66 PHYSIOLOGY OF NUTRITION 



nitrogen of ammonium salts forms the smallest part. The ammonia of the soil 

 is derived partly from the decomposition or organic nitrogenous compounds 

 and partly from the air. According to Schlösing's investigations, ammonia 

 gas is vigorously absorbed from the air by both dry and moist soils. Dry soils, 

 it is true, soon become saturated with ammonia, but this is not so for moist 

 soils, for the ammonia absorbed is gradually converted into nitric acid. A soil 

 surface of i hectare (2.5 acres) can absorb yearly from 53 to 63 kg. of ammonia 

 from the air. 



Besides organic compounds and ammonia, every soil also contains nitric 

 acid or its salts. According to Boussingault's exact investigations nitric acid 

 is formed in the soil at the expense of other nitrogenous compounds. A known 

 quantity of damp soil, of known composition, was placed in a large carboy, 

 which was sealed in 1859 and not reopened until 1871. At the conclusion of the 

 experiment the soil in the carboy was again analyzed. The results are presented 

 in the following table. 



The nitric acid was at least mainly formed from other nitrogenous compounds 

 present in the soil. Moreover, during the progress of the experiment a part 

 of the nitrogen of the soil diffused into the air of the enclosed space. Bous- 

 singault showed in later experiments that very many kinds of organic materials 

 (e.g., meat, blood, horn, bone, wool, etc.), if added to the soil, serve as sources 

 for the formation of nitrates. Conditions thus exist in the soil which render 

 possible the transformation of a great many kinds of nitrogen compounds 

 into nitric acid or nitrates. 



Now the question arises, how is it that, in spite of the continuous formation 

 of nitric acid, there is never more than a small quantity of this substance present 

 in the soil? An answer is obtained from a consideration of the phenomena 

 of absorption of various compounds by the soil." The soil takes substances 

 out of solution and retains them, so that a solution filtered through a soil layer 

 becomes less concentrated. The first investigator to direct his attention to 

 this phenomenon and to recognize its importance in agriculture was Bronner 

 (1836), who describes the following experiment. A bottle with a small opening 

 in the bottom is filled with fine sand or with half-dry, sifted garden-soil. Dark 

 ill-smelling manure extract is gradually poured into the bottle until the entire 

 soil-mass is saturated. The liquid issuing below is almost entirely odorless and 



9 This is partly the phenomenon now generally^termed adsorption.— Ed. 



