20 SOILS AND FERTILIZERS 



duction of grass and grain a larger amount of water 

 is required than for corn. The yield is determined 

 largely by the amount of water which the soil con- 

 tains. 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, me- 

 chanically, 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 

 closer texture are required than for other small grains. 

 There are three classes of wheat soils. The first (i 

 in Fig. 10) contains from 30 to 50 per cent, of clay 

 particles, these being mostly disintegrated limestone. 

 The soil of the Red River Valley of the North belongs 

 to the first class. The surface soil contains from 8 to 

 12 per cent, of vegetable matter and the subsoil about 

 25 per cent, of limestone in a very fine state of division. 

 For the production of wheat, the subsoil should con- 

 tain 20 per cent, of water. A crop can, however, be 

 produced with less water, but a smaller yield is ob- 

 tained. 



The second type of wheat soil (2 in Fig. 10) con- 

 tains less clay and more silt. Many prairie subsoils 

 which produce good crops of wheat contain about 20 



