634 



SCIENCE 



[N. S, Vol. XLII. No. 1088 



agencies whereby this food will be replaced 

 when that which is now present is consumed. 

 We must also recall that water, air and tem- 

 perature greatly influence the bacterial con- 

 tent of the soil and they are probably more 

 sensitive to changes in respect to these three 

 requirements than are the plants themselves. 

 As will be seen later, it is possible to meet aU 

 the demands of the physicist and yet have an 

 unproductive soil. 



The single standpoint of the chemist is also 

 open to criticism, for, granting that the right 

 amounts of the necessary elements are present 

 in the soil and are in an available form at the 

 time of exaipination, they will soon become 

 exhausted unless replaced by bacterial activity. 

 The amount and kind of material in the soil 

 solution is one that has caused considerable 

 discussion and for which we can set no arbi- 

 trary standards, knowing that this is inti- 

 mately related to the composition of the soil, 

 which is dependent upon the original nature 

 of the rock of this place and the care of the 

 land since it has been cultivated. Would it, 

 remembering that different animals require 

 different kinds and amounts of foods to repair 

 their protoplasms, be quite correct to assume 

 that all plants require exactly the same 

 amounts of various substances to repair their 

 equally diversified protoplasms? 



While it may be claimed that the bacterio- 

 logical content of a soil is a very delicate index 

 of its fertility, we must not forget that the 

 chemistry and physics of the soil are also im- 

 portant. It is true that from the type of bac- 

 teria present we can form a good idea of the 

 fertility of the soil, but without the knowledge 

 gained from a physical and chemical examina- 

 tion we have no means of knowing how long 

 these conditions will persist. 



The problem of soil fertility then is a com- 

 posite one which needs for its solution a knowl- 

 edge of the interrelated subjects physics, chem- 

 istry and bacteriology. With these points in 

 view, it becomes a very simple matter to har- 

 monize statements which on the surface seem 

 conflicting. We now know that it is necessary 

 to provide the proper laboratory (physical con- 

 ditions) and the necessary raw material (chem- 



icals, etc.) in order that the particular bac- 

 terial cell which we desire may do its work in 

 increasing the fertility of the soil. In order 

 to show the dependence of the bacterial cell 

 on physical factors, we may cite the physicist's 

 demand for a porous soil and remember at the 

 same time that the organisms beneflcial to 

 plant life require oxygen in order to continue 

 their reactions. Again the necessity for mois- 

 ture is clear when we remember that bacteria 

 can use only food that is in solution, and the 

 question of a proper temperature is explained 

 when we remember that at the optimum tem- 

 perature of the bacteria in question, their re- 

 actions are greatly accelerated. 



To discuss in detail the tenets of the chem- 

 ist would require too much space here, but one 

 simple illustration may suffice to show the close 

 relationship between the chemical composition 

 of a soil and its bacteriological content. The 

 adding of chemicals to a soil affects the phys- 

 ical and bacteriological nature of the soil as 

 well as the chemical content. When we re- 

 member that the activity of bacteria often pro- 

 duces acid end-products, which if not neutral- 

 ized will inhibit their activity and finally cause 

 their death, the reason for the addition of lime 

 to a soil is easily understood. 



If we provide all the conditions above out- 

 lined and the soil continues to be unfertile, 

 we may be sure that we have not the proper 

 bacteria present or else some enemy such as 

 certain protozoa are present and are preying 

 on those bacteria which make plant life pos- 

 sible upon the earth. As animals are in the 

 last analysis dependent upon the plants for 

 food, and as plants are dependent upon the 

 nitrifying bacteria, we can easily see that no 

 life could long exist upon the earth without the 

 aid of these organisms. 



J. E. EusH 



Department op Biology, 

 Carnegie Institute or Technology 



TSE THIBTEENTH NEW ENGLAND INTEE- 

 COLLEGIATE GEOLOGICAL EXCUESION 



The annual meeting of the Geologists and 

 Geographers of the New England Colleges and 

 Universities was held under the direction of 



