456 Introduction to the Study of Science 



fields the plow seldom goes as deep as that. The layers beneath the 

 soil are known as the subsoil. This is much more compact, harder, 

 and contains no humus ; and, if plowed up, would be destructive to 

 profitable crops. 



220. Classification of soils : On the basis of texture. Soils 

 are classified in several ways. It may be observed that all 

 soils are largely composed of rock fragments of different sizes. 

 Classification on the basis of size of particles is common. Test 

 different soils by rubbing between the hands or fingers. If a 

 soil feels smooth, its particles being very fine, it is clay. This, 

 if wet and worked, will retain any shape given it. If a soil 

 feels gritty when rubbed, it is silt. The particles of silt are so 

 fine, however, that they can be seen only with the aid of a 

 magnifying glass. Soil which is mostly made up of particles 

 about the size of a period (.) is sandy, and is called sand. When 

 it is composed of rock fragments about the size of the letter o, 

 it is gravelly. If the soil is of particles many times the size 

 of o, it is gravel. Perhaps at the bottom of the test hole 

 is found rock, such as sandstone or limestone, or just a bowlder 

 of rugged shape. 



Soils are more than mixtures of rock fragments. They con- 

 tain humus, and this adds to the above classification. Soil 

 containing humus usually varies from a dark shade to black, 

 according to the amount. If the humus is mixed with sand, 

 it is known as sandy loam. If the mixture is of clay and humus, 

 it is a clay loam. 



On basis of origin. Soils are classified also with respect 

 to their origin. For example, the soil of the great flood plains 

 of the Mississippi, the Nile, the Sacramento in California, and 

 other rivers, is alluvial. The soil was transported by the rivers 

 from the upstream country and deposited on the flood plains. 

 Its fertility seems to be almost inexhaustible. 



The soil of a large part of North America is glacial in origin. 

 Thousands of years ago great ice sheets covered an extensive 

 area, as far south as the Mason and Dixon line in the eastern 



