386 MICROBIOLOGY OF SOIL 



tested proved to be ammonifiers. Similarly, a number of species tested 

 by the writer, among them B. coli, B. choleras suis, B. (Proteus) vulgaris, 

 B. subtilis, B. megatherium, etc., all produced ammonia in meat infusions. 

 A mass of additional data, accumulated by different investigators, 

 furnishes further proof that ammonia production is a common function 

 of soil bacteria. 



The more prominent ammonifiers, including members of the B. 

 subtilis group and certain strep tothrices, are numerically important in 

 all arable soils. Their numbers are affected, however, by the amount 

 and composition of the soil humus. It has been found, for instance, 

 that additions of straw and of strawy manure increase markedly the 

 numbers of B. subtilis and of other members of the group. An increase 

 in the numbers of certain ammonifiers is caused also by additions of 

 lime or of green manure. For example, in experiments carried out by 

 Lipman and his associates portions of fertile soil inoculated with B. 

 mycoides were found to contain, a month later, 2,000,000 of bacteria per 

 g. of soil. In similar soil portions that had also received additions 

 of grass the number was twice as great. 



More recent investigations (Temple, Waksman) have shown that 

 ammonification tests are of little value in determining the nature of the 

 microbial soil flora, since the rate of ammonia production is largely 

 controlled by the soil medium. If the soil is suitable, there will usually 

 be found enough microorganisms capable of changing the protein nitro- 

 gen into ammonia. Temple has suggested the use of ammonification 

 as a test for soil fitness. 



Ammonification should be studied not only in the light of decompo- 

 sition proteins and protein derivatives in the soil, but also from the 

 point of view of energy sources in the soil. Microorganisms can use 

 both carbohydrates and proteins as sources of energy. There is a great 

 deal more of the carbon compounds oxidized to supply the required 

 energy than there is nitrogen consumed in the normal metabolism of 

 the microbe. The addition to any soil of definite amounts of protein 

 with varying amounts of available carbohydrates will lead to the 

 following results: ammonia will be accumulated in the soil to which the 

 protein alone has been added, the amounts of ammonia increasing with 

 the period of incubation up to a certain point; where only small quanti- 

 ties of carbohydrates have been added there will be at first no ammonia 

 produced, but soon the ammonia will begin to accumulate, so that the 

 actual quantities of ammonia may become in a few days even greater 



