THE ORGANISMS OF THE SOIL 157 



When we come to the sulphates the evidence is clearer. 

 There are definite bacteria which feed on sulphur and separate 

 it both in the form of oily globules of the element and as 

 sulphuretted hydrogen. The effect of feeding the bacteria of 

 the soil with gypsum (hydrated sulphate of lime) is most 

 marked. Pichard states the fact in the following way : the 

 nitrification in the soil by bacteria is stimulated by — 



Magnesium carbonate 125 times proportionately 1 

 Calcium carbonate . 1 33 „ ,, 



Potassium sulphate 35*8 „ „ 



Sodium sulphate . 47*9 ,, „ 



Gypsum . . . ioo'o ,, „ 



In the case of the carbonates the action is probably simply 

 due to the neutralisation of acids which act deleteriously on 

 bacteria ; but the action of sulphates is certainly direct and 

 is due to their forming food-stufT for the organisms. 



Though no direct evidence is as yet available as to the action 

 of sulphur bacteria in the soil, there are the researches of 

 Zelinsky and Brussilovsky on the bacteria in the Black Sea, 

 which leave very little doubt that the reaction on land is a 

 similar one to that in the sea. The surface waters of the Black 

 Sea contain free oxygen and support an abundance of organic 

 life ; but the deeper and denser waters are charged with sulphu- 

 retted hydrogen and the only organisms present are the bacteria. 

 The amount of sulphuretted hydrogen increases with depth. 

 At 100 fathoms there are 33 cubic centimetres in 100 litres ; 

 at 200 fathoms, 222 c.c. ; and at 1185 fathoms, 655 c.c. Several 

 species of bacteria have been observed but only one, the 

 Bacterium hydrosulphuricum politician, has been studied in 

 detail. This bacterium possesses the power of liberating 

 sulphuretted hydrogen, not only from organic matter con- 

 taining sulphur but also directly from sulphates and sulphites. 

 All authors are agreed that the sulphates of the sea-water 

 are acted upon but there is some divergence of views as to 

 whether the changes are due solely to bacteria or whether 

 they are in part purely chemical. Changes of an opposite 

 kind take place in the zone where water containing sulphu- 

 retted hydrogen comes into contact ^with that containing 



1 E. W. Hilgard, Soils in Humid and Arid Regions, New York, 1906, p. 147. 



