234 THE SMALL GRAINS 



The soil requirements may therefore be summarized as 

 follows: (1) at least 5 feet of soil in the proper physical 

 condition for storing a large percentage of the rainfall ; 

 (2) mineral foods in sufficient abundance in the same 

 depth to furnish quickly the amount of these necessary 

 in the short life of a cereal crop ; (3) plenty of lime to 

 help render available other foods ; (4) humus as a source 

 of nitrogen and for increasing the water-absorbing and 

 retaining capacity of the soil. 



245. Black prairie soils. The conditions just described 

 are found to exist most perfectly in prairie soils. They 

 are, in general, a deep, black silt, clay, or sandy loam, or 

 heavy clay, rich in humus and mineral bases, and sup- 

 ported by a subsoil, varying greatly, but often little dif- 

 ferent from the upper soil. The depth of black soil in 

 the Red River-of-the-North Valley and in extreme eastern 

 Russia reaches several feet. The bulk of the small-grain 

 production in the two principal cereal regions of the 

 world, the Great Plains of North America and the Black 

 Earth belt of Russia, is supported by a prairie soil forma- 

 tion. The more important of these, the Russian Black 

 Earth, is characterized by a more perfect development of 

 this formation. 



246. Mineral bases, phosphates, and nitrogen. In 

 all cases where a soil is found to be naturally imperfect 

 in its food supply for cereal production there is a lack of 

 one or more of the substances, potassium, phosphorus, 

 and nitrogen. These, with other mineral bases, such as 

 calcium and magnesium, are usually abundant in prairie 

 soils, and are very important for the production of grain 

 of good quality. It is a fact well known to Russian agri- 

 culturists that in the Black Earth belt the hardness of 

 the wheat kernel and its protein content increase in the 



