BACTERIA IN SOIL 153 



The numbers of bacteria in the soil vary very much. Accord- 

 ing to Houston's results, fewest occur in uncultivated sandy soils, 

 these containing on an average 100,000 per gramme. Peaty soils, 

 though rich in organic matter, also give low results, it being 

 possible that the acidity of such soils inhibits free bacterial 

 growth. Garden soils yield usually about 1,500,000 bacteria 

 per gramme, but the greatest numbers are found in soils which 

 have been polluted by sewage, when the figures may rise to 

 several millions. In addition to the enumeration of the numbers 

 of bacteria present, it is a question whether something may not 

 be gained from a knowledge of the number of spores present in 

 a soil relative to the total number of bacteria. This is a point 

 which demands further inquiry, especially by the periodic investi- 

 gation of examples of different classes of soils. The method is to 

 take 1 c.c. of such a soil emulsion as that just described, add it 

 to 10 c.c. of gelatin, heat for ten minutes at 80 C. to destroy 

 the non-spored bacteria, plate, incubate, and count as before. 



Besides the enumeration of the numbers of bacteria present in 

 a soil, an important question in its bacteriological examination 

 lies in inquiring what kinds of bacteria are present in any par- 

 ticular case. Practically this resolves itself into studying the 

 most common bacteria present, for the complete examination of 

 the bacterial flora of any one sample would occupy far too much 

 time. Of these common bacteria the most important are those 

 from whose presence indications can be gathered of the con- 

 tamination of the soil by sewage, for from the public health 

 standpoint this is by far the most important question on which 

 bacteriology can shed light. 



Bacillus tnycoides. This bacillus is 1'6 to2'4 /u. in length, and about 

 9 JJL in breadth. It grows in long threads which often show motility. 

 It can be readily stained by such a combination as carbol-thionin, and 

 retains the dye in Gram's method. All ordinary media will support its 

 growth, and, in surface growths on agar or potato, spore formation is 

 readily produced. Its optimum temperature is about 18 C. On gelatin 

 plates it shows a very characteristic appearance. At first under a low 

 power it shows a felted mass of filaments throwing out irregular shoots 

 from the centre, and later to the naked eye these appear to be in the 

 form of thick threads like the growth of a mould. They rapidly spread 

 over the surface of the medium, and the whole resembles a piece of wet 

 teased-out cotton wool. The gelatin is liquefied. 



Cladothrices. Of these several kinds are common in the soil. The 

 ordinary cladothrix dichotoma, is among them. This organism appears 

 as a colourless flocculent growth with an opaque centre, and can be seen 

 under the microscope to send out into the medium apparently branched 

 threads which vary in thickness, being sometimes 2 /* across. They 

 consist of rods enclosed in a sheath. These rods may divide at any 

 point, and thus the terminal elements may be pushed along the sheath. 



