EXPERIMENT STATION BULLETINS. 489 



in brief, that the climatic conditions and their consequence as change in 

 moisture and ventilation, are not included. 



While the data of this paper must be translated to field conditions, 

 they apply directly to such tests of bacterial eflSciency as the Remy or 

 the Stevens method. 



VI. CONCLUSIONS. 



For the comparison of bacterial efficiency in soil, and for the study 

 of the influence of soil upon microorganisms, it is necessary to use equal 

 volumes of soil solution as the basis of comparison. 



The amount of oxygen that diffuses into a solution under ordinary 

 laboratory conditions, i. e. in a test tube or flask, is by no means sufficient 

 to allow maximum growth of aerobic bacteria. The experiments with B. 

 mycoides, Bad. aceti and Azotobacter show that the development in- 

 creases with decreasing thickness of the layer of moisture, the maximum 

 being reached at a thickness of 10 — 20 fx. Experiments concerning the 

 oxygen requirements and oxygen tolerance of bacteria should be car- 

 ried on with very thin liquid films, as they are found in a coarse sand 

 with 5 to 10% moisture. 



If the moisture film becomes less than 10 /x in thickness, the develop- 

 ment of bacteria is retarded, because though the oxygen supply is abund- 

 ant, the diffusion of food to the cells and the diffusion of metabolic 

 products from the cells is not sufficient to allow the fastest metabolism. 

 In this case, the growth will be slower, but the endpoint of decompo- 

 sition will be the same if the moisture film is thick enough to ]iermit a 

 complete though slow exchange of food and products through the whole 

 soil solution. The rate of decomposition in such soils depends conse- 

 quently upon the number of cells and their even distribution. If the 

 film becomes verj' thin, the diffusion ceases almost completely and the 

 cells may die from starvation. 



Aeration and thickness of the moisture film are the two controlling 

 factors in quartz sand cultures. They may be considered as the tr^^o 

 main physical factors of the soil. They depend both upon the average 

 grainsize and the moisture content. 



Aeration increases with the square of the grainsize, while the moisture 

 film increases in direct i)roportion to the grainsize. A coarse soil is, 

 therefore, of advantage to aerobic bacteria. Aeration decreases with 

 increasing moisture, and the moisture film increases, consequently mois- 

 ture is desirable only to a limited degree, namely, until the surface film 

 reaches the optimum thickness. 



The grainsize of the cultivated soils is so small that the optimum 

 moisture film is reached only in the waterlogged state. Such soils will 

 never reach the maximum rate of decomposition, because the ideal con- 

 ditions of maximum aeration and optimum moisture film cannot be ful- 

 filled at the same time. The rate of decomposition is necessarily slow, 

 but the final endpoint of decomposition may be the same as in coarse 

 grained soils. 



Anaerobic bacteria are favored by increasing moisture, since it in- 

 creases the film diameter and decreases aeration. They are also favored 

 by a large grainsize. The grainsize of soils may be so small that even 

 saturation with water does not afford optimum conditions for anaerobic 

 bacteria on account of too slow diffusion. 



Absorption plays a minor r61e in the bacterial activity of quartz sand 

 cultures. 



