20 EXPEBIMENT STATION EECOED. 



The bacterial activity in soil as a function of grain size and moistore 

 content, O. Rahn (Michigan Sta. Tech. Bui. 16, pp. S-Jfl, fig. 1). — The buDetin 

 records results of extensive experiments covering the ammonia formation by 

 Baoillus mycoides in five different soils, three different sands, in solutions and 

 soil extracts ; a more detailed study of the mechanism of the ammonia pro- 

 duction, and experiments with other aerobic bacteria ; and on the requirements 

 of anaerobic organisms. 



The author concludes that " for the comparison of bacterial efficiency in soil, 

 and for the study of the influence of soil upon micro-organisms, 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 labora- 

 tory 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, 

 Bacterium, aceti, and Aaotobacter show that the development increases with 

 decreasing thickness of the layer of moisture, the maximum being reached at a 

 thickness of from 10 to 20 microns. Experiments concerning the oxygen re- 

 quirements and oxygen tolerance of bacteria should be carried on with very thin 

 liquid films, as they are found in a coarse sand with 5 to 10 per cent moisture. 



" If the moisture film becomes less than 10 microns in thickness, the develop- 

 ment of bacteria is retarded because, though the oxygen supply is abundant, 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 end point of decomposition will be the same if the 

 moisture film is thick enough to permit a complete though slow exchange of 

 food and products through the whole soil solution. The rate of decomposition 

 in such soils depends consequently upon the number of cells and their even 

 distribution. If the film becomes very thin the diffusion ceases almost com- 

 pletely 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 main physical factors 

 of the soil. They depend both upon the average grain size and the moisture 

 content 



"Aeration increases with the square of the grain size, while the moisture film 

 increases in direct proportion to the grain size. A coarse soil is, therefore, of 

 advantage to aerobic bacteria. Aeration decreases with increasing moisture, and 

 the moisture film increases, consequently moisture is desirable only to a limited 

 degree, namely, until the surface film reaches the optimum thickness. 



" The grain size of the cultivated soils is so small that the optimum moisture 

 film is reached only in the water-logged state. Such soils will never reach the 

 maximum rate of decomposition, because the ideal conditions of msLximum 

 aeration and optimum moisture film can not be fulfilled at the same time. 

 The rate of decomposition is necessarily slow, but the final endpoint of decom- 

 position may be the same as in coarse-grained soils. 



"Anaerobic bacteria are favored by increasing moisture, since it increases the 

 film diameter and decreases aeration. They are also favored by a large grain 

 size. The grain size of soils may be so smaU that even saturation with water 

 does not afford optimum conditions for anaerobic bacteria on account of too 

 slow diffusion. 



"Absorption plays a minor role in the bacterial activity of quartz sand 

 cultures." 



The bacterial activity in the soil as a function of the size of the soil par- 

 ticles and water content, O. Rahn (Oentbl. Bakt. [etc.], 2. AM.. 35 (1912), 

 No. 11-19, pp. 429-^65, fig. 1; abs. in Chem. ZentU., 1912, II, Jfo. 26, pp. 



