4SS STATE BOARD OF AGRICULTURE. 



The data of the two tables confirm generally the deductions in the 

 beginning of this chapter. Strain IV proves more sensitive against oxy- 

 gen than Strain IT. Both thrive best in the saturated sand of coarse 

 and medium grain size, while the development in the fine sand is not 

 as good. This is })robably caused by the too thin moisture film. In the 

 very beginning, the difference in the various grainsizes is not very 

 noticeable, because there is plenty of food in the near surroundings of 

 each cell. l>ut when this is all used up. the food has to diffuse farther 

 awav, at the same time the acid accumulates around the bacteria. The 

 large sized grains have large sized intervals between them, and the 

 channels of diffusion are wide. With the small sized sand, the moisture 

 films between the sand particles are very thin, and diffusion is slow, re- 

 tarding growth and fermentation. Strain I\' shows even at the last de- 

 termination no probability of the fine sand finally reaching the stand- 

 ard of the coarser sands. 



In the sands with 10% milk, the growth and fermentation is retard- 

 ed in each case, and the inhibiting effect increases with decreasing 

 grainsize. The reasons for this have been given in the beginning of this 

 chapter. The differences between fine and medium sand are greater than 

 between medium and coarse sand, corresponding to the greater differ- 

 once in grainsize and inner surface. Some of the data do not conform 

 with the rest, due probably to irregularities in growth or analysis. It 

 must be remembered, too, that for each determination had to be used a 

 separate culture, 



V FINAL CONSIDERATIONS. 



The experiments in this paper were undertaken with the object in 

 view to study in what way the soil as a culture medium varies from 

 the solution. The chemical side of this question has hardly been touched 

 in this pai^er, and the physical side has been discussed only in relation 

 to grainsize and moisture. One important phase of physical influence, 

 namely, the humus bodies, have been omitted entirely. They are quite 

 essential, however, on account of their spongy nature, influencing the 

 available water. The author expects to attack this problem in the near 

 fjiture. 



A development of these studies is possible in two directions, extending 

 either into the chemical influences of the soil upon the bacteria or into 

 symbiosis. The possibilities of chemical influences can hardly be all 

 enumerated because they are so manifold. The experimental side should 

 make no difliculties. The study of symbiotic decompositions, however, 

 will require great experimental skill, cautious work, and very much 

 time. Yet, it is one of the most urgent needs of soil bacteriology. Only 

 such organisms can be tried in symbiosis whose requirements for ex- 

 istence are well known and whose metabolism can be followed closely by 

 chemical analysis. 



It is very evident that the results of these laboratory experiments are 

 not directly transferable to field conditions. The statement that the 

 coarsest grained soil will give the best conditions for aerobic bacteria is 

 true only if the water content remains constant. A coarse-grained sand 

 in a field will dry out so fast that bacteria find optimum conditions 

 only for a few days during the year. Ground water changes and drain- 

 age will bring oxygen into the deeper layers of the soil occasionally, 

 and as a matter of fact, the conditions in fanning soils are changing 

 from aerobic to anaerobic conditions continuously. It may be stated, 



