DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 325 



It appears, therefore, that ammonia production in soils rises or falls 

 as the rainfall or irrigation is increased or decreased, or as the soil water 

 is more or less thoroughly conserved by proper methods of tillage. In 

 the same way, seasons of high temperature favor ammonification while 

 seasons of low temperatures discourage it. This point is well illustrated 

 by the observations of Marchal that at 0° to 5° only traces of ammonia 

 were formed in his culture solutions; that at 20° ammonia production 

 was quite marked, and that at 30° the maximum was reached. More- 

 over, apart from the seasonal variations in any one locality, there is a 

 wide range in ammonia production, as we pass from the torrid to the 

 temperate and from the latter to the frigid zones. 



Species and Numbers. — Ammonia production is a function common 

 to most soil bacteria. Already in the earlier experiments of Marchal, 

 seventeen out of the thirty-one species tested were found capable of 

 producing ammonia. Prominent among these ammonifiers were B. 

 mycoides, B. {Proteus) vulgaris, B. mesentericus vulgatus, B. janthinus, 

 and B. subtilis. Of a considerable number of soil bacteria tested by 

 Chester all but one were observed to produce ammonia. In Gage's 

 experiments with sewage bacteria, seventeen out of twenty species 

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

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

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

 A mass of additional data, accumulated by different investigators, 

 furnish 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 streptothrices, are numerically important in 

 aU 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. 



