138 BACILLUS SUBTILIS AND ITS CONGENERS 



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 alternation of excessive moisture and dry- 

 ness, 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 naturally 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. 



