COLOUR OF SOIL 73 



60. Observation of Colour. 



Soil samples to be compared should be in the air- 

 dry condition. The colour is noted and the sample 

 gradually heated to dull redness. During heating the 

 following points should be noted : — 



{a) The colour at 100° C, i.e. after removal of the 

 moisture. 



{U) The extent of carbonisation, and whether a 

 nitrogenous odour is produced. 



{c) Any change of colour which may ensue. 



{d') The colour of the residue both in the dry 

 condition and after moistening with water. 



Soils may be roughly classified according to colour, 

 as follows : — 



High humus content. 



Black or dark brown whenl 



moist ... J 



Red or reddish brown . Ferric oxide. 



Bluish tint, changing to al Ferrous compounds 



reddish shade on heating/ and organic matter. 



Light yellow or whitish] Preponderance of sand, 



tint .... J or else chalk. 



Specific Gravity. 



Since soil consists of a number of particles which enclose a 

 certain amount of air or water, it will have a " true " and an 

 "apparent" density. The former represents the weight per unit 

 volume of the solid particles entirely free from air, whilst the 

 latter is that of the soil with pore-spaces included, and is, of 

 course, considerably smaller than the true specific gravity. A soil 

 in good tilth, for example, will tend to have a low apparent specific 

 gravity. In general the specific gravity of a soil decreases 

 inversely to its humus content. The true density of ordinary soils 

 may be taken as 2-65 ; the apparent density varies, but averages 

 approximately 1*2. 



