622 PRINCIPLES OF SOIL MICROBIOLOGY 



of the soil is filled with water. In the case of light sandy soils this con- 

 dition obtains when the water content is about 8 to 10 per cent of its 

 weight and, in heavy silt or clay soils, when the water content is 16 to 

 20 per cent or more. As the soil dries out the activities of the organ- 

 isms are gradually reduced ; the larger forms, like the fungi, suffer most 

 while the smaller and more resistant forms, like the actinomyces and 

 spores of bacteria, 7 suffer least. Excessive moisture may prove un- 

 favorable to aerobic microorganisms by limiting the supply of oxygen, 

 while anaerobic bacteria are favored 



Since the microorganisms live largely upon the colloidal film surround- 

 ing the inorganic soil particles, the lower the colloidal content of a 

 soil the smaller is the amount of water necessary to bring it into a 

 condition favorable for the activities of microorganisms. An excess 

 above the optimum amount of moisture will prove injurious to the ac- 

 tivities of the aerobic organisms. The heavier the soil, the higher must 

 be the water content to bring about equal decomposition of organic 

 nitrogenous substances. Using the liberation of ammonia and nitrate 

 from horn meal as an index of the activities of microorganisms, Miinter 8 

 demonstrated (table 64) that in a sandy soil decomposition did not vary 

 within the moisture range of 6 to 18 per cent (decreasing at greater 

 moisture contents). The rapidity of decomposition reached its high- 

 est point during the first three weeks in the sandy soil. In a loam 

 soil, decomposition was twice as great at 16 per cent moisture as at 

 8 per cent, and was constant between 16 and 24 per cent. The in- 

 fluence of the moisture content was still more marked in a clay soil. 

 Only 0.63 per cent of the material was decomposed in three weeks at 

 8 per cent moisture, while at 18 and 28 per cent moisture and in the 

 same period of time, 30.80 and 36.36 per cent of the material was de- 

 composed respectively. After twelve weeks, 13.35, 63.21 and 57.94 

 per cent of the material was decomposed at the respective moisture 

 contents. 



The effect of moisture is also shown in the composition of the soil 

 flora and the nature of the processes occurring in the soil will conse- 

 quently be materially affected. The decomposition of cellulose can 

 readily illustrate this phenomenon. In the presence of sufficient avail- 

 able nitrogen and minerals, cellulose is decomposed at a medium mois- 



7 Hoffmann, C. Relation of soil bacteria to evaporation. Wis. Agr. Exp. 

 Sta. Res. Bui. 23. 1912. 



8 Miinter, F. Untersuchungen iiber chemische und bakteriologische Umsetz- 

 ungen im Boden. Landw. Jahrb., 55: 62-138. 1920. 



