33^ MICROBIOLOGY OF SOIL 



as analytical in that protein, carbohydrates and fats are split into more 

 simple compounds. At the same time, the microorganisms concerned 

 in the decomposition processes multiply very rapidly and fashion the 

 complex compounds of their cell-substance put of the simple cleavage 

 products in their medium. In other words, analytical and synthetical 

 reactions proceed hand in hand in the soil. 



While it is not definitely known how large a proportion of the soil 

 humus consists of the dead and living cells of microorganisms. there 

 is a mass of indirect evidence to show that these cells form a very con- 

 siderable proportion of the total quantity of organic substances in the 

 soU. For instance, it has been demonstrated that a large proportion of 

 the dry matter of solid animal fpces may consist of bacterial cells. At 

 various times and by difiEerent investigators the proportion of bacterial 

 substance has been estimated at from 5 to 20 per cent or more of the 

 total dry weight of faeces. A heavy application of barnyard manure 

 may introduce, therefore, several hundred pounds of bacterial cells per 

 acre of soil. Moreover, because of the extensive changes in the soil 

 humus itseU, as is evidenced by the rapid formation of nitrates, large 

 masses of bacterial substances are constantly being formed and dis- 

 integrated. 



Availability or Bacterial Matter. — Substances of micrborganic 

 origin are decomposed more or less rapidly, according to their com- 

 position. The extent of transformation under favorable conditions is 

 indicated by an experiment performed by Beyerinck and van Delden, 

 in which 50 per cent of the nitrogen in Azobacter cells was transformed 

 into nitrate in seven weeks. On the other hand, the humus of peat and 

 muck soils, or that of worn-out soils, may contain microorganic residues 

 of so inert a character as to yield but little available nitrogen to 

 crops. 



Transformation of Peptone, Ammonia and Nitrate Nitrogen. 

 — The cleavage of protein compounds into peptones, amino-acids and 

 ammonia, and the oxidation of the latter into nitrites and nitrates, may 

 be properly included among analytical reactions. It should not be 

 forgotten, however, that in the accompanying synthetical reactions the 

 compounds just mentioned may be transformed back into complex 

 proteins. It happens, thus, that large quantities of the available 

 nitrogen compounds may be withdrawn from circulation by micro- 

 organisms that use these as jjuilding material. Under extreme con- 



