Why do soil horizons form

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. 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. 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, feet at the summit of Mount Rainier in Pierce County and the average annual precipitation ranges from about 6 inches to more than 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. 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.

Next WA Soils Atlas section. Natural Resources Conservation Service Washington. Stay Connected. Loading Tree Soils Soil Health Soil Surveys. Topography Very few Washington landscapes are flat. Organisms Both living plants and animals including humans affect natural soil formation. Time Time is the last of the five soil forming factors to consider. If the topsoil or horizon at the planting depth has the nutrients and minerals a plant crop needs, then there will be less maintenance throughout the season.

However, if the soil is lacking but has other concentrations of minerals and nutrients, it may be a good idea to rotate the crops or combine nutrient-rich additives to the soil. Farmers are more likely than gardeners to need a thorough soil analysis. Typically, for home gardening soil , you do not have to view the lower soil horizons because of the fact that most garden plants remain in the topsoil. Soil nutrients and pH are common factors that gardeners check and adjust before planting their seeds.

A soil profile, if extracted correctly, should show multiple of the soil horizons in one piece, or adjoining pieces. Soil profiles often come from one area of the ground and will not give accurate results if pulled from more than one location. The goal of extracting a soil profile is to find out the mineral content of a specific area of the soil.

To create a soil profile, you must dig a hole. Find a spot in the soil where it will be the least difficult to dig up a decent-sized hole. Pay attention to nearby plants to avoid breaking any roots and dig a hole that is at least a foot in diameter.

Gardeners may only need a small piece of soil, but the larger the soil profile is, the more it will tell you in the analysis. Once the hole is deep enough, take a shovel or a gardening tool and scrape one side of the hole to make one long, flat piece.

This method will help with comparing the different soil horizons and measuring their depths, but you can also pull a small amount of soil from each horizon as you dig the hole and place them in separate contains for examination.

Some soil test kits allow for a quick analysis of certain aspects of the soil. For example, there are numerous test kits for testing soil pH, but not as many for testing nutrients. Here are a few soil test kits that you should consider using for your soil profile analysis. Be sure to write down your observations before using the soil in a test kit, so that you do not have to extract a second soil profile.

A: Horizon R, which is at the very bottom of soil the bedrock is impermeable because of the compact rock that forms the horizon. However, other soil horizons can be impermeable as well, if there is a dense structure of soil, rock, and no cracks or gaps through which water could seep. A: Horizon B is the subsoil. It is rich in minerals due to contents that have moved further down into the soil from the upper layers.

Horizon B can contain high levels of iron, aluminum, gypsum, and silica clay. A: Soil horizons form because of the effects of nature. If all the water, wind, and animals remained completely still on Earth, all the time, soil horizons would not form because there would be no means for movement of the soil.

Plants and animals of all sizes are equally important to the formation of the soil. The elements in soil horizons can provide knowledge to anyone who is looking to learn more about their soil. Gardeners can use the information they gain from horizons to grow taller plants, and farmers can use it to grow healthier crops. The analysis of soil is easy, and it is possible to extract and examine a soil profile right at home. However, the deeper layers of soil are harder to reach and are not necessary for the soil analysis of small areas of land.

The most useful information comes from the top soil layers, such as horizon O and horizon A. Soil horizons are informative because they are a collection of everything that has gathered in a specific area of soil since its original formation. The types of parent materials and the conditions under which they break down will influence the properties of the soil formed.

For example, soils formed from granite are often sandy and infertile whereas basalt under moist conditions breaks down to form fertile, clay soils. Soil formation is influenced by organisms such as plants , micro-organisms such as bacteria or fungi , burrowing insects, animals and humans. As soil forms, plants begin to grow in it.

The plants mature, die and new ones take their place. Their leaves and roots are added to the soil. Animals eat plants and their wastes and eventually their bodies are added to the soil. This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter.

This may take the form of peat, humus or charcoal. Temperature affects the rate of weathering and organic decomposition. With a colder and drier climate, these processes can be slow but, with heat and moisture, they are relatively rapid. Rainfall dissolves some of the soil materials and holds others in suspension. The water carries or leaches these materials down through the soil.

Over time this process can change the soil, making it less fertile. The shape, length and grade of a slope affects drainage. The aspect of a slope determines the type of vegetation and indicates the amount of rainfall received.

These factors change the way soils form. Soil materials are progressively moved within the natural landscape by the action of water, gravity and wind for example, heavy rains erode soils from the hills to lower areas, forming deep soils.

The soils left on steep hills are usually shallower. Transported soils include:.