ARTIFICIAL FERTILIZERS AND SOIL MICROBES 223 



TABLE 55 



Influence of Phosphates and Sulfates on Formation of Carbon 

 Dioxide in Soil (after Fred and Hart) 



Carbon dioxide produced 

 Treatment during 6 days 



(comparative figures) 



Untreated 100 



0.1%K2HPO4 194 



0.1% (NH4)2S04 152 



0.1% Ca3(P04)2 123 



0.1% CaS04-2H20 105 



0.1%MgSO4 107 



0.1%K2SO4 101 



salt concentration as a result of their addition to the solution. 

 (4) The salts may change the reaction of the soil solution, making 

 it either more basic or acid as a result of physical-chemical action, 

 selective adsorption, or transformation of certain portions of the 

 compounds by soil organisms. (5) Certain catalytic effects may 

 be exerted, as in the stimulation produced by compounds contain- 

 ing such elements as manganese or radioactive compounds. (6) 

 Probably the most pronounced effects upon microbes exerted by the 

 addition of inorganic fertilizing substances are brought about indi- 

 rectly, namely, through higher plants, particularly where fertihz- 

 ing practices have been in operation for extended periods of time. 

 The response of plants to the fertilizers is transferred to the soil 

 population, as the plant residues are incorporated in the soil. 

 The resultant effects of these interwoven reactions is apparent 

 in the data given in Table 11. Changes in the physical conditions 

 of the soil accompany the various soil treatments. 



Certain fertilizers may affect specific groups of soil organisms to 

 a far greater extent than the soil population as a whole. This is 

 particularly the case where certain constituents of the fertilizers 

 may serve directly as foods for the organisms. The ammonia in 

 ammonium salts is used by the nitrifying bacteria as a source of 

 energy. Urea favors the development of specific urea-decompos- 

 ing bacteria which transform the urea into ammonia. Dicyano- 

 diamid, frequently an impurity of cyanamid, may even prove 

 injurious to various important soil bacteria. The numerous 

 organic compounds of nitrogen which are contained in fertilizers 



