Soil surveys.—Soil may be "surveyed" from different points of view. The early pioneer looked out over the broad prairies and extensive woodlands of Illinois, and from the general appearance of the landscape selected a future home. If he appreciated the possibilities of future land values he acquired title to large tracts, sometimes thousands of acres in extent. The eastern farmer who sold his improved holdings at good values came to Illinois while lands were still cheap and made sufficient examination of the soil and its crops to con vince him that the new farm was obtained at a favorable price.
After years of heavy cropping on soils of exceeding fertility, the farmer became convinced that what was once con sidered a soil of inex haustible fertility was producing, even under improved methods of cultivation, smaller acre yields than were obtained from the virgin soil. This led to the inaugu ration of the scientific soil survey of Illinois. A gen eral soil survey of the state was first made and a, map prepared showing fourteen soil areas. De tailed county surveys have been made for a number of counties, and these will be continued until the detailed survey of all the counties is completed.
In the Illinois Experi ment Station at Urbana many laboratory experi ments on soils are being constantly carried on, solving problems, and adding much to our knowledge of the soil.
These scientific surveys are carried on by soil experts who inspect every ten-acre area, and enter on maps, while in the field, the areas covered by each soil type. Each surveyor carries a small auger 40 inches in length with which to obtain soil samples to the depth of 40 inches. The surface soil is sampled to a depth of 63 inches. Next, the subsurface soil is sampled to a depth of 63 to 20 inches. Below this the sub soil is sampled to a depth of 20 to 40 inches. Thus the thickness assigned to the surface soil is one-half that of the subsurface soil; and the combined thickness of the surface and subsurface strata is equal to the thickness of the subsoil. These samples, carefully analyzed at the laboratories of the Experiment Station, give accurate, scientific knowledge of the plant-food elements in each soil type.
The soil-survey map shows the fourteen soil urcas of the state. These are readily located on the map. They are briefly mentioned in chapter iii, "The Glacial Period." A soil area may contain a number of types of soils. The
Experiment Station has defined the different general groups of soil types as follows: Peat—Consisting of 35 per cent or more of organic matter, sometimes mixed with more or less sand or silt.
Peaty looms—Soils with 15 to 35 per cent of organic matter mixed with much sand. Some silt and a little clay may be present.
Mocks—Soils with 15 to 25 per cent of partly decomposed organic matter mixed with much clay and silt.
Clays—Soils with more than 25 per cent of clay, usually mixed with much silt.
Clay learns—Soils with from 15 to 25 per cent of clay, usually mixed with much silt and some sand.
Looms—Soils with from 30 to 50 per cent of sand mixed with much silt and a little clay.
Sandy looms—Soils with from 50 to 75 per cent of sand.
Fine sandy loams—Soils with from 50 to 75 per cent of fine sand mixed with much silt and a little clay.
Sands—Soils with more than 75 per cent of sand.
Gravelly loams—Soils with 25 to 50 per cent of gravel with much sand and some silt.
Gravels—Soils with more than 50 per cent of gravel and much sand. Stony looms—Soils containing a considerable number of stones over one inch in diameter.
Rock outcrop—Usually ledges of rock having no direct agricultural value.
More or less organic matter is found in all the foregoing groups.
Required plant food.—Ten different chemical elements are required for plant growth. These are: carbon, hydrogen, oxygen; sulphur, iron; magnesium, calcium; nitrogen, phos phorus, potassium. If any one of these elements is not avail able, the plant fails to develop. With all present in proper proportions, and under favorable climatic conditions, a large crop is assured. The problem of a permanent and profitable agriculture in Illinois, therefore, is the problem of maintaining soil fertility so that these ten elements of plant food shall always be available for the production of maximum crops year after year throughout the centuries.
Carbon, hydrogen, and oxygen come from air and water in unlimited amounts except in times of drought. These three elements constitute about 95 per cent of the weight of the mature crop.
The seven remaining elements, constituting but 5 per cent of the crop, are obtained from the soil. Two of these, sulphur and iron, are required in such small amounts, and they are present in nearly all soils in such large amounts, that they need not be considered of importance in maintaining soil fertility.