318 ANNUAL REPORT OP THE Off. DoC. 



writer, eight of them grew only in the open end of the fermentation 

 tube, thus showing, in addition to the character of the growth in 

 gelatin stab cultures, their distinctly aerobic habit. 



In Fig. 4 A, is shown a culture in a fermentation tube to which 

 was added 1 c. c. of a watery infusion of soil from the Delaware Sta- 

 tion experimental grounds. In this 1 c. c. of infusion, something like 

 10,000 bacteria were introduced, out of the total of 1,000,000 present 

 in the same, or one out of every hundred. It is therefore reasonable 

 to assume that in this way at least the most important and predom- 

 inating bacteria were introduced. As a result it is seen that all of 

 the growth is confined to the open end of the tube. Hence we may 

 believe that at least the predominating hacteriaof this soil were dis- 

 tinctly aerobic in habit. 



2. The Kelation of Bacteria to Moisture. 



One of the primary requisites of bacterial growth is the presence 

 of moisture. If a soil becomes perfectly dry, not only do bacteria 

 cease to multiply therein, but a large proportion of them die. It is 

 the organic and in a slight measure, the inorganic materials in solu- 

 tion in the soil moisture which supply food for bacteria, hence the 

 maintenance of soil moisture is one of the essential requisites for 

 bacterial development. In short, those moisture conditions which are 

 most favorable to the plant are likewise equally favorable to the 

 bacteria of the soil. A free and uniform distribution of soil moisture 

 is furthermore essential to a uniform distribution of bacteria, and 

 hence to the active elaboration of plant food in all parts of the soil. 

 It is clear that when the soil becomes dry to a considerable depth 

 bacteria cease to develop and with it the digestion of plant food 

 ceases; hence the maintenance of soil moisture by proper methods 

 is important. 



Relation of Soil Bacteria to Organic Matter and Humus 



in the Soil. 



Inasmuch as organic matter and humus furnish food for soil 

 bacteria it might be presumed that the greater the amount of such 

 materials present the greater would be the number of bacteria and 

 hence the greater the amount of plant food digested. Such is the case 

 only within limits. If a solution be prepared containing one per cent, 

 by weight of beef pepton, and be seeded with a soil organism capable 

 of converting the pepton into ammonia and other decomposition 



