MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 305 



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 Actimonyces form 

 40 to 80 per cent of the total micro-organic 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^^1- 

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

 meadow soil. As to the activities of Actinomyces in the soil, Beyer- 

 inck has shown that the Act. chromogenus produces an oxidizing sub- 

 stance, quinon (C6H4O2) which may play an important part in the 

 oxidation of organic matter in the soil. Munter, Krainsky and Scales 

 have demonstrated that many Actinomyces are able to decompose cellu- 

 lose in the soil, and that in some instances this ability is very marked. 

 Krainsky records that soil Actinomyces need very little nitrogen for 

 their life activities, and that they can get it from any available source. 

 If nitrates are present, these are reduced first to nitrites, and then 

 utiUzed. Waksman and Curtis working with soil sterilized by steam, 

 did not find any great accumulation of ammonia through the activities 

 of Actinomyces, although different species seemed to show marked varia- 

 tion in their power to accumulate ammonia. 



Alg^. — At times the influence of algse in changing the character of 

 the soil as a culture medium for bacteria is quite considerable. As 

 chlorophyl-bearing organisms they are enabled to manufacture sugar 

 and starch with the aid of sunlight, and to favor thus the development 

 of Azotobaeter and of other microorganisms dependent for their energy 

 on the organic matter in the soil. Investigators both in France and 

 in Germany have found that the fixation of nitrogen in sand used for 

 pot culture experiments occurs in the surface layer possessing a growth 

 of algse. The advocates of bare fallows attribute the greater pro- 

 ductivity of fallowed land to the growth of algse, the accumulation of 

 nitrogen through their influence and to other changes affecting the soil 

 bacteria. 



Protozoa. — It has been shown for a long time that certain species 

 of protozoa are common in soils and that their food consists of bacteria. 

 To what extent protozoa play a part in soil fertility has not yet been 

 fully explained, even though Russell and Hutchinson and of the 

 Rothamsted Experiment Station have maintained that these^minute 

 animals are extremely important in that they maintain a certain bac- 

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