22 SOILS AND FERTILIZERS 



on silt soils, it is essential that a good supply of 

 organic matter be kept in the soil so as to bind 

 the soil particles. The special peculiarities of the 

 different grain crops as to soil requirements are con- 

 sidered in connection with the food requirements of 

 crops. 



17. Sandy, Clay, and Loam Soils. In ordinary 

 agricultural literature, the term l sandy,' * clay,' or 

 ' loam ' is used to designate the prevailing character of 

 the soil. Sandy soils usually contain 90 per cent, or 

 more of silica or chemically pure sand. The term 

 light sandy soil is sometimes used to indicate that 

 the soil is easily worked, while the term heavy clay 

 means that the soil offers great resistance to cultiva- 

 tion. Many soils which are clay-like in character are 

 not composed very largely of clay. They are sub- 

 soils in the western states which have clay -like char- 

 acteristics but contain only about 15 percent, of clay, 

 the larger part of the soil being silt. A loam soil is a 

 mixture of sand and clay ; if clay predominates the 

 soil is a clay loam, while if sand predominates it is a 

 sandy loam. 



RELATION OF THE SOIL TO WATER 



18. Amount of Water Required by Crops. Ex- 

 periments have shown that it takes from 275 to 375 

 pounds of water to produce a pound of dry matter in 

 a grain crop. In order to produce an average acre of 

 wheat, 350 tons of water are needed. The amount of 

 water required for the production of an average acre 

 of various crops is as follows : I2 



