THE ORGANISMS OF THE SOIL 



151 



estimates are given lyy Ramann of the relative proportions in 

 the various types of soil. 1 



These numbers may seem enormous but it must be remem- 

 bered that the bacteria and spores are very small : a bacterium 

 is about one-thousandth of a millimetre in diameter and the 

 " saturation " point in soil is only reached when there are six 

 hundred million bacteria in a cubic centimetre. 



The manner of estimating such germs is by growing 

 cultures in nutrient gelatine and agar-agar and then counting 

 the developing colonies of bacteria ; the results are approximate 

 only and there are doubtless many that do not germinate in 

 the particular medium employed. Others again are too small 

 to be recognised under the highest powers of the microscope. 

 Another factor which increases the difficulties of estimation is 

 the enormous powers of multiplication which these organisms 

 possess: a bacterium divides into two every thirty-five minutes; 

 one bacterium, therefore, at the end of twelve hours will have 

 four million descendants, so that the numbers in the soil vary 

 from moment to moment. 



Whilst actual numbers cannot be given definitely, the pro- 

 portions are more or less correct. It has been found that 

 bacteria are more abundant in the first foot of soil. 2 Thus 

 Adametz found in one gramme at the surface 38,000 bacteria, 

 whilst at a depth of ten inches there were 460,000. In this 

 particular sample there were only forty to fifty fungus germs, 

 of which six species were true moulds and four were ferments, 

 including the yeasts of wine and beer. At three feet down the 

 numbers decrease rapidly in proportion to the aeration of the 

 soil. Fraenckel, however, found that even in the soil beneath 



1 E. Ramann, Bodenknndc, Berlin, 1905, p. 

 Type of Soil 



1. Pines with beech undergrowth 



2. Pines in boggy ground 



Bacteria 

 in 1 grm. of dry substance 



3. Beech-leaf mould . 



4. Old leaf mould below No. 3 



5. Leaf mould in oak coppice 



6. Pine-needle mould 



7. Loamy soil . 



8. Sandy soil . 



9. Soil below humus 



35,000,000 



1,647,000 



3 1 ,000,000 



264,000 



40,000,000 



50,000,000 



4,860,000 



2,500,000 



247,000 



120. 



Fungi 

 in 1 grm. of dry substance 



6o,O0O 



343,000 



560,000 



800,000 



3,430,000 



Uncountable 

 4,000 — 277,000* 

 66,000 — 566,000* 

 35,000—350,000* 



• From one cubic centimetre. 



* Adametz, Inaug. Diss. Leipzig, 1876. 



