CHAPTER XXXI. 
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 the}' 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 2 or 3 cm., which is 
exposed to sunlight and freciuent 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 micro- 
organisms diminishes rapidly, as FraenkeP showed many years ago. 
At a depth of from 1 to 2 feet 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.- 
Actively cultivated soils frequently contain one to several millions of 
bacteria.-^ Soil permanently covered with grass is usually relatively 
poor in bacteria. ■* The dust of streets may contain from 1,000,000 to 
10,000,000 bacteria per gram,^ and soil intimately contaminated with 
manure may exliibit as many as 78,000,000 bacteria per gram.'' 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 
contain large numbers of microorganisms, and sand, which is notori- 
ously infertile, contains relatively few. 
The normal bacterial flora of fertile soil consists essentially of at 
least two distinct tjpes of organisms; they may be classified accord- 
ing to their chemical activity into those which eft'ect a rapid deep- 
seated decomposition of dead organic matter into simple combinations 
1 Ztschr. f. Hyg., 1887. 2, 521. 
2 Adametz: Untersuch. ii. niederen Pilze der Akerkrume, 1886. 
3 Chester: Delaware Agr. Coll. Exp. Sta. Rep., 1900-1901. 
4 Chester: Bacteria of the Soil, etc., Bull. No. 98, U. S. Dept. Agric. 
6 Manfredi: Atti della R. Acad, della Science di Napoli, 1891, vol. 2. 
* Maggiora: Roy. Acad, di med., 1897, No. 3. 
