CHAPTER 11. 



THE ATMOSPHERE AND SOIL. 



The Atmosphere. — Its composition — The carbonic acid, ammonia and 

 nitric acid which it supplies — The quantity of combined nitrogen, 

 chlorides, and sulphuric acid contained in rain. The Soil. — Its 

 physical constituents— Tenacity — Relations to water — Relations 

 to heat— Formation of soils — Organic constituents — The plant 

 food contained in soil, its quantity and condition — Oxidation in 

 the soil, nitrification — Deoxidation in the soil — Movements of salts 

 in soil, losses by drainage — Absorption of atmospheric nitrogen — 

 Absorption of bases and acids— Influence of tillage and draining — 

 Clay burning. 



The Atmosphere.— One hundred volumes of air con- 

 tain nearly 78 of nitrogen and 21 of oxygen, with 1 of 

 argon and other gases. 



The free nitrogen of the atmosphere is apparently 

 made available to leguminous crops through their root 

 tubercles (p. 11) ; it may also be assimilated by cer- 

 tain low organisms in the soil under special conditions. 



We have already stated that the whole of the carbon 

 of plants is obtained from the carbonic acid present in 

 the atmosphere ; 10,000 volumes of air contain nearly 

 3 volumes of carbonic acid, or about 1 lb. in 1,057 

 cubic yards of air. An acre of a good wheat crop will 

 obtain from the atmosphere in four months 1 ton of 

 carbon — a quantity corresponding to a column of air 

 3 miles in height. The small amount of carbonic 

 acid in the atmosphere is made sufficient by the action 

 of winds, which bring an enormous quantity of air in 

 contact with both soil and plant, 



