14 FIRST PRINCIPLES OF AGRICULTURE. 



root to penetrate the soil without injury. The acid sap 

 or fluid which is produced by the root, hair, or cell, when 

 it comes in contact with soil particles, has a solvent effect 

 upon them ; thus is insoluble matter in the soil, containing 

 ash ingredients, made soluble to the plant. 



These elements derived from the soil are, however, not 

 taken up in the form of individual chemical elements, but 

 chiefly as acid or salts. Nitrogen, for instance, is com- 

 bined with oxygen to form nitric acid ; which, united with 

 bases like sodium or calcium, forms sodium or calcium 

 nitrates. The very weak solutions of the compounds 

 taken up by the roots are concentrated in the upper part 

 of the plant, owing to the rapid evaporation of the water 

 through the leaves, and are employed in the formation of 

 new tissues. 



Supply of Pood Constituents. — The atmosphere 

 contains relatively a very small proportion of carbonic 

 acid, from which the carbon is obtained ; it is less in the 

 open country and over large bodies of water, and more in 

 the vicinity of towns, yet the supply in the aggregate is 

 enormous, and is sufficient to meet all the demands of 

 vegetation for carbon. 



The atmosphere also contains small quantities of nitro- 

 gen, existing both as ammonia and as nitrates ; these are 

 brought to the earth by means of dew and rain, and thus 

 act as a direct source of this element to plants. The 

 amount so provided is, however, insufficient for the 

 entire needs of vegetation. 



The moisture, or " water vapor " in the atmosphere does 

 not, to any great extent, serve to supply the plant with 

 water, the absorption of water being a function of the 



