90 Physics of the Soil 



there are other processes which reverse these operations and 

 set free again the nitrogen of organic compounds and of ni- 

 trates so that it is again returned to the atmosphere as free 

 nitrogen gas. 



(1) Dr. Angus Smith showed in 1867 that nitrates in 

 sewage waters are decomposed and the nitrogen set free 

 as a gas. (2) Scblosing showed that when moist humus- 

 bearing soils are placed in an atmosphere free from oxygen 

 they quickly lose all traces of nitrates. (3) Warington 

 demonstrated that sodium nitrate in a water-logged soil 

 is decomposed and the nitrogen liberated as a gas. (4) 

 So great is the demand for oxygen in rich water-logged 

 soils that according to the experiments of Mtintz even such 

 compounds as chlorates, iodates and bromates are deprived 

 of their oxygen, leaving iodides, chlorides and bromides in 

 their place. (5) When black marsh soils are stirred up 

 with water and allowed to stand Prof. J. A. Jeffery and the 

 writer have shown that the nitrates rapidly disappear and 

 nitrogen gas is set free. 



In all of these cases there are microscopic organisms in 

 the soil and water whose needs for oxygen are so great that 

 when that which is free in the soil-air or water-air is not 

 sufficient they have the power of decomposing nitrates and 

 even some organic compounds for the oxygen they contain 

 and in this way liberate free nitrogen. 



(6) There is still another condition under which denitri- 

 fication takes place in which the loss is large, rapid and 

 nearly complete. It is when human excrements are covered 

 with pulverized dry soil, as is done in the dry-earth closets. 

 The late Colonel Waring kept two tons of dry earth for a 

 number of years, having it used over and over again in or- 

 der to see how long it might be used without losing its effi- 

 ciency. The closets were filled with the dry earth and excre- 

 ment about 6 times each year, and when they were emptied 

 the material was thrown in a heap on a floor of a well venti- 

 lated cellar to dry. After the same soil had been used over 

 not less than 10 times it was analyzed for the amount of 

 nitrogen it contained, and in 4,000 Ibs. of the soil was found 



