428 



SCIENCE. 



[N. S. Vol. XX. No. 509. 



are also helpers and colaborers to be fa- 

 vored. 



The total number of bacteria in any soil 

 capable of sustaining luxuriant vegetation 

 is always very great and they vary in some- 

 what close proportion to the degree of fer- 

 tility. In the first foot of rich garden 

 earth about one to five millions per cubic 

 centimeter commonly exist, though many 

 times the larger number may be present 

 when there is much decomposable matter 

 and the conditions are favorable for rapid 

 multiplication. The numbers rapidly de- 

 crease in the deeper layers, so that at three 

 feet comparatively few are usually found, 

 and at two or three times this depth they 

 are practically absent. The numbers of all 

 kinds have, however, less meaning than has 

 the presence and activity of certain specific 

 kinds— those which have to do with the 

 original formation of soil and some active 

 and efficient helpers in the maintenance of 

 soil fertility. 



ROCK REDUCEES. NITRIFIERS. 



We all understand that what we term 

 soil is not simply the crust of the earth in 

 its original condition. The old-time rocks 

 through long eons of time, by various agen- 

 cies, have been crushed and comminuted, 

 and the more or less pulverized materials 

 have been mixed, transported and deposited 

 by titanic forces and upon a scale of gigan- 

 tic proportions. Mechanical agencies have 

 been operative in the stupendous changes 

 which have taken place in the reduction of 

 the original rocks to the forms of sand and 

 clays in strata as we find them, but only in 

 recent times has it become known that cer- 

 tain microscopic living beings have had a 

 share in the process. In spite of their 

 diminutive size and of their low organiza- 

 tion, in spite of their invisibility and of 

 their individual nothingness, they are help- 

 ers in the accomplishment. It is custom- 



ary to speak of the crumbling of exposed 

 surfaces of rocks as due to the weather, 

 and the process is called weathering. But 

 in these later days it has been found that 

 this change is largely due to living organ- 

 isms, just as the decay of fruits and the 

 putrefaction of the flesh of animals, for- 

 merly supposed to be brought about by 

 mere exposure to the air, are now well 

 known bacterial operations. We know now 

 that the process of canning fruits and meats 

 succeeds not by keeping out the air as we 

 formerly thought, but by keeping out bac- 

 teria. In the so-called weathering of rocks 

 and the formation from them of soil, it is 

 not so much the air, the rain, the frost, etc., 

 which perform the work as it is our minute 

 friends the bacteria. 



Until recently it was supposed to be im- 

 possible that any bacteria could live with- 

 out organic food supplies. It is and has 

 been well known that green vegetation 

 builds up organic matter from inorganic 

 compounds under the energizing a'gency of 

 sunshine. For instance, starch is formed 

 in green leaves from carbon dioxide and 

 water— the former absorbed from the air 

 and the latter taken up by the roots; and 

 there is no other way known to man by 

 which starch is formed from elementary 

 materials. A very large proportion of our 

 food and that of all animals consists of 

 starchy products so primarily produced, 

 hence the operation is an immensely im- 

 portant one and all due to green leaves. 

 They are almost miracle workers, but it 

 had not been imagined that colorless bac- 

 teria could in any measure share with these 

 higher forms of plant life as miracle-work- 

 ing agencies in this respect. Yet these 

 bacteria that live on bare rocks get their 

 sustenance from these and from the air, 

 and in their life processes actually store 

 up organic matter. That is, they are soil 

 makers, fertility producers, advance agents 



