266 MICROBIOLOGY OF 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 

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

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

 various times and by different 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 itself, as is evidenced by the rapid formation of nitrates, large 

 masses of bacterial substance are constantly being formed and disin- 

 tegrated. 



AVAILABILITY or BACTERIAL MATTER. Substances of microorganic 

 origin are decomposed more or less rapidly, according to their composition. 

 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 Azotobacter 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 compounds may be withdrawn from circulation by microorganisms that 

 use these as building material. Under extreme conditions microorgan- 

 isms may become serious competitors of higher plants for available 

 nitrogen food. 



Manure stored in heaps not infrequently deteriorates in quality, 

 even when losses by leaching are excluded This deterioration is largely 

 due to the change of the water-soluble ammonia and amino-compounds 

 into insoluble protein substances. While the extent of the change into 

 protein compounds is variable it may range from less than a tenth of the 



