22 STATE BOARD OF AGRICULTURE. 



dominates; at another, with the changing reaction, a new set of life- 

 operates, owing to the exceeding susceptibility' of bacteria to a change 

 of reaction in environi.ng medium. This is an agent which should 

 receive attention in securing proper or improper conditions for the de- 

 velopment of certain types of micro-organisms. 



Two classes of soil bacteria stand out prominently in the microbial 

 consideration of the soil — not that they are alone in executing important 

 physiological functions in connection with the soil, but because they 

 have opened up a new field of practical research and have answered 

 some very important economic questions in the short time they have 

 been under the eye of investigators. These two classes are the Nitrify- 

 ing and Denitrif^'ing Bacteria: Under the former class may also be 

 considei-ed those which utilize and fix free nitrogen. 



It is, I believe, an accepted fact that nitrogen in the form of organic 

 substances and as ammonia compounds is not the best combination for 

 plant assimilation. For long years the process of changing ammonia 

 compounds into nitrates, the most suitable form of nitrogen for plant 

 assimilation was unknown until in 1877 !r?chlossing and Muntz ventured 

 the statement that nitrification was the result of vital acidity, for the 

 operation was almost stagnant at 41 degrees F., became apparent at 54 

 degrees F., and was most active at 97 degrees F. and gradually grows 

 weaker as the temperature rises, and at 131 degrees F. the action ceases 

 entirely. Moisture increases action if it does not prevent perfect aera- 

 tion. A slightly alkaline reaction is also favorable to this class of bac- 

 teria. Sometimes the process stops with imperfect nitrification if the 

 temperature is low and the supply of oxygen is short. 



It is a recognized fact that an abundance of organic matter does not 

 favor the growth of this nitrifying micro-organism. Winogradsky has 

 given us two distinct classes of these micro-organisms, those which con- 

 vert ammonia into nitrous acid or nitrites, partial oxidation, and those 

 which do not act upon ammonia but attack the nitrous acid or nitrites 

 formed by the former class and convert them into nitric acid and 

 nitrates. 



(N H,),O+30,=NA+4HA 



The above bacteria are spoken of in general as the nitroso and nitro 

 bacteria. The nitroso bacteria are easily affected by dessication. 



In the formation of saltpetre, the action of these micro-organisms 

 plainly indicates their role in nature. The wall saltpetre which appears 

 frequently on the walls of stables and closets (especially in troincal 

 climates), results from the action of these germs upon the ammonia salt 

 formed from urea. Calcium and alkali are in the wall and oxygen is 

 present in abundance. The conditions for growth are favorable and 

 saltpetre is formed. The masonry suffers accordingly from the corrod- 

 ing action of the acids formed. 



Many of the large deposits of saltpetre, especially the soda saltpetre 

 of South America, have been accomplished through the gradual nitrifica- 

 tion of the organic matter accumulated there in large quantities through 

 the instrumentality of birds. 



This very natural means may be simulated by selecting a proper 

 impervious clay bed and mixing with, the organic matter chalky earth. 

 Brush is placed in the heap to allow free circulation of air. The calcium. 



