Chapter 3: Soil classification and survey.
After completing this chapter, you should be able to:
* describe the current USDA soil classification system
* explain how soil surveys are prepared and used
* list soil capability classes
TERMS TO KNOW
land capability classes
land capability subclasses
At the end of the 1800s, public leaders began to realize that land in the United States was being damaged by poor land policies. This realization led to public efforts to conserve soils--efforts that continue today. A start was made in the early 1900s when the government began to survey and classify the soils of the United States.
Soil survey depends on a system of grouping soils of like properties. Soil classification helps us to understand, remember, and communicate knowledge about soils.
The Russian soil scientist V. V. Dokuchaev first suggested a way to classify soils around 1880. He proposed that soils were natural bodies created by soil-forming factors. This proposal formed the basis of a classification system that soil scientists began using to survey United States' soils.
Over the years, the United States has used several, constantly evolving soil classification systems. Early in the 1900s soils were grouped based on the soil-forming factors that created them, using terms like "brown forest soil" or "black prairie soil." Further systems were developed in 1938 and 1949, and familiar names like podzols and chernozems came into use. The USDA introduced the current classification system in 1960 with the publication of Soil Classification, a Comprehensive System. Continued modifications lead to its republication in 1975 as Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys, the basis for modern soil classification in the United States. This system too continues to evolve, with changes like the addition of a twelfth soil order, the Gelisols, in 1998.
Earlier systems generally were based on the presumed history of a soil, that is, its process of formation under the five soil-formation factors. The system of the Soil taxonomy is based, rather, on the properties of the soil as it can be observed in the field or laboratory.
It resembles the way plants and animals are grouped according to a system known as taxonomy--a grouping of objects at several levels to show how they relate. Figure 3-1 compares the taxonomy of plants with the taxonomy of soils. However, unlike taxonomy for plants and animals, soil classification is not universal. While the USDA's soil taxonomy can be applied outside the United States, other nations employ their own systems to serve their own purposes.
As shown in figure 3-1, the new system has six levels of classification. The highest level, the soil order, is the broadest group. The system recognizes twelve soil orders, described in figure 3-2. These orders are based mainly on the presence or absence of certain key horizons in the soil profile, called diagnostic horizons, and on average temperatures and rainfall. An Alfisol, for instance, has a subsurface horizon with a clay accumulation, a medium to high base supply, and moisture at least ninety days of the growing season. An Entisol shows few signs of soil development, with little horizon development. Note that the names of all soil orders end in the suffix "ol." Appendix 3 presents a map of the soil orders of the United States.
Each order is divided into several suborders, the next highest level of the soil taxonomy. Suborder members of the same order differ most often in soil moisture or temperatures but may differ by other factors. A Psamment, for instance, is a suborder of Entisols that is highly sandy. The name of a suborder includes a Latin or Greek root that provides information about the suborder and ends in several letters that identify the order to which it belongs. A Psamment, for instance, is an Entisol. The letters "psamm" come from the Greek word for sand.
Suborders, in turn, are divided into great groups, which are often based on the presence of certain key horizons but may differ by other traits like soil moisture and temperature. A great group is named by adding a prefix to the suborder name. A Udipsamment is a Psamment (sandy Entisol) that is usually moderately moist, expressed by the prefix "udi."
Great groups are further divided into subgroups, based on how close a soil is to the "central concept" of its great group. That is, there is a core image of what that great group should be, but there are gradations within it. A subgroup that matches the central concept is called Typic, while other words express the variations. We affix the subgroup name as a separate word in front of the great group name, for instance, a Typic Udipsamment.
Subgroups are themselves divided into families, which are units of a subgroup with similar properties important to the growth of plants and soil use, such as subsoil particle sizes or the minerals found in the soil. Family names are composed of a string of descriptive words placed in front of the subgroup name, for example, a frigid, mixed Typic Udipsamment. This is now the full, taxonomic name of this soil, and you will find such names in soil surveys. The naming system for families is quite complex and will not be further discussed here.
All the words and syllables used to create these names are listed in Soil Taxonomy, and the reader should refer to that publication for further details about these levels of the system. Those who use soils at the local level, like growers, builders, or county extension agents, are more concerned about the lowest soil grouping, called the soil series.
Soil Series. Soil scientists divide soil families into smaller units called soil series. The soil series is the taxonomic unit with the narrowest range of features, and all pedons within a series have very similar soil profiles. Each of these units is distinct from other units and is the same as the polypedon described in chapter 2.
In the United States, each series is given the name of the town, county, or other location near where the series was first identified. The Mahtomedi soil is named after a town in east central Minnesota and is an example of the soil family just classified above. Other examples of soil series include the Saybrook (a Mollisol), found near the central Illinois town of Saybrook, or the Ontario (an Alfisol), named after a town in New York. A series name may be followed by the surface texture of the soil, as in the Saybrook silt loam.
The series is the lowest official category in the soil taxonomy. However, in practice, a series may be subdivided further into phases. A phase is a variation of a series based on some factor that affects soil management, such as slope, degree of erosion, or stoniness. One might have, for example, an Ontario loam, 3 percent-6 percent slope phase. Soil series with their phases become mapping units for the most detailed soil surveys--the next topic in this chapter.
The USDA developed the soil classification system for use in soil surveys. Soil surveys classify, locate on a base map, and describe soils as they appear in the field. These soil surveys are performed under the auspices of the National Cooperative Soil Survey Program, a joint effort of the USDA Natural Resources Conservation Service, or NRCS (formerly Soil Conservation Service), and state Agricultural Experiment Stations (see chapter 20). Most of the actual surveying is done by soil scientists of the NRCS.
Field Mapping. For detailed mapping, a soil scientist walks the land to survey it (figure 3-3). Frequently he or she stops to probe the soil. By studying the soil profile, that spot can be placed in the correct series. The surveyor also notes slope, evidence of erosion, and other interesting features. With this information, the surveyor draws the soil series or phase on a base map.
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The NRCS uses aerial photographs as a base map (figure 3-4). Aerial photographs make good base maps because they show landscape features, including ponds, woods, and sand pits. Figure 3-5 shows a map of a small farm as it was hand-drawn by an NRCS surveyor. Note that the aerial photograph base map shows much of the farm as wooded.
When the survey is complete, the resulting map is copied neatly. Maps show the boundaries of the mapping units, with each unit identified by codes that vary from state to state. Note the codes shown in figure 3-5. These codes may have one, two, or three parts. The main group of digits refers to the soil series. The mapping unit labeled 169, for instance, is a member of the Braham series. In addition, a unit may be labeled with codes to indicate the slope and erosion. If the latter two codes are absent, one assumes a nearly level relief with no erosion.
Figure 3-6 gives common codes and other symbols that indicate different features in the field. The following codes are used on the map in figure 3-5: 158 Zimmerman series, no slope or erosion 158C Zimmerman series, 6 percent-12 percent slope, no erosion
Modern technology is becoming an increasingly useful tool for soil surveys. Even satellites are employed for remote sensing of earth forms. Such technologies aid, but do not replace, the activities of a soil surveyor doing field mapping.
Mapping Units. Different mapping units are used in soil surveys, depending on how large an area the map or survey covers. For small areas, the mapping units are detailed phases of soil series. For larger-scale maps, the units may be higher levels of the soil taxonomy, like families or great groups. Soil orders are the mapping units on the national soil map in appendix 3. For most county maps, phases of soil series are the basic mapping unit.
For some land, different soils are so mixed that many cannot be separated on the scale of the map. For instance, tiny pockets of one soil may be mixed into a larger soil unit. Therefore, many mapping units contain more than one series, family, or whatever level is being used. One such mapping unit is the soil association. An association consists of one or more major soils and one or more minor soils. For instance, Zimmerman soils usually appear beside two other series called the Isanti and Lino. In this area, glaciers carved out a landscape of fairly level outwash soils (Zimmerman), with scattered poorly drained low spots (Isanti), and other soils. Because they are so mixed with each other, they appear as one mapping unit on some maps as the Zimmerman-Isanti-Lino association.
Soil survey maps are drawn to scale appropriate to the area and detail needed. The larger the scale, the larger the area covered but the less detail offered. Most county surveys are drawn to scales of 1:12,000 to 1:31,680. At the smaller scale, there are about 0.2 miles per map inch (5.28 inches per mile), and the smallest size area that can be practically noted is about one and a half acres. Therefore, small pockets of soil may well differ from what is mapped, a warning to be considered when interpreting a map. Where finer-textured detail is needed, a more exhaustive survey and a map drawn to finer scale may be required.
Soil Survey Reports. A completed soil map becomes part of a soil survey report. A soil survey report has four major parts: a set of soil maps, map legends that explain the map symbols, descriptions of the soils, and use and management reports for each soil. All these parts provide much useful information about the soils, including:
* Taxonomy of the soil--telling the order, suborder, and other classes.
* A brief description of the soil. For instance, for mapping unit 544 in figure 3-5, the Cathro series, the description reads: "The Cathro series consists of very poorly drained soils formed in deposits of herbaceous organic material over loamy sediments in depressions. This soil is black muck 23 inches thick. The substratum is a grayish brown sandy loam. Slopes are less than 2 percent. Most areas are used for woodland."
* Soil properties of each horizon, including texture, bulk density, permeability, available water, pH, salinity, and other features. Engineering properties are also listed.
* Rating of suitability for engineering projects like landfills, buildings, and roads. Problems are mentioned. For instance, the Cathro is listed as poor for most projects because of ponding.
* Suitability for water-management projects like reservoirs, drainage, and irrigation. Problems are mentioned. The Cathro, for instance, is poor for digging aquifer-fed ponds because of a low refill rate.
* Suitability for recreational development like playgrounds and campgrounds. Problems are mentioned. The Cathro is listed as poor because of ponding.
* Potential for cropping, including capability class and projected yields for common crops grown under high management. The Cathro, for instance, cannot be cultivated unless drained. If drained, one can expect 50 bushels of corn per acre, or 55 bushels of oats per acre.
* Woodland suitability, including problems and suggested trees to plant. The Cathro is rated as poor for woodlands, but certain trees that are tolerant of wet soil may be planted.
* Information about good plants for windbreaks.
* Potential as a habitat for wildlife. The Cathro is rated as good for wetland plants and animals, but poor for others.
Survey Report Uses. Soil maps are the heart of good land-use planning. Soil maps give the information needed to make good land-use decisions--whether the decision maker is a national planner or a farmer, home builder, or landscape designer. At the national level, for instance, the USDA has inventoried soil resources of the United States and kept track of them from soil maps.
Engineers also need soil maps. Civil engineers planning a new road will study maps to find routes with good soils for roadbeds. Planning commissions searching for new landfill sites will begin with soil maps.
New growers or growers planning to expand find soil maps useful for choosing new land. Instead of driving all over a region searching for the right land, one can target certain prime areas on soil maps.
Growers can use soil maps in many other ways. The information in soil surveys helps in planning irrigation or other engineering projects. For instance, the grower who owns the farm in figure 3-5 dug a pond in the wet Cathro soil in hopes of irrigating out of the pond. Had he read a soil report first, he would have known the pond would refill too slowly to be used for this purpose. Surveys also give farmers guidance as to what yield they should be getting and other useful information.
For a grower, an important use of soil maps is to prepare the field map, a most useful planning tool.
Field Maps. An NRCS map makes a good base for a grower's own field map. He or she traces the NRCS map on a sheet of clean paper, redrawing it at a larger scale if needed. With this map, the farmer divides the farm into fields and labels each part. By basing the fields at least partially on soil mapping units (or capability classes, to be covered next), fields will be uniform for cropping and for soil sampling. The grower can now use these blank maps as record-keeping and planning tools for numerous uses, including:
* noting crop rotations
* tracking manuring or other practices that affect fertilization
* recording pesticide applications
* making notes of problem spots in the field, like wet areas or large rocks
* mapping locations of irrigation and drainage systems
Land Capability Classes
Soil maps provide the basis for placing soils into land capability classes. This system indicates the best long-term use for land to protect it from erosion or other problems. The uses include cropping, pasture, rangeland, woodland (for lumber), recreation, and wildlife. The classes are not designed for all horticultural crops or crops that need very special management.
For example, flat land with deep rich soil can sustain long-term heavy cropping without erosion. It has few limitations and can be used for any of the listed uses. Sloping land, on the other hand, must be managed carefully to avoid destructive erosion and should not be "overfarmed." Sloping land has more serious limitations.
Capability Classes. The United States NRCS recognizes eight land capability classes. These are numbered by Roman numerals I to VIII. Class I soils have the fewest limitations and Class VIII soils are so limited as to be totally unsuitable for agriculture. Erosion hazard due to slope is the main criterion, but other criteria are used as well. Figure 3-7 shows sample uses for each class. Note that there are fewer safe uses for each succeeding class.
Class I soils have few limitations. They can be heavily cropped, pastured, or managed for woodlands or wildlife. Crop cultivation is the most profitable use of Class I soils (figure 3-8). These soils are well-drained and nearly level (0 percent to 2 percent slope). They have good water-holding capacity and are fertile. Ordinary cropping practices such as liming, fertilizing, and crop rotation keep these soils productive.
Class II soils are also suitable for all uses, but they have mild limitations that need moderate soil conservation or other measures when cropped (figure 3-9). Problems include:
1. gentle slopes (2 percent to 6 percent slope);
2. moderate erosion hazards;
3. shallow soil;
4. less than ideal tilth;
5. slight alkali or saline conditions; or
6. slightly poor drainage.
Class III soils can grow the same crops as Class I and II soils. However, serious problems need to be addressed, such as:
1. moderately steep slopes (6 percent to 12 percent slopes);
2. high erosion hazards;
3. poor drainage;
4. very shallow soil;
6. low fertility;
7. moderate alkali or saline conditions; or
8. unstable structure.
Special conservation methods are needed. Growers should limit the number of row crops grown and favor close-growing crops. This is the lowest soil class that can be used safely for all crops, but only if used carefully.
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Class IV soils are marginal for cultivated crops (figure 3-10). Limitations are those listed for Class III but are more severe. Slopes may be 12 percent to 18 percent. Row crops cannot be grown safely but close-growing crops may be. Crops that cover the soil completely, like hay crops, are best. Careful erosion control measures must be practiced.
Class V soils are not suited to cultivated crops but may be used for range, pasture, woodlands, and recreation. These soils are level, have little erosion hazard, but are limited by factors such as (1) flooding; (2) short growing season; (3) rockiness; or (4) wet areas that cannot be drained.
Class VI soils are unsuitable for cultivated crops but may be used for pasture, range, wildlife, and woodland. Problems may include (1) steep slopes (18 percent to 30 percent slope); (2) severe erosion hazard; (3) established severe erosion; (4) stoniness; (5) shallowness; or (6) drought.
Class VII soils have the same problems as Class VI but are more severe. It is difficult to maintain high-quality pasture, but the land may be used for range, woodlot or forest, recreation, or wildlife if it is carefully managed. Slopes may be greater than 30 percent.
Class VIII soils cannot support any commercial plant production, even timber. They may only be preserved for recreation, wildlife, or for beauty (figure 3-11). Sandy beaches, rock outcroppings, and heavily flooded river bottoms are examples of Class VIII land.
A soil class may be upgraded if the problem is removed. For instance, a Cathro soil (figure 3-5, mapping unit 544) is so wet as to be placed in Class V. An artificially drained Cathro, however, may be moved to Class IV. Permanent irrigation, land leveling, and other practices may also upgrade a soil class.
The eight classes can be simplified to soils that can be used for cultivated crops (Classes I-III), marginal land for cropping (Class IV), and lands not suitable for cropping (Classes V-VIII).
Land Capability Subclasses. All classes except Class I have one or more limitations. Land capability subclasses indicate factors that limit soil use by means of a single letter code added to the class number. A Class IIe soil, for instance, is slightly limited by erosion hazards; a Class VIe soil is very limited by erosion hazards. The letter codes are as follows:
* e-Runoff and erosion. Land with slopes greater than 2 percent are those that need some form of water control.
* w-Wetness. These soils may be poorly drained or occasionally flooded (figure 3-12). Some such soils may be drained; others are classed as wetlands and are best left as is.
* s-Root zone or tillage problems. These soils are shallow, stony, droughty, infertile, or saline. Wind and water erosion may be problems.
* c-Climatic hazard. Areas of rainfall or temperature extremes make farming difficult. Examples include deserts or the Far North.
Soil Use Maps. Land capability classes rate soils for their use in agriculture. One could modify a soil map to replace soil series identifications with use classes and have a map that would show suitability of the area soils for agriculture. This would be a type of soil-use, or interpretive, map.
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Similarly, a variety of other soil-use maps could be derived from soil surveys. For instance, one could draw a map of suitability for home drain fields, or woodlot production. The soil scientist, using the survey, rates each soil for the amount of hazard involved in that soil use. The map is then drawn and coded, for example, as low, medium, or high hazard for the intended use.
Appendix 5 shows how one might judge soils for a number of uses. From this appendix, students could judge soils as a lab exercise or even try to prepare their own land-use maps.
Computer Applications. Soil-mapping information can be assembled in a computer for faster, more knowledgeable natural resource and land management decisions. Information on mapping units in a state or county, for instance, can be computerized for easy access, or even put on the Internet.
Soil researchers are beginning to use computer software called Geographic Information Systems (GIS) in studying soils and making management decisions. GIS, as used in soil work, pulls together information about soils, topography, watersheds, and geology from such sources as soil surveys, the United States Geologic Survey, and even satellite images, to create a database. The GIS integrates all this information to generate a variety of customized interpretive maps. These maps are invaluable for land-use planning.
Lands of the United States. The United States is fortunate to have a great deal of good farmland. Figure 3-13 summarizes the capability of U.S. soils, excluding Alaska. Approximately 43 percent of our soil is rated in Classes I to III. This is soil on which nearly any crop can be grown. Most of the rest of U.S. land is suitable for some form of commercial production like grazing or woodlands.
Good farmland is not evenly distributed over the United States. Corn Belt states have the highest percentage of good farmland, followed by Northern Plains states and Delta states. Much of the land of the West is too mountainous to be useful for cultivated crops.
Generally, land use in the United States follows capability. Land in the top three classes is used primarily for cropland. The remaining land is used for pasture, range, or forest. However, the most severe erosion in the nation occurs on marginal land that is cropped by farmers. As figure 3-14 shows, soil loss increases when farmers use less suitable land.
Soil survey efforts in the United States began around 1900. At first, a simple soil class system based on soil formation factors was used. This system was refined over time until the current system came into use.
Soil scientists presently classify soils according to their soil properties and profiles. This soil taxonomy has six levels. The top level consists of twelve soil orders. Each order is further divided into sub-orders, great groups, subgroups, families, and series. The important level to an individual grower is the soil series and its subdivision, the soil phase.
Soil scientists survey land and prepare a soil map based on this classification system. The surveyor studies the soil profile and notes slope, erosion, and other features. A soil survey report includes the map plus printed information about the soils on the map and their suitable uses. These reports are then used by regional planners, engineers, growers, and others.
The information in a soil survey places the land into one of eight capability classes. Classes I, II, and III are suitable for cultivated crops. Class IV is marginally useful for cultivation. Classes V to VIII are restricted to noncultivated uses. A number of factors are used to classify the soil, principally erosion hazard. Other factors include drainage, droughtiness, and extreme climates.
1. How do the five soil-forming factors interact to produce an Alfisol?
2. Explain what a soil interpretive map is. How could you draw one from a soil survey map?
3. The Saybrook silt loam, mentioned in the text, is a "fine-silty, mixed, superactive, mesicOxyaquic Argiudoll. "Try to identify which syllables or words apply to family, subgroup, great group, suborder, and order. What soil order is it?
4. At the time colonists arrived in the New World, most of the northern New England states were covered by forests. They were cut down and replaced by farms. Later, forests grew back when the center of agriculture shifted to the Midwest. Explain differences in soil that could have contributed to this shift. Hint: look at the soil map of the United States.
5. As soils develop over time, they may move from one soil order to another. What might be examples of young, "middle-aged," and older soil orders? Explain.
6. What are soil survey maps drawn on? What are the advantageous of this as a base map?
7. Describe the suitability of soils of the United States for crop growing. Give numbers.
8. What are the major soil orders of your state? How did the soil-forming factors interact to put them there?
9. Go back to Review Question 10 in chapter 2. Speculate on what soil order that soil would have been. Is there more information you need to be certain? What soil order is there now?
1. The official inventory of soil series of the United States is maintained at the National Soil Survey Center at Iowa State University (NSSC), and is accessible on the Internet at <http://www.statlab.iastate.edu/soils/nsdaf/>. This site carries a wealth of data, including the classification and description of all the soil series. Search for the Ontario series under "Official Soil Series Descriptions." Examine the classification and information. What would you say about this series? Much of this information will be more clear after chapter 4.
2. The NSSC's Soil Science Education Web site has links to a listing of the order, suborder and great group "formative elements"--the combinations of letters that make up names. It is under "Soil Formation and Classification." It also has a link to a glossary of soil terms, which you may need to interpret the lists. So for deeper study of the naming system, go to: <http://www.statlab.iastate.edu/soils/nssc/educ/Edpage.html>.
3. For photos and descriptions of soils of the twelve orders, visit <http://soils.ag.uidaho.edu/soilorders>or <http://www.geobop.com/paleozoo/Soils/index.htm>. Compare and contrast the appearance of a Spodosol and a Mollisol. Where are they mostly located?
4. Examine the soil survey report for your county. What are the main chapters and what information do they contain? If you don't have a copy of your county report, the National Soil Survey Center has several on-line (note: many are PDF files). Pick one on the list found at <http://www.statlab.iastate.edu/soils/soildiv/surveys>.
5. For a detailed description of soil survey, study this on-line booklet from the USDA (a lengthy PDF file) at <http://www.statlab.iastate.edu/soils/nssc/nsscprod/surdown.pdf>.
FIGURE 3-1 The USDA soil classification system lists six levels of soil classes (left). The approximate number of each is provided but continues to increase. Phases are also listed but are not an official level of the soil taxonomy. The classes of plants taxonomy are listed for comparison. Soil Classes Plant Classes Order (12) Kingdom Suborder (66) Division Great Group (>320) Class Subgroup (>1,400) Order Family (>8,000) Family Series (>19,000) Genus & species (Phases) (Variety) FIGURE 3-2 This simplified listing of the soil orders provides the order names, description, and main uses in order of importance. The boldfaced letters in the order names are used in suborder names. Some terms are explained in later chapters. Soil Order Description Use Alfisol Usually deciduous forest soils of temperate Cropland, moist climates, light colored, slightly to forest, moderately acid with illuvial layer high in range silicate clays. Medium to high base saturation (35%). Fertile soil. Especially north central states. Typical profile: O-A-E-Bt-C Andisol Geologically recent volcanic materials. Cropland, Dark, fertile, high CEC and OM, low forest density, often on volcanic slopes and high altitude. Pacific Northwest, Hawaii, Alaska. Aridisol Arid soils of cool to hot deserts and dry Range, shrublands, often alkaline with salted irrigated horizons, thin or no O or A. High base. cropland Western states. Typical profile: A-Bt-Ck or Ckm, Cy, Cz Entisol Soils lacking well-developed horizons, Range, often young, or under conditions that cropland, inhibit horizon development like being forest, sandy, wet, alluvial, or steeply sloped. wetlands Least developed soil order. Often difficult to use. Widely scattered in US. Typical profile: A-C Gelisol Very cold soils of the tundra, cold None safely deserts, or high peaks with subsoil except permafrost. Often with muck or peat surface wildlife soil. Alaska mostly. Very fragile. Typical profile: O-A-Cf Histosol Organic soils, usually of wetlands. Organic Wetlands, matter 20-30%. Very low density. Must be forest, drained for use, then prone to subsidence horticulture and fire. Northern Midwest and fuel Atlantic/Gulf coastal areas. Typical profile: O1-O2-O3-C Inceptisol Soils with minimal horizon development, but Cropland, more than Entisols. Often young. May have forest, weak B horizon visible by color or range structure; no illuviation. Extremely variable, and widely scattered in US. Typical profile: A-Bw-C Mollisol Mostly grassland soils. Dark, thick, high Cropland, organic matter and base A horizon. Low to range moderate rainfall. May have illuvial or calcareous subsoil. Highly fertile and productive. Great Plains and Northwest states. Typical profile: A1-A2-A3-Bw-C Oxisol Highly weathered tropical soils, often Cropland, under rainforests. Subsurface horizon low forest, in weatherable minerals but high in shifting aluminum or sesquioxide clays. Low native agriculture fertility, but can be fertilized. Hawaii and Puerto Rico. Typical profile: A- Bo (or Bv)- C Spodosol Light colored, acid coarse soils, typically Forest, under coniferous forest. Usually of cool pasture, humid regions, but not always. Illuviation cropland of iron or aluminum-humus complexes in B horizon. Low base saturation, infertile. Upper Midwest to Northeastern states. Typical profile: A-E-Bs (or Bhs)- C Ultisol Highly weathered soils of humid warm Forest, climates, often under forest. Low base cropland saturation (<35%), acid, leached. Subsoil layer with illuviated silicate clays. Surface layer light colored, subsoil often red clay. Can be productive if properly fertilized and limed. Southeast states mostly. Typical profile: A-E-Bt-C Vertisol High in swelling clays in climates with a Range and dry season. When dry, large, deep cracks pasture, form that surface soil falls into, mixing cropland the soil. Unstable for engineering uses. Most common in Southcentral states, especially Texas, some in upper plains states. Typical profile: A-AC-C FIGURE 3-5 A soil map of a farm in Minnesota prepared by an NRCS surveyor. The numbers are codes for the following soil series: 75 Blufton 123 Dundas 132 Hayden 158 Zimmerman 169 Braham 225 Nessel 540 Seelyville 544 Cathro (Courtesy of USDA, NRCS) FIGURE 3-6 A sample of soil-mapping symbols that give information about slope, erosion, and landscape features. Slope Legend Percentage of Slope Description A 0-2 Nearly level B 2-6 Gently sloping C 6-12 Sloping D 12-18 Strongly sloping E 18-30 Very strongly sloping F 30-60 Steep More than 60 Very steep Erosion Legend Description 0 No erosion 1 or P Slight, 0 to 1/3 topsoil gone 2 or R Moderate, 1/3 to 2/3 topsoil gone 3 or S Severe, 2/3 or more topsoil to 1/3 subsoil gone 4 Heavy subsoil erosion, deposition of eroded soil FIGURE 3-7 Suitable uses for soil capability classes. The higher the class number, the more limited is the number of safe uses. A single slash indicates that very careful management is needed or that the soil cannot be safely used for this purpose every year. Class Use I II III IV Row crops X X / Hay, small grains X X X / Pasture X X X X Range X X X X Woodland X X X X Recreation, wildlife X X X X Use V VI VII VIII Row crops Hay, small grains Pasture X X Range X X / Woodland X X / Recreation, wildlife X X X X FIGURE 3-13 Land capability of nonfederal rural land in the United States, excluding Alaska. Land not suitable for Land suitable Land suitable cultivation: for occasional for continuous 600 million acres 143 million acres 599 million acres Clas I 2% (31 million acres class II 21% (285 million acres) Class III 20% (281 million acres) Clas IV 14% (193 million acres) Class VI 19% (263 million acres) Class VII 275% million acres) Class VII 2% (30 million acres) Source: USDA 1997 National Resource Inventory) Note: Table made from pie chart. FIGURE 3-14 Average rate of sheet erosion by capability class in 1982 for cultivated cropland. A loss of about 5 tons per acre is generally the highest acceptable level. Most erosion occurs when farmers cultivate marginal land. Note that even Class IIe land nearly exceeds the acceptable rate. (Source: USDA Preliminary Report 1982, National Resource Inventory 1984) I 2.4 II 3.3 IIe 4.5 III 4.8 IIIe 6.5 IV 6.6 IVe 8.6 V 1.7 VI 10.1 VIe 13.4 VII 12.7 VIIe 21.8 Note: Table made from bar graph.