162 ATMOSPHERE IN ITS RELATIONS TO VEGETATION. 



THE ATMOSPHERE IN ITS RELATIONS TO VEGETATION. 



Few of our readers need to be told that the greater part of the nutriment 

 of plants is derived from the atmosphere. The fabled food of the chameleon 

 is the real food of the whole vegetable world. It may indeed be asserted 

 that all the constituents of plants, except the ash, are of atmospheric origin. 

 Not that crops draw their supply of nitrogen directly from the air, or per- 

 haps in all cases their entire supply of carbon ; but the ammonia, nitric 

 acid, and carbonic acid which they get from the soil are themselves derived 

 from the atmosphere. By nitrogenous manuring we are in fact simply 

 furnishing our crops with nitrogen previously obtained from the air under a 

 different set conditions. 



The atmosphere is, however, to a limited extent an original source of 

 food ; it merely forms part of a great circle, and is itself supplied with car- 

 bon, and partly also with nitrogen, from the exhalations of the earth. The 

 organic parts of plants and animals are resolved by the process of decay 

 into gases, which pass into the atmosphere and are universally diffused for 

 the benefit of growing vegetation. The nature and amount of the various 

 constituents of the atmosphere, and the manner in which they are assimi- 

 lated by plants and the soil are clearly questions of the highest interest to 

 the student of scientific agriculture. 



The atmosphere furnishes our crops an abundant supply of carbon in the 

 iorm of carbonic acid gas. At Rothamsted, England, wheat crops contain- 

 ing on an average about a ton of carbon to the acre were taken from the 

 land for twenty-thee successive years, though no carbon was returned in the 

 manure, and the experiment might have been continued for an indefinite 

 period with similar results. This vast amount of carbon has certainly been 

 obtained from the atmosphere. 



The percentage of carbonic acid present in the air has repeatedly been 

 made the subject of scientific investigation. Herr Sehulze at the experi- 

 mental station at Eostock determined the amount of carbonic acid present 

 each day for three years. The variations observed were not great, the 

 largest quantity of carbonic acid found being 3.44 volumes, and the small- 

 est quantity 2.25 volumes in 10,000 volumes ot air, the mean of all his re- 

 sults being 2,92 volumes of carbonic acid in 10,000 of air. This proportion 

 — 3 to 4 volumes in 10,000 — is the quantity which has most usually been ob- 

 tained. If we take 3.5 as a mean number, then 1 lb. of carbon will be con- 

 tained in about 3500 cubic yards of air of the ordinary temperature and 

 pressure. As we have just seen that a good wheat crop is capable of remov- 

 ing one ton of carbon from the air in the course of a few months, it is evi- 

 dent that the constant renewal of the air by winds perfectly compensates 

 for the small fraction of carbon it contains. 



The amount of carbonic acid in the air is rather less at the surface of 

 the sea than on land. It is at sea greater in the day than in the night, but 



