262 PRINCIPLES OF SOIL MICROBIOLOGY 



turn begins to decompose, the native grasses again invade the soil and 

 develop luxuriantly, because of the abundant supply of readily available 

 nitrogenous materials. 82 



Influence of reaction upon the growth of fungi. A detailed discussion 

 of the various phases of the physiology of the fungi is out of place here, 

 since it is given in the standard texts on plant physiology. Attention 

 may be called to only some physiological properties of fungi which are 

 important from the point of view of the growth and activities of these 

 organisms in the soil. 



It has been pointed out above that acid soil conditions favor the de- 

 velopment of fungi. It need not, however, be construed that fungi 

 grow only under acid conditions or even that they have their optimum 

 growth at distinctly acid reactions; they have a rather wide range of 

 reaction optimum, as shown in table 17. 83 



Fungi are thus shown to be much more resistant to acidity than the 

 other groups of soil microorganisms. On the alkaline side, however, 

 they are not more resistant than the bacteria. On the acid side there 

 will, therefore, be no competition for the available plant food. 



The reaction of the medium has an important influence upon the 

 germination of fungus spores 84 and upon the respiration of the organ- 

 isms. 85 Increasing acidity favorably influences the germination of the 

 spores; a maximum of germination is exhibited by the majority of the 

 spores tested at a pH of 3.0 to 4.0. Inhibition of germination is evi- 

 denced only at pH 1.5 to 2.5; the alkaline limits vary with the organism 

 and with the medium. 



Fungi as a rule modify greatly the reaction of the medium, by the 

 production of organic acids from available carbohydrates, by the 

 consumption of organic acids (leaving the medium less acid), or by the 

 formation of ammonia from proteins. 86 



82 Shanz and Piemeisel, 1917 (p. 282). 



83 Johnson, H. W. Relationships between hydrogen ion, hydroxyl ion and salt 

 concentrations and the growth of seven soil molds. Iowa Agr. Exp. Sta. Res. 

 Bui. 76. 1923. 



84 Webb, R. W. Germination of the spores of certain fungi in relation to hydro- 

 gen-ion concentration. Ann. Mo. Bot. Gard., 8: 283-341. 1921. 



84 Molliard, M. Influence de la r6action du milieu sur la respiration du 

 Sterigmatocystis nigra. Compt. Rend. Soc. Biol., 83: 50-51. 1920. 



86 Butkewitsch, Wl. Umwandlung der Eiweisstoffe durch die niedere Pilze. 

 Jahrb. wiss. Bot., 38: 147-240. 1903. 



