UNIVERSITY 



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COLOUR OF SOIL 49 



by weather. On the surface soil the full action of wind and 

 rain, and the disintegration by frost are experienced. It is 

 the finer particles which are first removed, both by mechanical 

 and chemical agents, and the large stones thus gradually 

 become a larger proportion of the whole. The Woburn arable 

 land does not show this accumulation of stones at the surface ; 

 this land was in fact in pasture at no very distant date. 



In the case of soils derived from an underlying soft rock, 

 say an oolitic limestone, the distribution of stones will be the 

 reverse of that experienced at Rothamsted. The stones, in 

 this case, are easily disintegrated by atmospheric influences, 

 and will therefore be fewer and smaller in the surface soil, 

 and will increase in size and number as we proceed down- 

 wards. 



Colour of Soil. Soil owes its colour chiefly to two con- 

 stituents humus, and ferric oxide. Humus alone causes a 

 soil to be grey when dry, but nearly black when wet. Ferric 

 oxide is the colouring matter of all red soils ; the tint varies 

 with the condition of the oxide, and in some cases may be 

 brown or yellowish. The admixture of humus of course 

 modifies the colour. 



The intensity of the colour is not a certain indication of the 

 proportion of iron or humus present. If the soil consists of 

 large particles, as a coarse sand, a little ferric oxide or humus 

 may strongly affect the colour, owing to the small extent 

 of surface to be coloured ; while a soil consisting of fine 

 particles will need a much larger amount of colouring matter 

 to produce the same tint. 



The colour of * blue lias/ and probably of other blue clays, 

 is due to finely divided iron sulphide, FeS 2 . When lias is 

 treated with strong hydrochloric acid the pyrites remain as 



