448 



PEACTICAL BOTANY 



concerned. Carbon dioxide comes from the air, water from 

 the soil, and with the water there are carried into the plants 

 compounds of nitrogen, potassium, phosphorus, magnesium, 

 calcium, iron, sulphur, etc. Uncombined nitrogen exists in the 

 air, but as such it is unavailable to green plants. In the soil, 

 when nitrogen exists in combination with oxygen as a nitrate 

 (NO 3 ) green plants may use it. The following tables show 

 the amount of these substances contained in the soil and the 

 amount used in a given quantity of plant product. 



RELATIVE ** SUPPLY AND DEMAND " OF SEVEN ELEMENTS l 



MINERAL PLANT FOOD IN WHEAT, CORN, OATS, AND CLOVER 2 



1 After Hopkins, C. G., Table 8, Soil Fertility and Permanent Agricul- 

 ture, 1910. 2 Ibid., Table 13. 



