Every time you step outdoors and look to the horizon, you are gazing upon one of Virginia's greatest natural resources - soil. Soil is the "backbone" of life. At the base of every building, every highway, and every farm road, airport, and bridge, you will find soil. And just as the human backbone holds our frames in place, soil holds the nutrients and water needed to grow our crops, livestock, and timber.
Have you ever stopped and looked at the soil beneath your feet? Have you ever thought of soil as a living, breathing thing? It really is! There are millions of small plants and animals in the soil, and you are walking on their homes (ouch!).
Soil is made of both living and dead plants and animals (organic matter), air, water, and mineral particles such as sand, silt, and clay. The diagram below shows the four main "ingredients" of ideal soil. Note that, except for doted-upon home gardens, most Virginia soils do not contain 5% organic matter.
Soil features such as thickness, texture, and color can easily be observed and studied. While contemplating these features as you look at a soil sample, ask yourself the following questions:
The best way to observe how soils develop in layers is to visit a road cut, building site, or ditch. Each layer, or "horizon," looks different and has unique physical and chemical properties. A cross-section cutting down through these different horizons is called a soil profile.
The following information will help you identify the different layers, or horizons, in the soil profile. A layer of leaf litter (decaying leaves, bark, nuts, and twigs) is found on top of the soil. Layered below the leaf litter are the following four horizons, which make up the soil profile.
The A Horizon, or surface layer, is usually darker than the lower layers. It is most often loose and crumbly and contains more organic matter than deeper layers. In the A Horizon, water soaks or leaches into the soil with ease. Clay and other dense compounds are missing; they have been carried by water deeper into the soil profile.
The B Horizon is the subsoil. Subsoils are usually light colored and dense and contain little organic matter. Materials washed or "leached" from the A Horizon collect in the B Horizon. Therefore, this horizon holds more clay, iron and other mineral compounds.
The C Horizon, or parent material, is the layer of very little weathering. (This means that forces of weather, such as rain and wind that cause erosion, or ice that causes freezing and thawing, have little effect on this horizon.) The C Horizon has very few roots. It is usually low in clay content and often contains pieces of rock.
The R Horizon, or bedrock, contains layers of solid rock.
Texture tells you a lot about soil. Texture affects the ability of soil to absorb and hold both water and plant nutrients. The texture, or feel, of soil is determined by the amounts of sand, silt, and clay it contains. Sand particles are the largest; silts are in-between sand and clay in size; and clay particles are the smallest - so small, in fact, that individual particles cannot be seen without a microscope!
With a little practice, most people can figure out the general texture of their soil quite easily. Sand makes a lump of soil feel gritty, silt makes it feel powdery, and clay makes it feel sticky.
|Length Of Ribbon||Textures|
|0 to 1/2 inches||loams|
|1/2 to 1 inch||clay loams|
|more than 1 inch||clays|
To understand the texture of a soil sample, place a small amount in the palm of your hand. Add a few drops of water to the soil and break apart the clumps. Next, work the soil into a ball and place the ball between your thumb and forefinger. Gently push the soil with your thumb and squeeze it upward, into a ribbon. Keep trying to make the ribbon longer until it breaks from its own weight!
The length of the ribbon that can be formed is used to determine the soil's texture. Measure the length of your ribbon. Use the chart below to decide the correct soil texture.
Slope "gradient," or steepness, is very important. It affects water runoff, erosion, and the use of farm machinery and lawn and garden equipment on the land.
Slope is defined as the number of feet the land rises or falls over a distance of 100 feet. Slope is read as a percent (%). The formula used to determine slope is: "rise over run times one hundred."
% slope = feet in rise/feet in run x 100
young soil scientist uses a line of sight, distance, and a tree to determine the grade or slope of the hill. First, she extends her arm to eye level and finds a mark on a distant tree that is in her line of sight. Then she walks down the hill toward the tree. She finds that the tree is 100 feet from where she started. Also, she discovers that her line of sight at eye level is now 8 feet lower than her original mark. This means she is 8 feet lower in elevation than the point where she started. Therefore, the slope of the hill is 8%.
Practice by placing two stakes 100 feet apart. The slope between the two stakes is the slope to be measured. You find that the land rises six feet from the bottom stake to the stake at the top of the hill. What is the slope?
Answer: X (% slope) = 6' / 100' x 100 = 6%
|Descriptive Term||Percent Slope|
|nearly level||0 - 2%|
|gently sloping||3 - 7%|
|sloping||8 - 15%|
|moderately steep||16 - 25%|
Soil structure is the arrangement of individual soil particles (sand, silt and clay) into larger pieces of soil, called "peds." Each ped has a particular shape or size and, together, determines the type of soil structure.
Different types of soil structures are shown in the chart below. The most common structures found in soils are granular and blocky.
In nature, rain and wind washes or blows loose soil off construction sites, bare spots in the lawn, and plowed farm fields. Where water runs downhill over the ground surface, it carries with it loose soil and pollutants. During storms, water moves so fast it doesn't have time to penetrate into (percolate) the soil. Instead, it gathers strength and picks up more loose soil and pollutants along its path.
We call this process erosion. Eroded soil eventually ends up as "sediment" in waterways - a form of nonpoint source pollution. It clouds the water, chokes fish and other animals, blocks sunlight that underwater plants need to grow, and makes it harder to clean up our drinking water.
The amount of loose soil "on the run" can be reduced through many actions that control erosion and sediment from leaving construction sites, road projects, and even our own backyards.