COLOUR AND ODOUR OF SOILS. 57 



Clay, containing from 70% to 95% clay and less than 

 3% calcium carbonate. 



Marl ,, fvom 5% to 20% calcium carbonate, 



the rest mainly clay. 



Calcareous ,, more than 20% calcium carbonate. 



Peaty or liumic more than 20% of humus. 

 The colour of a soil depends mainly upon the amount of 

 humus and of oxide of iron which it contains and upon the 

 quantity of moisture present. Organic matter tends to pro- 

 duce a black colour when moist and a grey when dry. Oxide 

 of iron gives a well-known reddish tint, which, however, varies 

 with its state of oxidation ; if a soil is poor in organic matter 

 and very porous, as is the case with sandy soils, although a 

 large amount of iron may be present, the colour will not be a 

 rich red but yellow. The rich red usually taken as a sign of 

 fertility is produced when both oxide of iron and a considerable 

 quantity of organic matter, and consequently moisture, are 

 present. The oxide of iron in such cases probably acts as a 

 carrier of oxygen from the air to the humus and so favours its 

 decay, even in the presence of an amount of moisture which 

 would interfere with direct oxidation by the air. 



The odour of soil. When soil is moistened a peculiar odour 

 is evolved. The cause of this was investigated by Berthelot 

 and Andre in 1891.* They found it to be due to a volatile sub- 

 stance which they were not able to isolate, but obtained in 

 aqueous solution by distillation with water. It possesses the 

 peculiar odour of moistened soil, is not an acid nor alkali, does 

 not reduce ammoniacal silver nitrate, and therefore is not an 

 aldehyde ; with potassium carbonate it gives a precipitate, and 

 with potash and iodine it yields iodoform. Its amount is ex- 

 tremely small. According to Berthelot and Andr6f clay soils 

 kept in a moist state slowly lose nitrogen by the evolution of 

 volatile nitrogen compounds ; they also state that soils con- 

 tain two classes of nitrogenous organic compounds, distin- 

 guished by their different rate of decomposition with cold 

 potash solution. 



Comp. Rend. 112, 598 ; J.C.S. 1891, abst. 858. 

 t C.R. 112, 195; J.C.S. 1891, 611. 



