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 \^ RiaHT-AfiS UB Saik IkxTuHB CHAirr row \ CkAssirvmo 3oii.s u b. 0 \^ Cl-A-r SAMO-r S11.TY \ Cl-AY CUAY \ SluTY CUAY LOAM\^ SAMDYCLATLOAMV LOAM i<^S. \^ LOAM SAMOY LOAM Sii-TY LOAM SAND \. SluT \ PERCEPiTAOB Of 311.T Figure 1. 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.