EXTENSION" COUESE IN SOILS. 71 



for pasture. This seriously lessens tlieir value for such crops as sugar 

 beets, cotton, or corn, which grow through the whole summer. On 

 the other hand, some small grains, which make their growth very 

 early in the season, are better adapted to such land. Crop rotation 

 for light soil should be short. Many of the best rotations are of but 

 three years' duration. 



Live-stoclc farming on sand. — The use of pasture is still, and prob- 

 ably will long remain, an important factor in most lines of live-stock 

 farming. This is partly because in grazing, stock harvest their own 

 feed, and in this way greatly lessen the expense for labor. Since 

 sandy soils, as we have seen, are poorly adapted to pasture grasses, 

 they are not as well suited to most lines of live-stock raising as are 

 heavier soils. However, it is frequently the case that considerable 

 quantities of produce, grown in connection with truck raising on sandy 

 soils, are not marketable and should be fed to some form of live stock. 

 A small number of live stock, therefore, should usually be kept, even 

 on sandy farms, the principal business of which is the growing of 

 truck or vegetable crops. 



CLAY SOILS. 



Formation and location. — Clay soils are commonly formed by the 

 settling out of fine sediment in standing bodies of water into which 

 streams carrying the sediment have run. Such areas of standing 

 water occur as lagoons along main river valleys like those of the 

 Mississippi, Ohio, Missouri, and other large rivers. They were also 

 formed in extensions of the Great Lakes which existed toward the 

 close of the glacial period. Broad belts of extremely heavy clay soils 

 were formed in this way along the southern shore of Lake Superior, 

 along Lake Michigan in Wisconsin, and on the southern borders of 

 Lake Erie and Lake Ontario. Many shallow lakes existed for a 

 comparatively short time at the close of the glacial period. In these 

 great areas heavy clay soils were formed. Lake Agassiz in Minnesota, 

 North Dakota, and Manitoba (long since dried up) is one of the best 

 illustrations of the formation of heavy clay soils. The clay soil of 

 the Champlain Valley in New York has its origin in the same way. 

 Some areas of heavy clay soil have also been formed along the sea- 

 shore as deltas and in bodies of salt water formed by shutting off the 

 main portion of the ocean. As stated in Lesson I, a residual soil from 

 limestone is also an extremely fine clay. This is because the soil is 

 made up of the insoluble portions of the rock, the soluble portions 

 having been dissolved and carried away by percolating water. 



Characteristics of clay soils (Rei. No. 7, pp. 95-99). — Clay soils owe 

 their special character largely to their very fine texture. Their large 

 water-holding capacity and poor underdrainage is the immediate 

 result of this texture. As a secondary result they often have poor 

 tilth and are liable under certain conditions to be cold during the 



