Effect of Changing Conditions 343 



nitrifying organisms, the nonsymbiotic nitrogen-fixing forms, and 

 some of the celkilose-decomposing organisms, are highly sensitive 

 to acidity and will nsnally fail to grow at a pH less than 6.0; other 

 bacteria, however, snch as some of the snlfur-oxidizing forms and 

 the facultative anaerobic bacteria, seem to be able to withstand 

 considerable acid concentration. The same is trne of the response 

 of different bacteria to the addition of specific organic substances, 

 to a change in soil aeration, and to other soil changes. The fungi 

 also show considerable variation in response to changing soil or 

 nutrient conditions: some are more sensitive than others to increasing 

 acidity or to diminished aeration; some attack the water-soluble sub- 

 stances more readily, others attack by preference the cellulose, and 

 still others prefer the lignins and the proteins. There is also consid- 

 erable \ariation in response to changes in environment and in food 

 supply among the various actinomycetes and protozoa. 



The stimulation of specific groups of organisms, whereby the nor- 

 mal microbiological equilibrium in the soil is interrupted and one 

 particular type or group, previously present only in limited numbers 

 or even in a latent state, becomes predominant, is due to the speciali- 

 zation of various microorganisms. Usually the energy source intro- 

 duced into the soil can be utilized only by the particular organism 

 under specific soil conditions, or the soil is modified to such a degree 

 as to favor the development of one organism in preference to others. 

 Winogradsky distinguished between the "autochthonous" bacteria, 

 or those organisms which attack primarily the organic substances of 

 the soil, and the "zymogenic" forms, or those which develop rapidly 

 as a result of addition of fresh organic substances. 



When complex plant and animal materials are added to the soil, 

 the stimulating effect upon the development of various bacteria or 

 fungi is difficult to analyze, because of the changing nature of the 

 organisms with the progress of the decomposition process. The 

 chemical composition of the material added, which varies with the 

 nature of the material and the degree of its maturity in the case of 

 a plant substance, the chemical and physical soil conditions, and the 

 environmental factors, all modify the microbiological response to 

 such treatments. As a plant matures, it contains smaller quantities 

 of water-soluble substances, such as sugars and amino acids, and it 

 becomes poorer in nitrogen and minerals and richer in cellulose and 

 lignin. The addition to the soil of residues of a young plant will 

 favor an abundant development of many bacteria, including the 

 lactic acid forms, which attack the sugars and other water-soluble 



