Initial Publication Date: January 13, 2021

Educator Background

Chapter Summary

Every classification system has a purpose that begins with data collection. In the U.S., soil scientists began collecting soil data at the beginning of the 20th century to help farmers better manage their land. Soil maps were created that showed how soils with similar properties occurred on the landscape. Soil scientists noticed the ClORPT factors affected the kinds of soils that formed on local and regional levels. By the 1930s soils were grouped into larger categories based on those factors. In the U.S., Soil Taxonomy has six levels, from the broadest category with 12 soil orders to the smallest unit with almost 20,000 soil series. Interpretations use data to predict how soil properties will affect the suitability of a soil for use in agriculture and recreation, as well as for things like buildings and roads and urban development.

Soil survey: Data and Mapping

Soil has physical, chemical, and biological properties that vary by depth and position on the landscape. Field soil scientists collect these data by describing soil profiles using soil pits, trenches, and core samples. The most obvious soil properties in the field are color, texture, and structure. Some soil is analyzed in the lab to determine more detailed soil physical and chemical properties. Biological properties often are inferred from field descriptions.

The next step in classification groups soils with similar properties and management practices and represents their distribution on a map. Scale is one of the most important features of a map. USGS 7.5' Quad topographic maps use a scale of about 1:25000, or 1 cm = 250 m. The scales for standard soil survey maps in the U.S. ranges from about 1 cm = 100 m (1:10000) to 1 cm = 480 m (1:48000). A scale of 1:10000 shows more detail, but still not enough for site-specific planning such as for the placement of an on-site wastewater treatment system (septic absorption field) or for a house in the country. In those cases, a soil scientist should be consulted to determine the suitability of the soil in that location for the intended purpose.

More about maps and scale see the following:

Classification: Taxonomy

There are several systems of soil classification used around the world; the UN-FAO (United Nations Food and Agriculture Organization), Australia, Canada, Russia, and Brazil all have soil classification systems, and all have common features, such as using a dichotomous key. Soil Taxonomy, the system used in the U.S., uses a series of dichotomous keys to sequentially classify the master horizons (O, A, E, B, as shown in Know Soil Know Life Table 2-10) of a soil series into diagnostic horizons based on their properties, then uses the master horizons to place the soil series into Orders, Suborders, Great Groups, and Subgroups. Soil physical and chemical properties are used to place the soils into Families.

Soil Taxonomy is an evolving system; currently, there are 12 Soil Orders and almost 20,000 series. Soil Orders are loosely based on the ClORPT factors; soils with similar ClORPT factors have similar diagnostic horizons. As more soils are mapped and soil scientists learn more about them, new categories have been added, and it is likely more will be added in the future. One of the newest areas of study focuses on urban soils and other drastically disturbed soils such as those reclaimed from mining operations. Since the current 12 Soil Orders are related to the ClORPT factors, human-altered soils do not fit nicely into any of the orders.

Classification: Land capability classification

The original purpose of soil surveys was to collect data that would assist farmers to better manage their land for crop and livestock production. The original Land Capability Classification system was published by the USDA in 1939 and had nine categories. In the 1940s, the most limiting categories were combined to create the eight categories currently used, and the subcategories describing the nature of the limitations were added. This system considers the highest agricultural use (intensive, annual cropping) for a plot of land, the nature of its limitations, and the amount of best management practices (see Know Soil Know Life Ch. 6, Table 6-1) required to conserve the land and maintain its productivity.
For more information see:
History of Land Classification

Wetlands

Wetlands are unique biomes that may exist as ecosystems in isolated areas and riparian zones (associated with rivers) within any other biome and climate, even deserts. The three diagnostic features of wetlands are hydric soils (based on soil profile characteristics), hydrology (presence of water in the soil), and hydrophytic vegetation (plants that can exist in saturated or near-saturated soils). Each state maintains a list of hydric soil series and diagnostic hydrophytic vegetation to assist in wetland delineation. The mapping scale in soil maps often precludes identifying the small areas of isolated wetlands (less than 0.4 to 5 ha), so soil scientists often must examine the soil profile characteristics and make hydric soil determinations.

Prime Farmland

Much farmland is lost every year to urban sprawl - building houses, roads, shopping malls, and industrial complexes. To feed the skyrocketing world population, it is important to keep the most productive soils producing food to feed the masses. Prime Farmlands are the most productive soils with the fewest limitations for crop production while requiring the least intensive best management practices. As these lands disappear, agriculture moves into more marginal lands that are more susceptible to human-accelerated land degradation. Those lands require more extensive Best Management Practices (BMPs) which increases the cost of food and fiber production.