CHAPTER IL 



THE SOURCES OF PLANT FOOD. 



The Atm.os2}here. — The carbonic acid, ammonia, and nitric acid which it 

 supplies — The quantity of combined nitrogen and chlorides contained 

 in rain. The Soil. — Its origin — Proj)erties of sand, clay, calcareous 

 matter, and humus ; their relation to water and heat — The plant food 

 contained in soil, its quantity, and condition — Losses by drainage — 

 The absorptive power of soils — Influence of tillage, drainage, and 

 burning. 



The Atmosphere. — 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 about 3 J volumes of carbonic acid, or about 1 lb. 

 of carbon in 3500 cubic yards of air. This small amount 

 is made sufficient by the action of winds, which bring an 

 enormous quantity of air in contact with both soil and 

 plant. 



The atmosphere also contains a very small and variable 

 quantity of ammonia. Schloesing found from 1 lb. in 

 6,000,000 cubic yards, to 1 lb. in 119,000,000 cubic yards. 

 The quantity is greatest, according to the same experi- 

 menter, in warm southerly winds. The ammonia of the 

 air is directly absorbed by plants to a very small extent, it 

 is rendered available chiefly through absorption by the 

 soil, and by means of rain, which brings it in solution to 

 the earth. 



