CHAPTER XXXII. 

 BACTERIOLOGY OF THE SOIL, WATER, AND AIR. 



SOIL. 



THE upper layers of the soil in arable regions of the Torrid and 

 Temperate Zones are densely populated with bacteria, many of which 

 occur with such regularity that they are properly regarded as the 

 normal bacterial flora of the soil. Others are of transitory or accidental 

 occurrence, reaching the soil from the air, from water, from excrement 

 and other waste products of man and animals, and from the dead 

 bodies of man, animals, and plants. 



The very uppermost layer of the soil, the first two or three centi- 

 meters, which is exposed to sunlight and frequent desiccation, usually 

 contains fewer bacteria than the next layer, from 15 to 20 cm. in depth. 

 Here the bacterial population is enormous, frequently reaching several 

 millions of organisms per gram earth. Below this level the number 

 of microorganisms diminishes rapidly, as Fraenkel 1 showed many 

 years ago. At a depth of from one to two inches in undisturbed soil 

 the bacterial flora is relatively insignificant in numbers and frequently 

 no microorganisms are found. 



The character of the soil and its state of cultivation are reflected in 

 the bacterial population which will develop upon ordinary media. 

 Thus sandy soil may contain but a few hundred thousand organisms. 2 

 Actively cultivated soils frequently contain one to several millions of 

 bacteria. 3 Soil permanently covered with grass is usually relatively 

 poor in bacteria. 4 The dust of streets may contain from one to ten 

 million bacteria per gram, 5 and soil intimately contaminated with 

 manure may exhibit as many as 78,000,000 bacteria per gram. 6 It 

 is not surprising, from these figures, to find that the fertility of the 

 soil is closely related to its bacterial population. Normal fertile soils 



* Ztschr. f. Hyg., 1887, ii, 521. 



2 Adametz, Untersuch. ii. niederen Pilze der Akerkrume, 1886. 



3 Chester, Delaware Agr. College Expt. Station Report, 1900-1901. 



4 Chester, Bacteria of the Soil, etc., Bull. No. 98, U. . Dept. of Agriculture. 



5 Manfredi, Atti della R. Acad. della Science di Napoli, 1891, ii. 



6 Maggiora, Roy. Accad. di Medicina, 1897, No. 3. 



