84 



SOILS AND MANURES 



various shades of colour seen in soils are due to the pre- 

 sence of organic matter, coloured crystals and the admix- 

 ture of black or coloured minerals in different propor- 

 tions. The shades most commonly produced by the different 

 combinations are as follows : 



Certain inferences may be drawn with regard to fertility 

 from the colour of the soil, but they are by no means reli- 

 able unless confirmed in other ways. The dark reddish 

 brown to black shades are generally due to organic matter 

 with more or less ferric oxide, and are regarded as a good 

 sign. Ferric oxide alone produces various shades of yellow 

 and red, and implies, at least, adequate oxidation. Blues 

 and greens generally indicate the presence of sub-oxides 

 of iron and possibly pyrites. These substances are poison- 

 ous to vegetation, and their presence implies imperfect 

 oxidation. These conditions are prejudicial to fertility, 

 and all dark colours, except when due to organic matter, 

 are to be regarded with suspicion. 



Odour. Soil, when moist, emits a well-known, distinc- 

 tive odour, but none of the attempts to isolate the compound 

 from which it arises have, as yet, met with success. The 

 substance appears to originate in the organic matter, is 

 volatile, of neutral reaction, and, when heated with potash, 

 gives rise to ( a resinous product, but it does not exhibit 

 other properties common to aldehydes. Until further 

 knowledge is gained about the compound it cannot be 

 regarded as of much importance. 



