Lecture 30

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Chapter 20

Soil Surveys

 

Shortly after World War II, the United States undertook an ambitious project to map, describe and inventory all soils in the nation. The methods for completing this task have evolved over time. The end result is a collection of Soil Surveys for most counties in the U.S. Some counties still have not been surveyed, and a few are presently being surveyed. These published Soil Surveys are of tremendous value to agriculture and to urban land users.

 

1. Who and Why? The National Cooperative Soil Survey was organized in 1952. This organization is a part of the USDA NRCS (formerly SCS). The U.S. Forest Service, BLM, and other federal and state agencies often cooperate in the production of Soil Surveys. The surveys are done to describe, evaluate and inventory soils. Each soil in a county is evaluated for its agronomic capability and its limitations for urban uses. It is also named and classified according to the system of soil taxonomy. Most U.S. counties have been surveyed, some twice. Crews work on a survey (usually one county) for a period of about 5 years.

 

2. Rating Soils. Soils are rated for agricultural capability and are placed into one of eight Capability Classes. The classes are described below:

Class I land can be used continuously for intensive crop production with good farming practices.
Class II land has more limitations than Class I land for intensive crop production, due to such characteristics as moderately steep slopes (2-5%).
Class III land has severe limitations and requires more special conservation practices than Class II land to keep it continuously productive. For example, the land may have shallow soil, slopes of about 6-10%, or shallow water tables.
Class IV land has severe limitations for cropping use and needs a greater intensity of conservation practices for cultivated crops than Class III land. Most of the time this land should be in permanent crops such as pastures.
Class V land is not likely to erode but has other limitations, such as boulders or wetness, which are impractical to correct, and thus the land cannot be cultivated. It should be used for pasture, range, woodland, or wildlife habitat.
Class VI land is suitable for the same uses as Class V land, but it has a greater need for good management to maintain production because of such limitations as steep slopes or shallow soils.
Class VII land has very severe limitations and requires extreme care to protect the soil, even with low intensity use for grazing, wildlife, or timber.
Class VIII land has such severe limitations (steep slopes, rock lands, swamps, delicate plant cover) that it can be wisely used only for wildlife, recreation, watersheds, or aesthetic appreciation.

Capability classes are divided into subclasses that identify the limiting factor for the soil. The Capability Subclasses are used as IIe, meaning, the land is Class II with "e" limitation. The symbols used and their meanings are:

e erosion (50% of U.S. land)
s shallow, stony, droughty, permafrost (25%)
w wetness (17%)
c climate (5%)

About 3% of land has no limitation for agricultural use.

Soils are also evaluated for engineering purposes. For examples see Table 20-5 in the textbook for dwellings, Table 20-6 for lawns and golf courses, and Table 20-7 for topsoil.

 

3. Contents of Soil Survey. A typical soil survey contains soil information provided in a systematic way. The surveys usually begin with general information about the area, such as history, climate, and geology. The survey will illustrate typical patterns of soil in the landscape. The surveys usually describe each soil twice; once in layman's terms, and again in more technical terms. Soil Survey books usually contain about 20 tables of data describing each soil in terms of physical properties (such as texture and bulk density), chemical properties (such as pH and salinity), water properties (such as water holding capacity), etc. The back of the survey books is a collection of maps drawn onto aerial photographs. These maps are usually on a scale of 1:24000, which means that one inch on the map equals 24000 inches on the ground.

 

4. Benchmark soils. Some soils are more important than others. Those soils that are extensive and of greater than average importance are designated as Benchmark Soils. In the U.S. about 1000 of our 15,000 series are benchmark soils. These soils are studied intensively. Examples of Benchmark Soils include:

Sharkey, Aquept in LA
San Joaquin, Xeralf in CA
Palouse, Xeroll in WA
Beltsville, Udult in MD
Amarillo, Ustalf in TX

 

Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.

 

Vocabulary

Land Capability Class
Benchmark soil
Soil survey

 

Web sites

The home page for the National Soil Survey Center.
URL: www.statlab.iastate.edu/soils/nssc/menu.html

A thorough description of the Houston Black, the state soil of Texas. This description illustrates the soil information contained in a published Soil Survey book.
URL: www.tx.nrcs.usda.gov/soil/hblack.html

 

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