MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 359 



reactions are more or less localized and diffusion slow, there are, un- 

 doubtedly, in the soil minute zones where individual species are more 

 prominent than they are in others. For example, Heinze has found it 

 convenient to isolate Azotobacter by inoculating suitable culture solu- 

 tions 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 calcium 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 com- 

 pared. 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 irriga- 

 tion water is about 4.55 g. (70 gr.) per gallon. Nevertheless, in Egypt 

 and the Sahara region irrigation water is occasionally used that con- 

 tains more than 13 g. (200 gr.) of soluble salts per gallon. Further 

 differences are introduced by the quality of these salts, e.g., the pro- 

 portion of sodium sulphate, magnesium sulphate, sodium chloride, 

 sodium carbonate, etc. Again, instances are on record, as in the investi- 

 gations 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. 



