THE SOIL ii 



higher levels. The total amount of soil thus carried away is 

 very great. It is claimed that if all the soil particles carried 

 by the waters of the Mississippi River in one year were 

 made into a solid rectangular block, it would cover one 

 square mile and be 268 feet high. The amount of soil carried 

 away by water varies with the slope of the land; the steeper 

 the slope, the swifter the water current, and therefore the 

 greater the quantity of soil material carried away. In hilly 

 lands the loss of soil from the uplands is considerable, so much, 

 indeed, that measures need to be taken to reduce the loss. 



Kinds of soil. The size of the particles which make up 

 the soil varies greatly. In some places most of the particles 

 are sand, as in a sandy soil. In others, clay particles are 

 most abundant, as in clay soil. Sometimes particles of clay 

 and silt make up about half and sand the other half. Such 

 soil is called a loam. If the mixture contains somewhat 

 more fine particles than sand, the soil is known as a clay 

 loam; but if more sand than silt and clay, it is called a sandy 

 loam. These names, sandy, clay, loam, clay loam and sandy 

 loam, are in common use in describing soils. 



We should now turn back and read again the first para- 

 graph of this chapter and notice especially the last sentence 

 in the paragraph. Rock particles alone, although they make 

 up the soil mass, do not constitute a fertile soil. Air, water, 

 and material for plant food are equally essential for plant 

 growth. For this reason each of these should be considered 

 in its relation to the soil, on the one hand, and to the plant 

 on the other. In the next few chapters we shall try to learn 

 something more about these relations. 



