742 



PRINCIPLES OP SOIL MICROBIOLOGY 



Soils dried gradually become more active than soils dried rapidly; 6 com- 

 plete drying results also in a greater stimulus than moderate drying. 

 Ritter suggested that this favorable influence of drying might be due to 

 a selective action upon the species of soil microorganisms. The greater 

 the period of time during which the soil is air dried, the more are the 

 activities of microorganisms stimulated, as indicated by numbers of 

 bacteria, evolution of carbon dioxide and increase in available 

 nitrogen (fig. 64). 



Drying of soil results in a decrease in bacterial numbers of 40 to 70 

 per cent, followed by a rapid increase, when the soil is moistened. 7 

 When a soil kept in pots becomes unfavorable for the growth of legu- 

 minous plants, it may be restored to normal condition by drying. The 

 protozoa (cysts) are not destroyed as a result of drying, 8,9 neither are 

 the fungi and actinomyces affected to any large extent. 



TABLE 82 

 Increase in total soluble salts due to drying 



SOIL LAYER 



Surface, to 8 inches 



Subsurface, 12 to 20 inches 

 Subsoil, 20 to 40 inches . . . 



Konig and associates 10 found that all soils except clays show a 

 small but definite increase in dialyzable materials, when heated 

 under diminished pressure. This is due to a change in the colloidal 

 condition of the soil, which causes the adsorbed materials to become 

 soluble. Air-drying of soil, under natural conditions, also causes a 

 partial destruction of the colloidal state, and, therefore, an increase 

 in the solubility of the nutrients held by the colloids. The influence 



6 Ritter, G. Das Trocknen der Erden. Centrbl. Bakt. II, 33: 116-143. 1912. 



7 Heinze, B. Bakteriologische Versuche. Landw. Jahrb., 65: 139-184. 1920. 



8 Goodey, T. Investigations on protozoa in relation to the factor limiting 

 bacterial activity in soil. Proc. Roy. Soc. B., 88: 437-456. 1915. 



9 Greig-Smith, R. Contributions to our knowledge of soil fertility. XII. 

 The action of toluene upon the soil protozoa. Proc. Linn. Soc. N. S. Wales, 

 39: 839-850. 1914. 



10 Konig, J., Hasenbaumer, J., and Glenk, K. tJber die Anwendung der Dialyse 

 und die Bestimmung der Oxydationskraft fur die Beurteilung des Bodens. 

 Landw. Vers. Sta., 79-80: 491-534. 1913. 



