MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 363 



in alkaline soils when cellulose in the form of filter paper is added. The 

 power to destroy cellulose is reported for a number of species of Peni- 

 cillia, A spergilli, Trichoderma and other organisms which belong to the 

 common soil forms. Though the fungi may play an important part 

 as cellulose destroyers also in alkaline soils, in acid soils where the 

 activity of bacteria is greatly inhibited, fungi probably play a pre- 

 dominant role. This fact led Marshall to conclude in 1893 that 

 fungi take an active part in the mineralization of the organic matter 

 in acid humus soils. 



Mycorrhiza. Apart from the so-called soil fungi, there exists another 

 group known as mycorrhizal fungi. These live symbiotically on the 

 roots of the higher plants. Many roots of forest trees, when examined 

 carefully, show that there is a union between the mycelium of certain 

 fungi, usually belonging to the fleshy fungi, and the root of the plant. 

 This union is called a " mycorrhiza" The fine filaments of the fungus 

 enter the cells of the root. These organisms were thought at first 

 to supply the roots with water and soluble plant food from the soil. 

 The power to fix atmospheric nitrogen has been ascribed to these organ- 

 isms by several investigators. But aside from these useful so-called 

 endotrophic Mycorrhiza^ there are also the ectotrophic Mycorrhiza 

 which probably live only parasitically upon the roots of plants. 



Actinomyces. The study of soil Adinomyces is nearly all of very 

 recent origin. Several years ago but two soil Actinomyces had been 

 definitely described, viz., Act. albus and Act. chromogenus. The work 

 of Krainsky, of Conn and of Waksman and Curtis has demonstrated 

 that Actinomyces are widely scattered in cultivated soils. The last- 

 named investigators have shown that while the absolute numbers of 

 Actinomyces decrease with depth of soil, their relative numbers are 

 materially increased so that if at a depth of 25 mm. (i inch) there 

 are only 6 to 10 per cent of Actinomyces and 82 to 93 per cent of 

 bacteria, at a depth of 750 mm. (30 inches) the Actinomyces form 

 40 to 80 per cent of the total microorganic flora of the soil. The 

 numbers of Actinomyces in the surface soil vary greatly with the types 

 of soil and abundance of plant food. In one instance 1,300,000 A cti- 

 nomyces were found in a total of 15,000,000 bacteria per g. of rich 

 meadow soil. The actinomycetes are present in the soil both in the 

 form of spores and vegetative mycelium. The same species have been 

 isolated from North America, Canada, Hawaiian Islands and newly 



