320 AKNUAL, REPORT OF THE Off. Doc. 



land, among other benefits, assists in the decomposition of organic 

 matter. This is true only indirectly. The lime neutralizes the acid- 

 ity of the soil and renders it a more favorable medium for the develop- 

 ment of those bacteria which are the true agents in the decomposi- 

 tion. 



In determining the number of bacteria in soils it is necessary to 

 use a medium the reaction of which is such that the maximum num- 

 ber will develop. Thus from a given dilution of a soil with water 

 an average of seveaty-two colonies developed in neutral gelatin, 

 while with the presence of 0.25 per cent, of free alkali an average of 

 thirty-four colonies developed, and with .07 per cent, of free hy- 

 drochloric acid an average of only one colony developed. Increas- 

 ing quantities of citric acid added to the medium had the same 

 retarding action, showing that with a distinctly acid condition of the 

 medium but few soil bacteria would grow. 



Of the different species of bacteria isolated by the writer from 

 soils, none grew in a medium containing one-tenth of one per cent, 

 of free hydrochloric acid, and either not at all or only feebly in one 

 containing one-half of this acidity. All, however, grew in neutral 

 media or in those feebly alkaline. A marked excess of caustic al- 

 kalies in the medium, approaching .02 to .03 per cent., had a retarding 

 action on the growth of the bacteria, but where the less caustic 

 bases like lime were added a considerable excess proved favorable. 



The valuable results from lime added to neutral gelatin media is 

 shown in the following table, in which is given the number of colonies 

 developing in media containing different amounts of milk of lime, 

 seeded with the same quantity of a one-ten-thousandths dilution of 

 a soil infusion: 



Table II. 



G. 



2. o 



u 



o (1> 

 ^ o . 



Calcium Hydrate (Ca (OH),) Present In Medium. 



n'O'o 



.44 grams per 100 c. c. 

 .OS grams per 100 c. c. 

 .12 grams per 100 c. c. 

 .16 grams per 100 c. c, 



S7 

 76 



>i 



