BACTERIAL CONTENT OF THE SOIL. 179 



C. FRAENKEL (III.) has given instructive examples. (A remarkable 

 exception to this rule was reported by S. ROHN and H. WICH- 

 MANN (I.).) Finally, the influence of aeration should not be for- 

 gotten. This, in the lower and comparatively undisturbed layers, 

 is almost nil, and the qualitative nature of the inhabitants of the 

 soil is greatly influenced by this factor, the upper layers being 

 relatively the richest in aerobic bacteria. 



The greatest percentage of organisms should not be expected 

 in the uppermost layer, since this is exposed to a too rapid alterna- 

 tion of excessive moisture and dryness, heat and cold, as well as 

 to the anti-bacterial influence of the sun's rays. For these reasons 

 the largest content of germs is always found at a depth of 10 to 

 20 inches below the surface ; a fact first shown by ROBERT KOCH (I.). 

 As the depth increases beyond this, the finds become smaller and 

 smaller, and approximate to nil at about 60 to 80 inches. 



It is manifest that the germ content of a soil is also dependent 

 on the presence of nutrient substances, and that a soil rich in 

 humus will be much more thickly infested than a poor sandy soil. 

 Thus BEUMER (I.) found in dune sand only some 1000 germs per 

 gram, which is very few ; and an almost identical result was 

 attained by A. MAGGIORA (I.) in the examination of a sample of 

 sandy soil from a hill near Turin. On the other hand, he found 

 in tilled agricultural soil some 1 1 millions of germs per gram, 

 and in the same weight of a sample of soil taken from a street in 

 Turin no less than 78 millions of bacteria. 



That the degree to which a soil is warmed, as also its condition 

 as regards moisture and meteorological factors, all influence its 

 bacterial population needs no further argument. It follows natu- 

 rally that the percentage of germs is higher in summer than in 

 winter, and that it falls in dry, but increases in wet weather. 



Any reader desirous of more closely studying the bacterial 

 content of the soil, especially from a hygienic point of view, will 

 find a good introduction thereto in Fodor's work, Hygiene des 

 Bodens (Hygiene of the Soil), forming the 4th part of the useful 

 Handbuch der Hygiene issued by Weyl (Jena, 1894 et seq.). 



In addition to bacteria, the soil harbours a large number of 

 higher fungi, comprising not only numerous innocuous mould 

 fungi, but also the spores of phytopathogenic Eumycetes. These 

 are of no immediate importance to agricultural chemistry, and 

 therefore do not need any further consideration here. E. Ch. 

 Hansen demonstrated that the wine yeasts winter in the soil, but 

 on this point reference must be made to the chapter devoted to 

 Saccharomyces apiculatus in the second volume. 



