MICROBIOLOGY OF SOIL 



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 substances are constantly being formed and dis- 

 integrated. 



AVAILABILITY OF BACTERIAL MATTER. Substances of microorganic 

 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 a vailable_ 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 nitrogen compounds may be withdrawn from circulation by micro- 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 building material. Under extreme con- 

 ditions microorganisms 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 

 water soluble material to more than three-quarters or four-fifths of it. 

 Also in the soil the same processes take place, but not so intensively. A 



