476 STATE BOARD OF AGRICULTURE. 



of the too thin moisture film, and if tlie moisture films are thick 

 enough to supply the food, the soil solution prevents the ventilation 

 and gas exchange entii-ely. The growth of aerobic bacteria in fine- 

 grained soils is a compromise. Table VI proves that this condition 

 actually exists. The highest anunonia production was obtained in the 

 sand cultures, and not in the soils, though these contained a good many 

 valuable food compounds. The soil extracts range higher than the 

 peptone solutions, but the sand cultures are distinctly superior to the 

 soil cultures. 



It can be deducted from the discussions of this chapter that the 

 optimal conditions for aerobic bacteria in soil are an appropriate mois- 

 true film (approximately 10 — 20 /u.) and the greatest possible ventila- 

 tion. Consequently, the life conditions will improve with increasing 

 grain size, because the ventilation increases with the square of the grain 

 diameter, the water film in direct ratio with it. In agricultural soils, 

 there is no danger that the moisture film becomes too thick from increase 

 of grainsize. Only if the grainsize exceeds 2 mm., this condition is apt 

 to take place, and such soils are not used for agricultural purposes. 

 It is therefore safe to state the following: Life conditions for 

 aerobic bacteria improve with increamig grainsize of the soil. 



The influence of the Water content cannot be stated in such a general 

 way. For agricultural soils of a grainsize less than 0..5 mm, the mois- 

 ture film should be as thick as possible and still allow good aeration. 

 Definite statements concerning the relation between bacterial develop- 

 ment, grainsize and moisture content cannot be made until a mathemati- 

 cal formula has been found for the dependence of ventilation from 

 grainsize and moisture content. 



II. THE ENDPOINT OF FERMENTATION. 



It has been generally assumed in the preceding pages that the great- 

 ly increased ammonia formation in the sand cultures is due to an in- 

 creased oxygen supi)ly. It remains to explain the biochemical mechan- 

 ism of this stimulating action of the oxygen. The endpoint of a fer- 

 mentation is ordinarily considered to be due to either the accumula- 

 tion of harmful products or the lack of food. In these experiments, 

 lack of food cannot figure as an explanation since less than 20% of 

 the total nitrogen in iwptone is changed to ammonia. Accumulation 

 of harmful products should then be the other alternative. Yet, it seems 

 highly improbable that bacteria in quartz sand can tolerate G to 8 

 times as much ammonia as in solution. An adsorption of ammonia 

 by the sand to such an extent would contradict all experience. 



Endpoint and Number of Bacteria. — In a previous paper* the author 

 has found certain lactic bacteria to grow faster and produce more acid 

 in the presence of peptone. This was explained by the theory of an 

 equilibrium between the zynuise in the cells and the acid produced. An 

 increase in the number of cells meant an increase in the zymase, and 

 this caused a disturbance of the equilibrium resulting in more acid 

 foi-mation. Possibly this theory would explain the ammonia increase. 

 The abundant oxygen supply might cause a greatly increased growth, 

 and the consequent increa.se of ammonia-forming enzyme would result in 



♦Michigan Agricultural College Technical Bui. No. 10 (1911) p. 30. 



