MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 361 



The plate count is not an index of the activity of the molds in the 

 soil, but merely indicates the number of spores present. An organism 

 that produces a large number of spores, particularly when these resist 

 drying, will be found by the plate method in much larger numbers than 

 another organism which, though causing a greater degree of chemical 

 change in the soil, produces fewer and less resistant spores. A method 

 was therefore suggested by Waksman which would permit the separa- 

 tion of molds w r hich produce mycelia abundantly and readily from 

 those that develop in soils only under special conditions of moisture and 

 temperature. The method consists of planting a lump of soil into the 

 agar in a Petri dish and observing the development of the mycelia in 

 the soil. It is obvious that the mycelia develop more readily than 

 the spores and grow out into the agar. In this manner we may separate 

 the organisms which actually live in the soil. Moreover, the fact that 

 the same species of molds have been isolated from soils in different parts 

 of the world would tend to indicate that, when conditions become favor- 

 able, molds vegetate in the soil, although at other times they may exist 

 there only in the form of spores. 



Ammonification. Miintz and Coudon, and after them Marchal, 

 working with pure cultures, proved conclusively that fungi decompose 

 organic matter and cause an accumulation of ammonia in the soil. 

 Wilson and McLean found that the forms of Monilia&re the most active 

 ammonifiers among the several groups of organisms studied, while the 

 Aspergilli showed the least ammonifying power. More recent work 

 has confirmed the earlier findings and has proved that fungi may 

 play an active part in the decomposition of organic matter, and the 

 accumulation of ammonia. 



The molds have been shown to be more rapid ammonifiers than the 

 bacteria and actinomycetes. Species of Trichoderma have been found 

 by Waksman and Cook to transform more than 60 per cent of the 

 nitrogen of dried blood and cottonseed meal into ammonia in a period 

 of seven to twelve days. This comparatively rapid ammonia produc- 

 tion is readily explained in view of the recent information on the energy 

 requirements of microorganisms. The molds decompose organic 

 matter more readily than do the actinomycetes and many bacteria. 

 They consume a great deal more energy and therefore liberate more 

 nitrogen as a waste product in the form of ammonia. 



