362 MICROBIOLOGY OF SOIL 



Nitrogen- fixation. Experiments on nitrogen-fixation by fungi were 

 carried on by Jodin as early as 1862. He observed a rich fungus growth 

 on nitrogen-free media, supplied with sugar, tartaric acid, or glycerin. 

 Berthelot, Saida, Ternetz, and others also reported- fixation of atmos- 

 pheric nitrogen through the activities of fungi, such as Aspergillus 

 niger, Alter naria tennis and several species of Monilia, Penicillium, 

 Mucorini and others. But other investigators, among them Wino- 

 gradsky, Czapek and Heinze, were unable to confirm these observa- 

 tions. The careful work of Goddard has also given negative results. 

 Duggar and Davis, eliminating all possible errors involved in this 

 study, could not demonstrate any nitrogen fixation for Aspergillus 

 niger, Penicillium digitatum, Penicillium expansum, and other fungi, 

 some of which commonly occur in the soil. Hence, nitrogen fixation 

 by soil fungi is at best of very little importance, since even in the case 

 of positive fixation the amounts are very slight. 



Nitrogen Utilization --The molds assimilate readily available 

 nitrogen compounds in the presence of available carbohydrates. In 

 this respect they may readily compete with higher plants in using up 

 the ammonia and nitrates formed in the soil by bacteria. 



Cellulose Decomposition. The destruction of cellulose in the soil is 

 due to a large extent, to the activities of soil fungi, as has been demon- 

 strated by several investigators. Cellulose decomposition by fungi was 

 first observed in the study of plant diseases. Van Iterson used filter 

 paper for the isolation of fungi, by exposing this medium to the air for 

 twelve hours. Thirty-five species of fungi were isolated thus proving 

 that a large number of cellulose-destroying fungi may be present in 

 the air. Appel found that certain species of Fusarium destroyed in 

 fourteen days 80 per cent of the filter paper used. Marshall Ward 

 and others recorded that a number of fungi are economically impor- 

 tant as wood-destroyers. Spores of a pure culture of Penicillium sown 

 on sterile blocks of spruce wood, germinated and grew normally. 

 Sections of the wood showed that the hyphse had entered the starch- 

 bearing cells of the medullary rays of the sapwood and consumed the 

 whole of the starch. MacBeth and Scales found that when the medium 

 is slightly alkaline, certain aerobic bacteria will play the principal 

 role in the destruction of cellulose. When the medium is acid, molds 

 and higher fungi become the active agents of destruction. They also 

 found that the cellulose-destroying forms multiply with great rapidity 



