4^ THE CHEMISTRY OF THE J'ARM 



particles filled with water, the nitrates present in it 

 are destroyed by certain kinds of bacteria, the oxygen 

 of the nitrate combining with carbon to form carbonic 

 acid, while the nitrogen escapes as gas. The process 

 of denitrification requires the presence of ferment- 

 able organic matter ; when this is present in 

 sufficient proportion denitrification may occur even 

 in the presence of air. 



In a peat bog, or in any water-logged soil, the 

 decomposition of the organic matter is brought about 

 by anaerobic organisms (see p. 52), and is of a putre- 

 factive character. Under these circumstances also 

 nitrogen may be lost as gas. 



10. Movements of Salts in Soil. — If water is allowed 

 to drain through a soil it carries with it a part of the 

 readily soluble matter which the soil contains. The 

 substances chiefly removed by the water will be car- 

 bonate of calcium, and the nitrates, chlorides, and 

 sulphates of calcium and sodium. When heavy rain 

 falls these substances are washed into the subsoil, and 

 partly escape by the nearest outfall into the springs, 

 brooks and rivers. The loss of nitrates from highly 

 manured land daring a wet season is very consider- 

 able, and will frequently be equal to several hundred 

 pounds of nitrate of sodium per acre. When dry 

 weather sets in evaporation takes place at the surface 

 of the soil, a part of the subsoil water is slowly 

 brought to the surface by capillary action, and the 

 salts it contains are concentrated once more in the 

 upper soil, forming in some rare instances a white 

 crust of salt upon the surface. Capillary action has 

 little influence in the case of coarse sandy soils. 



