142 



Science of Plant Life 



seven elements mentioned above, plants use, for the manu- 

 facture of food, carbon, hydrogen, and oxygen, which they 

 derive from water and carbon dioxid. 



Most plants grow best when the soil is neutral or slightly 

 alkaline. Red clover, alfalfa, and blue grass, for example, 

 cannot withstand an acid soil. By the addition of lime, acid 

 soils may be neutralized or made alkaline. This explains 

 the common practice of putting lime or wood ashes on lawns 

 where a growth of blue grass is desired. Some plants are 

 favored by an acid soil. Cranberries, blueberries, and redtop 

 grass are examples of such plants. In arid regions, the 

 evaporation of water may cause salts to accumulate in, the 

 surface layers of the soil to such an extent that most or all 

 plants are excluded. 



Another soil factor of great importance is humus. This 



U. S. Dept. of Agriculture 



FIG. 80. A Kansas cornfield. The soil is rich in humus, and the plants attain a 

 height of 12 to 14 feet. 



