6O SOILS AND FERTILIZERS 



when the substances are in solution. In fact, water is 

 necessary for nearly all the chemical reactions in the 

 soil which result in rendering plant food available. 



59. Joint Action of Air and Gases. In the disintegra- 

 tion of materials to form soil, air takes a prominent 

 part. By the aid of oxygen, carbon dioxide, and other 

 gases and vapors in the air, rock disintegration is has- 

 tened. The action of oxygen changes the lower oxides 

 to higher forms. All rock contains more or less oxygen 

 in chemical combination. The carbon dioxide of the air 

 under some conditions favors the formation of carbon- 

 ates. The disintegrating action of air, moisture, and 

 frost is illustrated in the case of building stones which 

 in time crumble and form a powder. Many of the 

 benefits of cultivation are due to aeration of the soil, as 

 air promotes chemical changes of mineral substances 

 and prepares the way for life processes in the soil. 



i 



60. Action of Micro-organisms. Micro-organisms, 

 found on the surface and in the crevices of rocks, are 

 active agents in bringing about rock decay, .deriving all 

 of their energy from the chemical changes which they 

 induce between minerals, and obtaining their carbon 

 from the air. Such organisms incorporate organic 

 matter with the rock residues. 21 Certain nitrifying 

 organisms can obtain their nitrogen also from the air, 

 and it is believed that they have largely prepared the 

 way for the production of agricultural plants, by incor- 



