CHAPTER III. 



THE PLANT AND THE ATMOSPHERE. 



Although the atmosphere is equally as important as the soil for 

 the production of plants, yet, since atmospheric conditions are 

 little subject to control and are less complex in relation to the 

 plant than soil conditions, much less attention need be given to it. 

 We have already found that the carbon of the plant comes from 

 the air. The atmosphere receives and transports water vapor, 

 and precipitates it in the form of rain or snow. The amount, 

 kind and period of rainfall which are dependent on atmospheric 

 conditions are highly important to agriculture. The atmosphere 

 moderates the variations in temperature. It tempers and stores 

 the heat of the sun. If there were no atmosphere, the tempera- 

 ture would be very hot in the direct sunshine and freezing in the 

 shade. 



The soil atmosphere is also of importance. The air penetrates 

 the soil, supplies the roots of plants with the oxygen they need, 

 and oxidizes deleterious substances. It aids in the preparation 

 of plant food so that the plant can take it up. It also aids in the 

 formation of soil from rock. 



Composition of the Air. The air consists chiefly of nitrogen 

 and oxygen. It contains, in addition, a small amount of carbon 

 dioxide, some water vapor, small quantities of argon and allied 

 rare gases, besides minute quantities of ammonia, nitric acid, 

 hydrogen peroxide, and marsh gas. It also contains suspended 

 solid matter, some of which consists of micro-organisms. 

 In towns, the air contains some sulphuric acid and hydrogen sul- 

 phide. The moisture in the air varies considerably. 



The composition of dry air varies but slightly, even when 

 sampled at widely distant localities. Its average composition by 

 volume is as follows : 



Oxygen 20.90 parts. 



Argon 0.90 parts. 



Nitrogen 78. 15 parts. 



Carbon dioxide 0.03 parts. 



Hydrogen 0.02 parts. 



100.00 parts. 



