CHAPTER II. 



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 con- 

 stituents Properties of sand, clay, calcareous matter, and humus 

 The relation of soil to water Its relation to heat The plant food 

 contained in soil, its quantity and condition Oxidation in the 

 soil, nitrification Movements of salts in soil, losses by drainage The 

 absorptive power of soils Influence of tillage, and draining Soil 

 burning. 



The Atmosphere. One hundred volumes of air con- 

 tain nearly 79 of nitrogen, and 21 of oxygen, with very 

 small quantities of: other constituents. 



The free nitrogen of the atmosphere is apparently 

 made available to leguminous crops through their root 

 tubercles (p. 10) ; there is no satisfactory evidence that 

 it serves as a plant food to other crops. 



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 Ib. in 3,200 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. 



