G44 BACTERIA IN THE SOIL. 



face soil, on the contrary, a multitude of colonies developed within 

 twenty-four to forty-eight hours, and, as many liquefying bacteria 

 were present, it was necessary to make the enumeration on the first 

 or second day, at which time, no doubt, many of the bacteria present 

 had not yet formed visible colonies. The results obtained have, 

 therefore, only a relative value. 



The most important fact developed by FrankePs researches is that 

 in virgin soil there is a dividing line at a depth of from three-quarters 

 to one and one-half metres, below which very few bacteria are found, 

 and that, consequently, the " ground- water region " is free from micro- 

 organisms, or nearly so, notwithstanding the immense numbers pre- 

 sent in the superficial layers. 



The extended researches of Maggiora, made in the vicinity of 

 Turin, led him to the following conclusions : 



1. The number of germs in desert and forest soils is much smaller, other 

 conditions being- equal, than in cultivated lands, and in these it is less than 

 in inhabited localities. 



2. In desert soils the number of germs bears a relation (a) to the geologi- 

 cal epoch to which the lands belong, and, within certain limits, to the height 

 above the level of the sea the older the soil and the greater the altitude, 

 other things being equal, the fewer the germs ; (&) to the compactness and 

 aeration of the soil the more compact and impermeable to air the smaller 

 the number of germs capable of developing in gelatin ; (c) to the nature of 

 the soil sandy soils contain fewer germs than soils rich in clay and in 

 humus. 



3. In cultivated lands the number of germs augments with the activity 

 of cultivation and the strength of the fertilizers used. 



4. In inhabited localities the number of germs in the superficial layers is 

 very great. In the deep layers it usually diminishes rapidly, as is the case 

 in all other soils. 



As to the kinds of bacteria present, and their biological characters 

 and functions in preparing organic material for assimilation by the 

 plants whose roots penetrate the soil, we have yet much to learn. 

 Frankel remarks that the species most frequently encountered in the 

 deeper strata of the soil were three bacilli which also abound in the 

 superficial layers viz., the " hay bacillus," the "wurzel bacillus," 

 and the "hirnbacillus." In all eleven bacilli were isolated and cul- 

 tivated. Micrococci were only found four times, and spirilla not at 

 all. Mould fungi were more abundant, and especially one previously 

 obtained from the air by Hesse and called by him "brauner Schim- 

 melpilz." Anaerobic bacilli, contrary to expectation, were not ob- 

 tained in Frankel's researches, and no pathogenic species were found 

 in the deeper layers of the soil. We have already referred to the 

 fact that the bacillus of malignant oedema and the bacillus of tetanus, 

 two pathogenic, anaerobic species, are common in rich surface soil in 

 various parts of the world. 



