156 SOILS AND FERTILIZERS 



dioxide, which is always present in stables in liberal 

 amounts as a product of respiration, and forms ammo- 

 nium carbonate, a volatile compound. When the 

 stable atmosphere becomes charged with ammonium 

 carbonate, some of it is deposited on the walls of the 

 stable, forming a white coating. The white coating 

 found on harnesses and carriages stored in poorly ven- 

 tilated stables, is ammonium carbonate. Accumula- 

 tions of manure in the stable and poor ventilation are 

 the conditions favorable to the production of this com- 

 pound. 



188. Human Excrements. — The use of human ex- 

 crements as manure is sometimes advised, and in some 

 countries they are extensively used. When fresh, human 

 excrements may contain a high per cent, of nitrogen 

 and phosphoric acid ; when fermented, a loss of nitro- 

 gen occurs. Heiden estimates that in a year 1,000 

 pounds of excrements per person are made, which con- 

 tain $2.25 worth of fertility. 58 For sanitary reasons, 

 human excrements should be used with great care. 

 It is doubtful with the abundance and cheapness of 

 plant food whether their extensive use as manure is 

 advisable. About 1840, Liebig feared that the essen- 

 tial elements of plant food would accumulate in the 

 vicinity of large cities and be wasted, and that in time 

 there would be a decline in fertilitv due to this cause. 59 

 Many political economists shared the same fear. 

 Since that time the fixation of atmospheric nitrogen 



