Soil maps are made by experienced soil scientists who are graduates of the State agricultural colleges or other colleges that offer courses in soil science. After graduation, the soil scientists usually receive several months of intensive training. They work in the field. First they observe the techniques of an experienced soil scientist. Then they practice, and their mistakes are pointed out by their supervisor. Finally, when they prove their ability, they begin work.

Ordinarily the soil scientists use aerial photographs as a base for plotting the soil boundaries. The scientist goes over the land and digs with a spade or auger as often as necessary to determine and evaluate the important characteristics of the entire profile. He identifies the kind of soil, locates its boundaries in the field, plots the boundaries, and places the identification symbol of each soil mapping unit on the map.

In making detailed maps, he follows or sees the boundaries between the kinds of soil through their entire length. In making reconnaissance surveys, he may not see the boundaries over their entire length; he merely identifies one when he crosses it and draws the boundary through to his next traverse, or crossing, on the basis of the information he can get from the aerial photograph.

The soil scientist makes simple chemical tests in the field to determine the degree of acidity and the presence of lime, salts, and a few toxic compounds. He measures slopes with a hand level. He usually takes samples of a few representative soils during the survey and sends them to the laboratory for detailed study.

All stages of the work from the mapping to the contents of the report are reviewed by the supervisors and representatives of the cooperating agencies.

Soil maps often are used before they are published. Each cooperator of the soil conservation districts is furnished a copy of the soil map of his holding.

County assessors and other users sometimes buy copies of such maps to use in their work before the publication of the completed survey. The local soils handbooks, available for reference at the Soil Conservation Service offices, give information needed to use and interpret the map.

Photographic copies of unpublished maps may be purchased through the Soil Conservation Service offices. These offices are usually located in county seats. Prices for the maps are fixed to cover the cost to the Government.

Soil maps are published by the Soil Conservation Service for all States except Illinois; in Illinois, the University of Illinois Agricultural Experiment Station publishes them.

Copies of available published maps and reports may be obtained through the State Extension Service or Soil Conservation Service offices. Files of unpublished maps are maintained in the Soil Conservation Service offices and may be examined there.

Interpretations of soil maps are physical and economic analyses of the alternative opportunities available to the users of the land. They indicate capabilities of the soils for agricultural use, adapted crops, estimated yields of crops under defined systems of management, presence of specific soil-management problems, opportunities and limitations for various management practices, and problems in nonagricultural use.

The main bases for interpretations are yield estimates, related to specific combinations of practices for soils in their climatic setting. Yield estimates for a soil are predictions of the average production of specific crops that a group of farmers could expect during the following 10 or 15 years if they followed the defined system of soil management. Yield estimates apply less closely to individual farmers, whose skills are variable, than to averages of groups. Sources of information are the results of research, the experiences of farmers, ranchers, and others who grow plants, and observations of plants growing on different kinds of soils.

The definitions and descriptions of the kinds of soil shown on maps provide information on their characteristics. We use these to infer the qualities of soils such as productivity and erosion hazard. We are able to make predictions about a soil whose behavior is unknown by comparing its characteristics with those of the soils about which we have basic information on behavior. Basic principles of soil management are another tool used to help us extend our predictions of soil behavior and responses to all kinds of soils.

Soil survey reports include a number of interpretations, especially the more permanent ones that concern soil qualities. New and additional research, new weeds or insects, changes in relative prices, new crops, new machines, and other changes in agricultural arts require us to revise interpretations from time to time. The interpretation of the soil units shown on soil maps is a continuing task if we are to make full use of available knowledge on the use and management of soils. Interpretations can be no more static than our civilization. Yet soil maps, with accurately plotted boundaries of carefully defined soils, can be reinterpreted as the situation demands in future years.

To go back to a specific kind of soil we cited earlier, some alternatives for Fayette silt loam, 8 to 14 percent slopes eroded, which will result in sustained production now and in the future are:

(1) Permanent pasture, which permits several choices of legumes and grasses, lime, varying amounts and kinds of fertilizers, and varying systems of grazing;

(2) Woodland;

(3) A 4-year rotation of corn-oats-alfalfa-alfalfa with contour cultivation, lime, and fertilizers; and;

(4) A 3-year rotation of corn-oats-red clover with contour strips, lime, and fertilizers.

Estimates of the yields of woodland, pasture, forage, and grain under these alternative uses permit the confident selection of the best management from the viewpoints of the desires of the operator and the conservation of the soil. If new crops, new pests, or new techniques of farming appear, the present "best management" may become undesirable. Yield estimates can be revised at that time, and will again provide the basis for selection of a suitable management.