Soils: What are they?
Soil. Earth. Dirt. No matter what we call it, it's the material that constitutes the outermost solid layer of the planet. We build on it. We raise food in it. We mine mineral resources from beneath it.
Apparently unchanging and lifeless, soils are dynamic mixtures teeming with life. One teaspoon of soil can contain billions of organisms ranging from microscopic bacteria and fungi to larger more recognizable forms such as earthworms, insects and spiders. Bedrock is continually fractured, dissolved, and changed into soil; but the process occurs slowly so we usually never notice.
Soil is a naturally occurring mixture of mineral and organic ingredients with a definite form, structure, and composition. Although soil composition varies from location to location, major soil ingredients by volume include:
- 45% Minerals (sand, silt, clay)
- 25% Water (the amount varies depending upon precipitation and the water ?holding capacity of the soil)
- 25% Air (an essential ingredient for living organisms)
- 5% Organic matter or humus (both living and dead organisms.
How is a Soil Formed?
Throughout the world thousands of different soils have been identified. Weakly developed, moderately developed, and well developed soils are formed through a combination of five important factors: 1) parent material; 2) climate; 3) living organisms; 4) topography and 5) time.
Parent materials are the earthy materials, both mineral and organic, from which soil is formed. Parent material can include things such as volcanic ash, sediment that has been deposited by wind or water or material depostied by glaciers. Weathered bedrock can also be parent material. Moderately and well-developed soils are made when a parent material is chemically and physically changed over time. See Map
Parent material is broken down into finer particles by a process called weathering, which is controlled by the climate of a given location. Temperature and water are major climatic forces that influence weathering.
Frequent freezing and thawing will cause water trapped in cracks to expand, exerting pressures that fracture the rocks and smaller materials even further. Alternate wetting and drying also break down particles because not all minerals expand and contract at the same rate. Further, water tends to dissolve certain minerals from parent material.
Both plants and animals help to create a soil. As they die, plants and animals add organic matter to weathered parent material to help form subsoil and topsoil. Plant roots also alter the soil in a combination of physical and chemical processes related to root growth and nutrient uptake.
As animals dig through the soil, they break it up permitting more air and water to enter. They mix the organic matter throughout the soil. Smaller plants and animals such as bacteria, fungi, and nematodes, for instance- also enrich soil by breaking down organic matter into simpler nutrients. The actions of plants and animals help form topsoil on moderately and well-developed soils.
Topography is the hilliness, flatness, or amount of slope of the land. Soils vary with topography primarily because of the influence of moisture and erosion. In many areas, moist, poorly drained soils are located in low areas and depressions of the land. In contrast, soils in sloping areas can be drier and well drained. Erosion can remove all or part of the topsoil and subsoil, leaving a weakly developed soil.
The previously mentioned factors, combined with time, result in the soils we see across the landscape today. Hundreds, if not thousands of years, may pass to form an inch of soil from parent material.
What are Soil Horizons?
Soils develop into layers. These layers, called horizons, can be viewed in areas where soil is exposed, such as road cuts. This cross-sectional view of the soil horizons is referred to as a soil profile. The thickness of each horizon varies with location. In some instances where the soil has been disturbed for agriculture, development, or due to severe erosion, not all horizons will be present.
The uppermost layer is called the organic horizon or O horizon. It consists of detritus, leaf litter, and other organic material lying on the surface of the soil. This layer is dark because of the decomposition that is occurring.
Below is the A horizon or topsoil. Usually it is darker than lower layers, loose and crumbly with varying amounts of organic matter. This is generally the most productive layer of soil.
As water moves down through the topsoil, many soluble minerals and nutrients dissolve. The dissolved materials leach from the topsoil and move downward in the soil profile.
The B horizon is known as subsoil. Subsoils are usually light colored, dense, and low in organic matter. The subsoil is a zone of accumulation since many of the materials leached from the topsoil accumulate here.
Deeper still is the C horizon. It is a transition area between soil and parent material. Partially disintegrated parent material and mineral particles may be found in this horizon. The final horizon is bedrock. In most of Waukesha County, dolomite forms the uppermost bedrock unit.
Land Use and Soils
Soil properties exert a strong influence on the manner in which land is used, since they affect the costs and feasibility of building site development and provision of public facilities. Soils are also an invaluable resource for agricultural and landscaping purposes. Information about the soils in a particular area is found in soil surveys published by the USDA-Natural Resources Conservation Service (formerly known as the Soil Conservation Service). Soil surveys contain aerial maps of the county with the soil types indicated. Soil surveys have provided definitive data on the physical, chemical and biological properties of soils and interpretations of the soil properties for planning, engineering, agricultural and resource conservation purposes.
Although development pressure remains high in Waukesha County and available cropland is reduced every year, there is still a considerable amount of cropland harvested. Grains, nursery, greenhouse, floriculture and sod, dairy, vegetables, corn silage and hay are the main commodities generating approximately $31.7 million in sales in 2002. According to the 2002 Census of Agriculture, there were 762 farms in Waukesha County in 2002. A farm is defined as “any establishment from which $1,000 or more of agricultural products were sold or would normally be sold during the year.” This is down from 817 farms in 1997, a decline of 55 farms or approximately 7 percent. Average farm size has also decreased, going from 142 acres in 1997 to 129 acres in 2002. Even with these declines, in 2002 Waukesha County farmers harvested 26,519 acres of corn, 22,871 acres of soybeans, and 1,073 acres of assorted vegetables. That same year, approximately 4,000 dairy cows on 52 farms produced 75,200,000 pounds of milk, an average of 18,800 pounds per cow. The most current numbers reported in Wisconsin Agricultural Statistics indicate that there are 42 Grade A dairy herds and 0 Grade B dairy herds in Waukesha County.
Most of the remaining large areas suitable for agricultural production are concentrated in the western, south central, and north central portions of the county with scattered cropland elsewhere, as shown in our soils Map II-5. This map was created by using soils information from the Natural Resources Conservation Service (NRCS), 2000 land use data from the Southeastern Wisconsin Regional Planning Commission (SEWRPC), and land parcel data from the Waukesha County Land Information System. Based on SEWRPC land use data, approximately 115,040 acres or about 31 percent of the county was in agricultural uses in 2000. Map II-5 subtracted off subdivision and condominium plats that were recorded up to March 2005, which reveals that approximately 104,475 acres or 28 percent of the county is now in agricultural uses. It should be noted that significant additional land conversions have occurred since 2000 that may still be shown as agricultural land on this map. These areas can only be accounted for through an updated land use inventory, which is not yet available for 2005. The soils Map II-5 also shows where Prime Farmland Soils and Soils of Statewide Importance are presently found within the county.
Soils vary dramatically across the landscape. In Waukesha County nearly 150 different soil map units have been identified. Soils also vary in their individual susceptibility to erosion depending on a number of factors including: parent material, vegetative cover, and position on the landscape. Tolerable soil loss or "T" for a particular soil is defined as the maximum average rate of soil erosion that will permit a high level of crop productivity to be sustained economically and indefinitely. In Waukesha County, "T" values for the different soil types ranges from 2-5 tons per acre per year.
According to information compiled by the Land Resources Division, nearly 90 percent of the cropland acres in Waukesha County are at or below "T" or the tolerable soil loss rate. It should be noted however, that "T" is not a water quality standard. In contrast, soil loss from an average construction site with no erosion control measures in place can reach 20-30 tons per acre. Much of this sediment can reach nearby waterways through road ditches and storm sewers.
Soil Limitations for Development
Not all soils are suitable for agriculture. Not all soils are suitable for building homes, roads, and factories. Soils have characteristics that may limit their suitability for different land uses.
The following series of maps show the primary features that present limitations for land development, including depth to water table, bedrock, and steep slopes. Hydric soils generally have seasonal depth to water table of 1 foot or less and are capable of supporting wetland vegetation. Poorly drained soils have seasonal depth to water table of 3 feet and are concentrated on the eastern part of the county where many of the soils have a high clay content, often causing a perched water table condition. Shallow water table conditions risk groundwater contamination from on-site septic systems and could cause wetness problems for dwellings with basements. Shallow bedrock conditions pose higher construction costs for basements and also risk groundwater contamination from on-site septic systems due to the lack of a filtering soil layer. Steep slopes represent possible increased grading costs and higher risks for soil erosion during land development activities.