20 SOIL PHYSICS AND MANAGEMENT 



contributes much toward the breaking down of rocks. Nitric icid is 

 formed under certain conditions in the atmosphere, and, although 

 the amount reaching the surface of the earth per acre per annum is 

 small, amounting at Rothamsted, England, to from 2.81 pounds to 

 2.98 pounds, yet the long-continued action of this acid during geo- 

 logical time has done a great deal toward breaking down rocks into 

 soil material. In some localities hydrochloric acid forms a very 

 active agent, especially upon limestone and marble. 



(b) Carbon Dioxide. The most effective acid in decomposing 

 rocks is that produced by the union of carbon dioxide and water, or 

 carbonic acid. Carbon dioxide is found in the atmosphere in all 

 localities, but, of course, in slightly greater quantities near cities 

 and factories than at other places. It is considered a weak acid, yet 

 because of the fact that it is always present, it exerts an immense 

 influence in breaking down rocks, especially those containing lime, 

 magnesia, potash, and soda. The soil air contains much larger 

 amounts of carbon dioxide than the air above, thus percolating water 

 becomes highly charged before coming in contact with the rocks 

 beneath. Carbonated water is an almost universal solvent. The 

 amount of carbon dioxide in air under different conditions is shown 

 by the following table: 



Amount of Carbon Dioxide in the Moil Air, and in the Atmosphere' 



Parts per million by weight 



Ordinary atmosphere 28i5 to (500 



Air from sandy subsoil of forest 3,800 



Air from loamy subsoil of forest 12,400 



Air from surface soil of forest 13,000 



Air from surface soil of vineyard 14,000 



Air from pasture soil . . . . 27,000 



Air fom soil rich in humus 54,300 



Fischer lias shown that in rain and snow water the amount of 

 carlxm dioxide varies between 0.22 and 0.4'5 per cent by volume of 

 water. These figures according to Merrill would give for the Atlan- 

 tic Coast States a depth of 3.75 mm. of carbon dioxide brought to 

 the surface in rain and snow, for the upper Mississippi valley 2.50 

 mm., for the lower Mississippi valley 4.50 mm., and for the North- 

 ern Pacific States 6.25 mm. Water percolating through soil would 

 absorb additional amounts. 



(c) Oxidation. The only element that free oxygen of the air 

 acts upon is iron when in the sulfide or ferrous condition. When 

 the iron of the sulfide is oxidized, iron sulfate is formed, which is 



