108
USE OF AGRICULTURAL SOIL MAPS IN MAKING SOIL SURVEYS
L. D HICKS, Chief Soils Engineer North Oil 'lina State Highway and Pubi Works Commission
SYNOPSIS
Soil surveys are made to obtain information relative to the type, extent of occurrence, and characteristics of the soils in a given area. The use of the pedological system of classification permits easy identification of the soils as to type, and knowledge of the characteristics of various soil types and previous experience with them can be utilized in planning and design. A large portion of many states has been surveyed by the Department of Agri• culture and maps are available showing the location of the various soil types. These maps may be used as guides in making soil surveys, and in many instances they contain all of the information desired. When agricultural soil maps are not available or when extreme accuracy is necessary, a soil survey must be made. The pedological system of classification can be used in making the survey by anyone with some knowledge of the system, assisted by a soil identification "key". This paper describes the use of agricultural soil maps by the North Carolina State Highway Department and a soil identification key used in making soil sur• veys IS included. The use of the key is described.
The first soil surveys in the United suitability for various crops given. In• States were made m 1899 by the Depart• cluded in each report is a map of the ment of Agriculture for agricultural pur• area surveyed, usually a county, showing poses. In the past decade soil surveys the various types of soils that occur. have been conducted by other organizations The typing of the soils follows a system for engineering purposes. Tlie soil sur• of classification known as the pedological veys conducted by the Department of Agri- system which is based on the features of culture are surficial, extending to a the soils themselves, including that of depth of three feet, and consist of clas• the parent materials. Soil surveys for sifying soils according to color, struc• engineering purposes are made for some ture, texture, physical constitution, specific project and cover only a limited chemical composition, biological charactEr- area. Such surveys are not published and istics, and morphology, while surveys con• their value is restricted to the particu• ducted for engineering purposes consist lar project for which they are made. of exploring soil profiles to specified Much of the work expended in making depths in which the strata of the differ• engineering soil surveys for highway pur• ent materials encountered are located as poses can be eliminated by intelligent to position and extent of occurrence and use of agricultural soil maps. Some the materials described and tested. The knowledge of the pedological system of Department of Agncul ture publishes reports classification IS necessary and character• of their surveys in which the different istics of the various types of soils must soils are described in detail and their be known. Each type of soil, as classi- 109 fied, has characteristics peculiar to it• diameter; fine gravel, particles with self which will be practically the same diameters between 1 and 2 mm.; sand, par• wherever that type of soil is encountered. ticles with diameters between 0.05 and 1 mm., silt, particles with diameters be• THE PEDOLOGICAL SYSTEM OF CLASSIFICATION tween 0.005 and 0.05 mm., and clay, par• ticles smaller than 0.005 mm. in diameter. Briefly, the pedological system of Sand IS subdivided into coarse sand, par• classification, as developed by the De• ticles ranging in diameter between 0.5 and partment of Agriculture consists of sep• 1 mm.; medium sand, particles ranging in arating soils into units, each unit re• diameter between 0.25 and 0.5 mm., fine presenting soils having the same texture, sand, particles ranging in diameter be• color, structure, physical constitution, tween 0.1 and 0.25 mm.; and very fine sand, chemical composition, biological character• particles ranging in diameter between 0.05 istics, and morphology. and 0.1 mm. Soil IS the result of the disintegra• Textural classification of soils con• tion, a mechanical process, and the de• sists of grouping them according to par• composition, a chemical process, of rock. ticle size range. Particle size range Some of the rock minerals, such as quartz grouping IS based on the amounts of par• and mica, are quite stable, and remain ticles belonging to certain size classes unchanged by chemical action as soil par• present in the soil. The particle size ticles visible to the eye, while other range groups used in textural classifi• minerals, such as feldspar, hornblend, cation of soils by the Bureau of Chemistry etc., are changed by chemical action into and Soils are defined as follows. secondary minerals of minute size which are discernible only with the aid of a Sands, soils containing less than 20 per• microscope. cent silt and clay, the rest of the mat• The movement of water from the surface erial being sand. Sands are classed as of the soil, downward, carries the finer coarse, medium, fine, and very fine. particles and deposits them at some level Coarse sand contains 35 percent or more of below the surface. The depth of this fine gravel and coarse sand and less than leaching action depends upon the amount 50 percent of other grades of sand. Med• of water, the permeability of the soil, ium sand contains 35 percent or more of and the length of time the process has fine gravel, coarse and medium sand and gone on. Hiis action produces layers of less than 50 percent of fine or very fine soil that are quite different. Hie sur• sand. Fine sand include 50 percent or face layer has been divested of its fine more of fine and very fine sand. Very material, with the coarser particles re• fine sand contains 50 percent or more of maining as the predominant constituent. very fine sand. The sub-surface layer has accumulated the Sandy Loams, soils containing from 20 per• fine material leached from the surface cent to 50 percent of silt and clay. They layer, and contains more fine material are designated as coarse, medium, fine, than it originally possessed. The soil and very fine in accordance with the pre• beneath the layers, where no water move• dominant sand class group present. ment has taken place, remains unchanged. Loams, soils containing 20 percent or These layers are called "horizons" and less of clay, from 30 to 50 percent of are designated as "A", " B", and "C", re• silt, and from 30 to 50 percent of sand. spectively. Silt Loams, soils containing 20 percent One of the requirements in soil classi• or less of clay, 50 percent or more of fication is texture. Texture, when ap• silt, and 30 percent or less of other plied to soils, denotes particle size classes. range. Particles of definite size are CZay Loams, soils containing from 20 to 30 placed in size classes which are: gravel percent of clay, from 20 to 50 percent of or stone, particles larger than 2 mm. in silt, and from 20 to 50 percent of sand. 110
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Clays, soils containing30 percent or more the textural classification of the soils of clay and 70 percent or less of other in an entire soil profile. classes. As stated before, the textural classi• Soils containing gravel or stone are fication of a soil as used by the Bureau designated as ' 'gravelly" or "stony". of Chemistry and Soils refers to the mat• Since the passage of water from the erial in the surface layer or "A" horizon. surface of the soil carries the fine par• This alone IS not of much value to any o.-'e ticles from the surface layer and deposits interested m the soils m the entire pro• them in a lower layer, it is obvious that file; however, when the textural classi• the textural classification of the soils fication IS gi^e^ in combination with the in the various layers of a soil profile soil series, one has information on the will be widely different. For this reason texture of the surface layer and the color, the textural classification of a soil structure, physical constitution, chemical type, as classified by the Bureau of Chem• composition, biological characteristics istry and Soils, refers to the texture of and morphology of the lower layers. A the surface layer only. When classifying soil classified in this manner is called soils for engineering purposes, however, a soil "type" which is the smallest unit it IS necessary to give the textural clas• m soil classification. An example of a sification of the soils in the various soil type is Cecil sandy loam. "Cecil" layers of the profile. Figure 1 is a designates the soil series and "sandy chart that may be used in classifying loam", the texture of the surface soil. soils according to texture. It will be Soils belonging to a particular soil type noted that this chart contains more groups are alike in all features, including the than used by the Bureau of Chemistry and parent material, so if a soil survey shows Soils, which makes it more applicable to the existence of only three types of soils, Ill one needs test data only on three samples of soil classification. Soil types are representing these three types. If this grouped into several categories that de• test data is available from another sur• note certain common features. For in• vey, It will be applicable, unless con• stance, a certain physiographical or geo• siderable detail and extreme accuracy is logical area may produce certain soil required. types, or certain areas that are exposed Except in cases when a soil material to very different climatic conditions may of a certain quality and texture is sought, produce other soil types. This grouping as in highway work when a soiKtype base, of soils m categories facilitates their sub-base, or surface is to be constructed, identification and study. the classification of a soil as to type is Soils of the,world are placed into two unnecessary. The series classification, Great Divisions, Pedocals and Pedalfers. which gives all of the features of the Figure 2 shows their distribution in the soil profile, including the parent mat• United States. Soils of the Pedocal Div• erial , except the texture of the surface ision are soils that have developed under soil, .is important. A knowledge of the and conditions which permitted the accu-
Figure 2, characteristics and reputation of soils mulation of calcium in their profiles. belonging to certain soil series and the Soils of the Pedalfer Division are soils possession of their test data eliminates that have developed under humid conditions the necessity of much sampling and testing and have had their soluble salts removed on the average highway project. About by leaching. Tlieir profiles contain high all of the information necessary on the percentages of silica, iron, and aluminum. average soil survey is the location of The two Great Divisions of soils may various soil series that occur, however, each be sub-divided into other groups in some instances soundings are made to whose features have been affected by de• determine the elevation of rock, certain velopment under variations in rainfall types of soils, and water table. within the and and semi-and regions, in In the foregoing, reference has been the case of the Pedocals, and by variations made to the soil type as being the unit in temperature in the humid regions, in 112 the case of the Pedalfers. Tlie following Brown Forest soils due to development is a list of the Great Soil Groups in under lower temperatures, characteristic each of the Great Divisions. of the high altitudes. In this State the soils are grouped into four soil provinces, 1. Pedocals. (Soils of and regions and the Atlantic Coastal Plain, the Piedmont semi-arid regions containing Plateau, the Appalachian Mountains, and accumulations of lime). the River Flood Plains. Divisions or sub-provinces are also recognized within Great Soil Group CI imate these provinces. Figure 3 shows the physiographic provinces of the State and 1. Tschernozem soils Semi-arid Figure4 shows the soil provinces and their 2. Chesnut Brown soils Semi-and (less sub-divisions. With the "Key" to the rainfall than identification of North Carolina soils is above) a geologic map showing the geological div• 3. Brown Grassland soils Semi-arid (less isions of the State. (Note: The appen• rainfall than dix, "Key to the Identification of North above) Carolina Soils", will be found folded in 4. Gray Desert soils And at the back of this bulletin.) Figure 5 IS a map of the State showing the normal II. Pedalfers. (Soils of humid regions annual precipitation. containing accumulations of iron and aluminum.) Soil surveys have been made and reports and soil maps prepared by the Department Great Soil Group CIimate of Agriculture for 90 of the 100 counties m North Carolina. Figure 6 is a map of 1. Tundra soils Frigid to sub- the State showing the counties that have frigid not been mapped. Figures 7 and 8 are 2. Podzol soils Cold temperate photographic copies of sections of soil 3. Brown forest soils Temperate with maps of two counties in the State and are forest typical of the other maps. On the original 4. Prairie soils Temperate with maps soil types are shown in different tall grasses colorswith letter symbols designating the 5. Red and Yellow soils Warm temperate types of soils. 6. Latentes Sub-tropical to Agricultural soil maps are of inestim• tropical able value to the Soils Department of thp
•-5IO&RAPHIC PflOVINCEa
WORTH CAROLINA
Figure 3.
NORra CAROLINA SOILS North Carolina State Highway and Public Works (;>)mmission. Many soil problems are With the exception of the true mountain solved by locating the proposed road on soils, the soils of North Carolina belong the soils map and noting the type and (or) to the Red and Yellow Great Soil Group. series of the soils that are traversed by The true mountain soils belong to the the road. Certain soil types are known 113 mm
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to be satisfactory materials for soil type cement requirement for soils belonging to base or surface construction, and if they the most common series in the State, it are shown on the maps, this information having been discovered several years ago is of great assistance to the material in• that the cement requirement for a definite vestigator sent to the project to locate horizon of a definite soil series was the these types of materials. Sand deposits same regardless of where the soil was lo• for sand asphalt pavements are also often cated. located in this manner. Soils belonging Many other uses are made of agricultur• to certain soil series are known to make al soil maps in North Carolina, among poor subgrades and their presence or them being the determination of the need absence can be ascertained by an examina• for pervious sub-bases to act as blotter tion of the soils map. Greater detail. courses beneath concrete pavements for the
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50 to 55.>,ch«s
Figure 5.
if required, must be obtained in the field, prevention of the detrimental effects of but the soils map may be used as a guide. pumping action. It is believed from ex• Much cement stabilization has been done perience that all subgrade soils that are in North Carolina, and practically all not granular type soils will eventually of the preliminary estimates of the cement cause trouble by permitting detrimental required have been determined from soil pumping to occur. (Granular type soils maps. The exact disposition of the cement are considered those soils belonging required was determined in the field after to the PBA Subgrade Groups below A-4). A grading operations were complete by a knowledge of the subgrade groups to which trained soils man capable of identifying the soils in the profiles of the various soils by their series group. Tliis pro• soils series that occur in the State be• cedure is possible because the Soils De• long permits a quick determination of the partment has previously determined the need of a sub-base beneath a concrete L14 pavement. to drainage, such as drainage well esta• When an agricultural soil map of a blished, fairly well established, and county IS not available or when more ac• poorly established. The organic soils curacy and detail are required than fur• division is sub-divided into two groups, nished by a map, it is necessary to make fibrous, partially decayed organic matter a soil survey. Surveys of this type are and well decayed organic matter. Some of made by men trained to identify soils by the sub-divisions are further grouped their series group. Soil identification according to the color of the "A" horizon keys have been prepared for this purpose material. and one will be found in the Appendix to The soils series of the Piedmont Pla• this article following the last paper teau provinces are grouped into three div• in this Bulletin. isions, the crystalline rock division, the
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Figure 6.
TTiis key consists of a geolopic map of slate belt division, and the sandstone and the State and a detailed description of shale division. The crystalline rock div• all of the soil series groups found in ision IS subdivided into soils series de• t;ii; four soil provinces of the State. rived from acid crystalline, basic crystal- The manner of describing the soils and line, (nixed acid and basic rocks, and mica their arrangement in groups, having cer• and quartz mica schist. The slate belt tain features in common, permits easy division is sub-divided into two divisions, identification. soils series derived from slates and fine The soil series groups of the Coastal grained volcanic rocks and mixed slates Plain Province are divided into sands, and basic rocks. soils with friable "B" horizons, soils The soils series of the Appalachian with plastic "B" horizons, organic soils, Mountain Province are grouped into four and miscellaneous soils materials. These divisions, high mountain soils, low moun• divisions, with the exception of the tain soils, old high terrace soils, and organic soils, are sub-divided according miscellaneous soils. The high mountain 115 ^
i
Figure 7.
soils division is sub-divided into soils termined by use of the geologic map with series derived from acid crystal 1 ine rocks, the key or the map of the soil provinces basic crystalline rocks, mica schists, in Figure 4. Second, if the soils in the slates and schists, schists and acid sub-province are separated according to crystalline rocks, and from sandstone, parent material, drainage, or both, or shale and quartzite. The low mountain color of the "A" horizon material, the soils division is sub-divided into soils separation fitting the soil in question series groups derived from acid crystal• must be determined. This procedure final• line rocks, basic crystalline rocks, ly eliminates all soils series groups ex• schists, from sandstone, shales, slates, cept a few and the exact identification is and quartzite, and from limestone. made from the detailed description of the The soils series of the River Flood color, arrangement, texture, and structure Plains Province, sometimes called Bottom• of the soils in the profile. lands, are separated into divisions, first Sometimes it may be difficult to deter• bottoms or soils subject to frequent in• mine the location of the River Flood undation by floods, and second bottoms or Plains Province, but the proximity, size, older deposits from floods that are rarely and flood area of the streams will de• inundated. Hie soils series of these two termine its boundaries. divisions are further separated according to drainage and origin of the parent mat• Example of Identifying a Soil as to Series erial . Group - A soil is located in the south- The procedure for identifying a soil central portion of Caswell County. Ac• as to series using the soil identification cording to the geologic map it is located key follows certain orderly steps which in the crystalline rock division of the finally eliminates all other soils except Piedmont Plateau Province. (The symbol the one to which the soil in question be• Cg designates the rocks to be carbonifer• longs. First, the soil province and sub- ous granite which are acid crystalline. province in which the soil occurs is de• Intrusions of basic rocks may occur, but 116 i
Figure 8. i an examination of the rocks in the area This paper covers the subject of how will check the type of rock as acid agricultural soil maps may be used in crystalline rocks are light colored, while conducting soil surveys for engineering basic crystalline rocks are dark to green purposes. The methods described have been in color.) The topsoil or "A" horizon used by the Soils Department of the North material is a brownish-grey sandy loam Carolina State Highway and Public Works and the sub-soil is a stiff, but brittle, Commission since 1938 with success, and red clay with mica flakes and free quartz. the data accumulated and experience gained This soil belongs to the Cecil series. so far are used constantly. Additional The type classification is Cecil sandy data and experience through the years will loam. enable the Soils Department to solve soils A discussion of the characteristics, problems quickly and economically by the uses, and treatments of the various soils use of agricultural soils maps and the occurring in this State is not within the pedological system of classification of scope of this article. This is material soi1s. for a Soils Manual which is being prepared by the Soils Department. The amount-of The author has drawn freely from the work and data necessary for such a manual "Atlas of American Agriculture", Part III, is large and its preparation requires a and Bulletin No. 293 of the Agricultural considerable expenditure of time, however, Experiment Station of the North Carolina when it is completed, it will contain State CxDllege of Agriculture and Engineer• valuable information on the soils of North ing for the description of the pedologi• Carolina from an engineering standpoint. cal system of classifying soils.