238 MICROBIOLOGY OF SOIL. 



lating suitable culture solutions with particles of calcium carbonate picked 

 out from the soil. Evidently these organisms were present in much 

 greater abundance on these particles than on others of non-calcareous 

 origin. Indeed, he occasionally obtained in this manner Azotobacter 

 membranes that constituted almost pure cultures. The more general 

 significance of this relation is apparent when it is remembered that nitro 

 bacteria are particularly favored by magnesium carbonate; tubercle 

 bacteria by gypsum and calcium carbonate; Azotobacter by cal- 

 cium phosphate and calcium carbonate; photo-bacteria by sodium 

 chloride, etc. 



Considerable as must be the local differences in any one soil, they are 

 undoubtedly even more pronounced when different soils are compared. 

 Extreme conditions are met with in certain irrigated soils in which a 

 marked concentration of salts occurs. In so far as crop production is 

 concerned, it is stated by Hilgard that the upper limit is practically reached 

 when the concentration of soluble salts in the irrigation water is about 

 4-55 g- (7 g r -) P er gallon. Nevertheless, in Egypt and the Sahara 

 region irrigation water is occasionally used that contains more than 13 g. 

 (200 gr.) of soluble salts per gallon. Further differences are introduced 

 by the quality of these salts, e.g., the proportion of sodium sulphate, 

 magnesium sulphate, sodium chloride, sodium carbonate, etc. Again, 

 instances are on record, as in the investigations of Headden in Colorado 

 and California, where the concentration of nitrates in the soil water is 

 so great as to kill even relatively resistant plants like alfalfa. It is to be 

 shown by future investigations what the effect of the concentration and 

 composition of such salts may be on the soil bacteria. 



In humid soils conditions are less extreme, yet even here the variable 

 concentration and composition of the soil solution are of direct moment 

 for the different microorganisms. Granite soils, for instance, are fairly 

 well supplied with phosphoric acid and abundantly with potash, but 

 when hornblende is lacking they are apt to be deficient in lime. Ill- 

 ventilated clay soils may contain reduction products of iron salts, while 

 green sand, chalk, slate, shale, sandstone and other soils may have their 

 individual peculiarities from the standpoint of a culture medium. 



BIOLOGICAL FACTORS. 



MOLDS. Soil bacteria must not only compete with other micro- 

 organisms and higher plants for their food, but must contend with unf avor- 



