22 SOILS AND FERTILIZERS 



ever, the amount of available plant food required for 

 each crop is not the same. The western prairie soils 

 which produce most of the corn raised in the United 

 States, are composed largely of silt. 



15. Medium Grass and Grain Soils. — For the pro- 

 duction of grass and grain a larger amount of water 

 is required than for corn. The yield of both is de- 

 termined largely by the amount of water which the 

 soil contains. For an average rainfall of about 30 

 inches, good grass and grain soils should contain 

 about 15 per cent, of clay and 60 per cent, of silt. 

 Such a soil ordinarily holds from 18 to 20 per cent, 

 of water. Many grass and grain soils have less silt 

 and more clay. A soil composed of about 30 per 

 cent, each of fine sand, silt, and clay, would also be 

 suitable, mechanically, for general grain production. 

 There are a number of different types of grass and 

 grain soils, with different proportional amounts of 

 sand, silt, and clay. Silt soils, however, form the 

 larger part of the grain soils of the United States. 



16. Wheat Soils. — For wheat production, soils of 

 a closer texture are required than for general grain 

 farming. There are three classes of wheat soils. The 

 first (1 in Fig. 10) contains from 30 to 50 per cent, of 

 clay particles, these being mostly disintegrated lime- 

 stone. The soil of the Red River Valley of the North 

 belongs to the first class of wheat-producing soils. 

 The surface soil contains from 8 to 12 percent, of veg- 



