CHEMICAL CHANGES 2O/ 



Number of bacteria 

 Inches in one gram soil 1 



2 1 ,330,000 



4 1,500,000 



6 1,900,000 



8 260,000 



10 265,000 



12 124,000 



The number of bacteria so counted appears to have no direct 

 relation to the ammonifying, nitrifying, or denitrifying power of 

 the soil. Important groups of soil bacteria, such as the nitrify- 

 ing, do not develop colonies at all. The bacterial count appears 

 to be more closely related to the organic matter content than to 

 anything else. If it is desired to study the bacteria further, the 

 desired medium is inoculated with a portion of a colony grown on 

 the plate. 



Another method- of estimating the number of bacteria consists 

 in inoculating a series of suitable media from different dilutions 

 of the soil, say equal to i mg., o.i mg., .01 mg., and .001 mg. of 

 soil. Suppose that with ten tubes inoculated from .01 mg. soil, 

 7 nitrify and 3 do not. Then we estimate that 7 bacteria were 

 present in 10 times .01 mg. soil, or 7,000 are present in a gram. 

 The solution for inoculating is prepared by shaking the soil with 

 sterilized water as described above. 



Kinds of Soil Bacteria. 3 The general tendency of bacterial 

 action in the soil is along well denned lines, although reverse 

 changes occur and complicate the process. Organic matter, by 

 decay or putrefaction, is finally converted into carbon dioxide, 

 water, ammonia or nitrates, and mineral salts. Some soil bacteria 

 produce organic matter from hydrogen or marsh gas and carbon 

 dioxide, or use other inorganic materials (sulphur, or sulphides) 

 as a source of energy, but, in spite of this, the general movement 

 is as indicated. The general movement of organic nitrogen is 

 towards the form of nitrates, through ammonia, in spite of the 

 presence of bacteria which act in the reverse direction and con- 



1 Chester, Delaware Bulletin No. 65. 



- Wiley, Principles and Practice, Agr. Chem. Anal., Vol. i. 



3 Kansas Bulletin 117 ; New Jersey Bulletin 40. 



