FOOD REQUIREMENTS OF CROPS 259 



agricultural crops have different powers of assimilating 

 food. This action of living plant roots upon soils is a 

 digestion process which is somewhat akin to the diges- 

 tion of food by animals. 



Plants not only possess the power of rendering a por- 

 tion of their food soluble, but they are also able to select, 

 and to reject that which is unnecessary. For example, 

 wheat grown on prairie soil with soda in equally abun- 

 dant and soluble forms as the potash will contain 

 relatively little soda compared with the potash ; also 

 many seaweeds contain more potash than soda, although 

 the sea water in which they grow has an excess of 

 sodium salts. 



For the feeding of crops, a nutritive soil solution is 

 desirable, and the soil should have a good stock of 

 reserve material that can be utilized either by action of 

 the plant roots or readily pass into solution in the soil 

 water. 



CEREAL CROPS 



309. General Food Requirements. -- Cereal crops con- 

 tain a high per cent of silica and evidently possess the 

 power of feeding upon some of the simpler silicates of 

 the soil, 74 liberating the base elements and using them as 

 food, while the silica is deposited in the outer surface of 

 the straw. As previously stated, cereal crops, although 

 they do not remove large amounts of total nitrogen from 

 the soil, require special help in obtaining this element. 

 There is, however, a great difference among the cereals as 



