50 COLOUR AND ODOUR OF SOILS 



Calcareous soils, containing more than 20 per cent calcium carbonate. 



Peaty or humic soils, containing more than 20 per cent of humus. 



The terms " sand " and " clay " are here to be understood in their 

 usual practical sense, i.e., sand refers to granular fragments, consisting 

 chiefly of silica, but also including some felspar, mica and other sili- 

 cates ; clay refers to the plastic material, consisting mainly of hydrated 

 aluminium silicate, but including also finely divided felspar, mica and 

 even silica. Soils, intermediate in character to those mentioned above, 

 are known by names which are self-explanatory, e.g., sandy loam, clay 

 loam, calcareous sand. 



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 mois- 

 ture present. Organic matter tends to produce a black colour when 

 moist and a grey when dry. Oxide of iron gives a yellow or reddish 

 tint, which, however, varies with the state of hydration ; 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 iron being probably present in the 

 state of limonite, 2Fe 2 O 3 .3H 2 O. 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, present as hcematite, Fe 2 O 3 , 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. 1 They found it to be due to a volatile substance which they 

 were not able to isolate, but obtained in aqueous solution by distilla- 

 tion 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 pre- 

 cipitate, and with potash and iodine it yields iodoform. Its amount is 

 extremely small. According to Berthelot and Andre 2 clay soils kept 

 in a moist state slowly lose nitrogen by the evolution of volatile nitro- 

 gen compounds; they also state that soils contain two classes of 

 nitrogenous organic compounds, distinguished by their different rate of 

 decomposition with cold potash solution. 



. Rend., 112, 598; Jour. Chem. Soc., 1891, Abstracts, 858. 

 2 Conipt. Rend., 112, 195; Jour. Chem. Soc., 1891, Abstracts, Gil. 



