204 Kansas Academy of Science. 



large enough to be seen by the unaided eye must be in the form of 

 a nitrate, such as Chili saltpeter or common saltpeter, or possibly 

 in some salt of ammonia. , 



Could cultivated plants use atmospheric nitrogen directly, the 

 chief problem of soil fertility would be solved. More than 83,000 

 tons of nitrogen rests on each acre of ground. Contained in Chili 

 saltpeter, this amount of nitrogen would be worth $4,000,000 per 

 acre, and the supply would be inexhaustible. So far as is now 

 known, bacteria alone, of all plants, can use nitrogen and the nitrites 

 as crude mineral foods. Fortunately they excrete the substances 

 as nitrates. The higher plants then absorb the nitrates and the 

 other mineral food materials and prepare the various proteids to 

 increase their cell protoplasm. 



Soil bacteria are of many kinds; and the number of individuals 

 of each kind, even in a handful of soil, is almost beyond compre- 

 hension. One gram (one-fourth teaspoonf ul ) of virgin soil con- 

 tains 53,436 bacteria; and the same amount of soil from a ceme- 

 tery supports 363,411 bacteria. Few are found below a depth of 

 six feet. Their rate of increase in numbers through cell division 

 is equally remarkable. Milk fresh from the cow contains on the 

 average 15,000 bacteria to the cubic centimeter (one-fourth tea- 

 spoonful); milk four hours old has in the same volume 100,000 

 bacteria, fifteen hours old, 6,000,000, and ten days old, 1,000,000,- 

 000 of these remarkable plants. 



Though the systematic study of bacteria is less than a half a 

 century old, and though each species varies in form and activity 

 through wide limits, in a few hours, in different environments, 600 

 or 700 kinds have been named and described. These kinds may be 

 grouped, for the purposes of this paper, in five or six classes : 



1. The bacteria of putrefaction. 



2. The air-hating, denitrifying bacteria. 



3. The air-loving, nitrifying becteria. 



4. The nitrogen- fixing, root-tubercle bacteria. 



5. The pathogenic bacterta. 



1. The bacteria of putrefaction reduce the bodies of plants and 

 animals to the condition called humus in soils. The complex or- 

 ganic compounds are broken down, resulting in a release of energy 

 and much cell-substance material for the bacteria and in the evo- 

 lution of various gases, such as carbon dioxid, dihydric sulfid, 

 ammonia, and probably some nitrogen. However, the work of 

 these bacteria seems to be necessary in order that organic matter 



