degree of soil development but also the parent material, climate, vegetation, and intensity of soil- forming factors during that time that ultimately determine soil development.

Washington State has a variable environment for soil development. Elevation ranges from 0 feet (sea level) at the shore of the Pacific Ocean to more than 14,000 feet at the summit of Mount Rainier in Pierce County and the average annual precipitation ranges from about 6 inches to more than 300 inches. Geologic formations and their rock types are also highly variable in composition as well as in age. Several thousand soil series are identified, mapped, and correlated in Washington State so soil variability is huge. Very few soils are suited to all uses without some kind of modification. Washington soils are a substantial resource supporting Washington’s economy through farming, ranching, recreation, and timber production. The soil-forming factors have produced a fascinating number of soil individuals in Washington for our use. It is important that we use them wisely and responsibly so that future generations can do the same.

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