THE MICRO-ORGANIC POPULATION OF THE SOIL 259 



apparently effect little or no decomposition of cellulose (292^). 

 The chemical changes involved are unknown, but it is unlikely 

 that the reaction would proceed by itself in the soil ; in all 

 probability the soil bacteria would participate. 



When ample supplies of carbohydrate (including cellulose) 

 are present, the most economical nitrogen nutrients are the 

 amino-acids (Czapek). 1 In absence of sufficient supplies of 

 carbohydrate, Waksman shows that the fungi can obtain both 

 energy and nitrogen from soil protein compounds, but in this 

 case more degradation occurs than is necessary to supply the 

 nitrogen required, and consequently ammonia remains over 

 (292^). Under suitable conditions certain fungi (e.g. Tricho- 

 derma Koningi) can produce ammonia more rapidly than 

 bacteria (McLean and Wilson, 1 88). Fungi have no power 

 of oxidising ammonia to nitrates. They can, however, readily 

 assimilate both ammonia and nitrates, and they have been 

 supposed by Ehrenberg (93^) to cause considerable locking 

 up of these compounds in the soil (see p. 210). 



It was at one time thought that fungi could fix gaseous 

 nitrogen, but Duggar and Davis, 2 in a careful series of ex- 

 periments, were unable to obtain evidence that any fungus 

 excepting Phoma Betae possessed this property. 3 



It is impossible on present knowledge to assess the im- 

 portance of fungi in soil fertility. The decomposition of cellu- 

 lose is undoubtedly beneficial and probably justifies the old 

 view that fungi are the humus formers of the soil (Ramann, 

 233) : on the other hand, the locking up of nitrogen compounds 

 is a disadvantage which, however, would be considerably 

 counterbalanced if the substance of the mycelium were readily 

 decomposable by bacteria on the death of the organism. 



3. Actinomyces. 



This group of organisms, sometimes included in the fungi 

 and sometimes in the bacteria, is of frequent occurrence in the 



1 Beit. Chem. Physiol. u. Path., 1901-2, I, 538 ; 2, 557 ; 3, 47. 



2 Ann. Mo. Bot. Gard., 1916, 3, 413-437. 



3 According to Ternetz (Jahrb. f. wiss. Bot., 1907, 44, 353-408) Phoma 

 radicis can also assimilate gaseous nitrogen. 



17* 



