EXPERIMENT STATION BULLETINS. 451 



THE BACTERIAL ACTIVITY IN SOIL AS A FUNCTION OF GRAIN 

 SIZE AND MOISTURE CONTENT. 



Technical Bulletin No. 16. 



BY OTTO RAHN. 



INTRODUCTION. 



The discussion concerning the best method for determining the micro- 

 bial efficiency in soils has entered a new stage since Stevens and Withers* 

 demonstrated by several series of experiments with different types of 

 soils that there is no direct relation between the microbial activity in the 

 soil and in the extract of this soil or in a similar nutrient solution 

 Stevens and Withers are not the first ones to advocate the use of soil 

 rather than liquids for soil bacteria. Practically all work coming from 

 tlie soil-bacteriological laboratory in Goettingen has been done with soil 

 in its natural condition. But we owe to Stevens and Withers the experi- 

 mental and conclusive proof that biochemical changes in soil and in 

 solutions inoculated with soil give not the same and not even comparable 

 data. There is no definite relation between the two, and no constant 

 factor will allow of the computation of one datum from the other. 



The object of this paper is the study of the physical factors in the soil 

 causing a different development of micro-organisms in soil and solution. 

 Three soil properties can be predicted a priori which are probably of 

 noticeable influence: , 



1. The water content. 



2. The actual soil surface giving opportunity for absorption ; and 



3. The surface of the soil solution giving opportunity for the ex- 

 change of gases. 



That physical soil properties have a great influence is, of course, known 

 through many experiments. However, the relations have never been 

 studied systematically. For a systematic investigation of the influence of 

 physical soil properties, it is necessary, above all, to exclude chemical 

 influences. This can be accomplished with certainty only if the soil 

 is substituted by an absolutely insoluble material, such as quartz sand. 

 It is further necessary to work with pure cultures, to avoid uncon- 

 trollable complications. 



While pure cultures and quartz sand have been used by several bac- 

 teriologists, this paper varies in one way distinctly from the general 



♦Stevens and Withers: Studies in Soil Bacteriology. 

 I. [Centralbl. f. Bakt. II Abt. Bd. 23, p. 355. 

 II. " " Bd. 23, p. 776. 



III. " " Bd.l25,'p. 64. 



IV. " « Bd.^27.fp. 169. 



