2l8 SOILS AND FERTILIZERS 



In the roots of all plants there are present various 

 organic acids and salts. Between the rootlet and the 

 soil there is a layer of water. The plant sap and the 

 soil water are separated by plant tissue which serves as 

 a membrane. All of the conditions are favorable for 

 osmosis. The sap from the roots finds its way into 

 the soil in exchange for some of the soil water. The 

 acid and compounds, excreted by the roots, act upon 

 the mineral matter, rendering portions of it soluble, 

 when it is taken up by the plant. Different plants con- 

 tain different kinds and amounts of solvents, as well 

 as present different areas of root surface to act upon 

 .the soil, and the result is that 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 digestion of food by animals. 



Plants not only possess the power of rendering their 

 food soluble but they are also able to select their food 

 and to reject that which is unnecessary. For ex- 

 ample, wheat grown on prairie soil containing soda in 

 equally abundant and soluble forms as the potash, 

 will contain relatively little soda compared with the 

 potash. 37 



CEREAL CROPS 



295. 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, 



