MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 239 



able changes caused in their medium by the competing organisms. In 

 the case of molds a sufficient degree of acidity is often produced in the 

 medium to retard or stop the growth of bacteria. This point is well 

 illustrated by the prominent development of molds in culture solutions 

 containing ammonium salts as the sole source of nitrogen. When the 

 ammonium salts are replaced by nitrates under certain conditions the 

 growth of molds is practically suppressed and bacteria come to the fore. 

 In the soil itself the rapid development of molds has been observed in 

 plots or pots receiving large additions of ammonium salts, a phenomenon 

 due to the acid residues of the ammonium salts. The nitrates, on the 

 other hand, give rise to alkaline residues since the "NO 3 " radical is used 

 up in greater proportion than the basic radical. Similarly, molds are 

 enabled to compete with bacteria when the proportion of sugar or of 

 starch to protein is increased, or where large amounts of fats are present. 



A-LGM. At times the influence of algae 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 favor thus the development of A zoto- 

 bacter 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 algae. 

 The advocates of bare fallows attribute the greater productivity of 

 fallowed land to the growth of algae, the accumulation of nitrogen by them 

 and to other changes affecting the soil bacteria. 



PROTOZOA. It has been known 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 of the Rothamsted 

 Experiment Station have maintained that these minute animals are 

 extremely important in that they maintain a certain bacterial equilibrium 

 in the soil. Their claim is mainly based on the fact that partially sterilized 

 soils (either by means of heat or antiseptics) soon come to contain enor- 

 mous numbers of bacteria. It is, therefore, assumed by them that this 

 abnormal increase is made possible by the destruction of the protozoa 

 that normally check the increase beyond a certain point. 



HIGHER PLANTS. Higher plants modify the soil as a culture medium 

 for bacteria in at least three ways. The root-hairs come in contact with 



