470 STATE BOARD OF AGRICULTURE. 



doubtless very unfavorable for aerobic bacteria. It must be further 

 considered that a larj;e portion of the moisture in peat is not available 

 f(ir bacterial development, and that the decomposition probably took 

 place in a water volume much smaller than the one used in the compu- 

 tation. This might easily double the amount of ammonia produced 

 per 100 cc. 



Influence of the Moisture Film. — The enormous increase of micro- 

 bial development in sand can be explained only by the enormously in- 

 creased ventilation of the culture. The chemical mechanism of the re- 

 lation between ventilation and develoi>nient is treated in detail in the 

 following chapter, because more experiments are necessary for this in- 

 vestigation. The data of the preceding experiments are fully sufficient 

 however, to discuss the influence of moisture upon the development of 

 bacteria. 



Table VII has shown B. mycoides to develop best in sand with 10% 

 moisture. If it grows better at 10% than at 15% moisture, this is 

 evidently due to better aeration. One should suppose therefore that 

 this Avould be the case in all soils. However, according to Table VI, 

 the best conditions are found at 20% in all the four soils, and at 10% 

 moisture content, the development is not as good though the aeration 

 is doubtless better. This seemed strange and contradictory at fii*st; 

 the too concentrated soil solution in the drier soils cannot serve as 

 an explanation because in these same soils, the addition of mineral 

 salts, i. e, an increase in the concentration, caused a higher yield of 

 ammonia. The supposition of harmful soil compounds exerting their 

 influence more strongly in the drier soils is perhaps possible, but with- 

 out any proof. 



A satisfactory solution of this problem was finally found by thQ 

 following reasoning : Decreasing moisture content increases the aeration, 

 and increasing aeration causes increased development of aerobic bac- 

 tei'ia. This relation is not unlimited, however, because in the perfectly 

 dry soil which has the greatest aeration, the bacterial growth is not 

 optimal, but zero. Consequently, soil can get too dry for bacterial 

 development. We are dealing with a film of moisture surrounding the 

 soil particles. If this film Ix'comes so thin that food can no longer 

 diffuse to the cells, the life manifestations of bacteria must cease. The 

 thickness of the moisture film grows in direct ratio with the grainsize, 

 and since the four soils under study have a considerably smaller grain- 

 size than the medium sand, it follows that their films are considerably 

 thinner than that of the sand cultures, or that more water is uecessar\ 

 to bring them up to the optimum film thickness. 



The moisture film as essential factor in the microbiology of soils 

 has ali-eady been mentioned nearly 30 years ago by Soyka*. His main 

 expei'iment is mentioned on page 36. 



♦Pettenknfrr nnd Ziemssen, HamJbuch cJcr Hygiene, 'Tcilll, Abteilung Il(Dor Bodtn, p. 228. 1885 



