1 62 PHARMACEUTICAL BACTERIOLOGY 



however been conclusively determined that all three groups of bacteria 

 are essential to the best yield of crop plants. 



Bacteria of group one are essentially rotting bacteria, causing the 

 disintegration of dead plants and animals, forming ammonia and other com- 

 pounds. They are the decomposers of organic matter. Bacteria of group 

 two are essentially elaborators of chemical compounds which the crop 

 plants can use directly as food, in which' work these bacteria make use of the 

 materials formed by the bacteria of group one. They are therefore build- 

 ers up, rather than tearers down, or decomposers; although they also form 

 certain decomposition products. Bacteria of group three possess the 

 remarkable power of converting the free nitrogen of. the air into nitroge- 

 nous compounds which the crop plant can utilize as food material. 



As to form (morphology) most of the soil bacteria are rod shaped, 

 some being comparatively small and others comparatively large, but even 

 the largest do not measure more than 9 microns (a micron being the 

 /f jOOO P ar t f one millimeter) in length. The smaller forms do not meas- 

 ure over i to 3 microns in length. Spherical forms also appear in the 

 soil and upon the roots of plants. Spirally twisted forms (Spirillae) are 

 fairly common in most soils. As has already been mentioned, certain 

 highly pathogenic bacteria (disease producing) may be found in soils. 

 Old, long cultivated soils are apt to contain the tetanus bacillus (the cause 

 of lock jaw). Pasture lands may be infected with the anthrax bacillus 

 (traceable to cattle dead from this disease). It is believed that certain 

 animals (dogs, coyotes, wolves) get the dread disease rabies or hydrophobia 

 from infected soils, although the usual source of infection (in humans as well 

 as in animals) is the bite of some animal already suffering from this disease. 



Soil and Crop Bacteria in Their Relationship to Cultural Operations. 

 It has now been conclusively proven that every agricultural operation 

 which results in an increase in soil productiveness or increase in crop yield, 

 has the effect of encouraging the development of those bacteria which 

 elaborate and set free (render available to the crop plant) those chemical 

 compounds required for the better growth of the crop plants. Turning 

 the soil and making it fine admits air and air (the free oxygen in it) is one 

 of the essentials for the development of soil bacteria. The rays of the sun 

 generate or create warmth and warmth is the second essential to the proper 

 development of soil bacteria. The other essential to bacterial develop- 

 ment is moisture and every farmer knows that a certain amount of soil 

 moisture is absolutely essential for the growth of plants. 



The activities of soil bacteria may therefore be discussed or stated 

 under three heads as follows: 



i . Soil Moisture and Bacterial Development. There are three states or 

 degrees of soil moisture generally recognized. First, that which consti- 



