ABSORPTIVE POWER OF SOILS. 67 



surface. By a slight addition of alkali to the water the 

 colouring matter may be discharged from tlie charcoal 

 which has been used to decolorise the fluid, and by 

 treatment with alcohol, the quinine or strychnine ab- 

 sorbed by charcoal from a fluid may be again extracted. 



The arable soil possesses, in these respects, the same 

 properties as charcoal. Diluted hquid manure, of deep 

 brown colour and strong smell, filtered through arable 

 soil, flows off colourless and inodorous ; and not merely 

 does it lose its smell and colour, but the ammonia, 

 potash, and phosphoric acid which it holds in solution, 

 are also more or less completely withdrawn from it by the 

 soil, and this in a far greater degree than by charcoal. 

 The rocks which by disintegration give rise to arable 

 soil, if reduced to a fine powder, are just as little pos- 

 sessed of this power as pounded coal. On the contrary, 

 contact with pure water or water containing carbonic acid, 

 deprives many siUcates of potash, soda, and other consti- 

 tuents, a clear proof that the former cannot possibly with- 

 draw the latter from the water. There is no perceptible 

 connection between the composition of a soil and its power 

 of absorbing potash, ammonia, and phosphoric acid. A 

 soil abounding in clay, with a small proportion of -lime in 

 it, possesses this absorptive power in the same degree as a 

 lime soil with a small admixture of clay ; but the amount 

 of humus substances will alter the absorptive relation. 



By a closer observation we perceive that the absorptive 

 power of arable soil differs in proportion to its greater or 

 less porosity ; a dense, heavy clay soil and a loose sandy 

 soil possess the absorptive power in the smallest degree. 



There can be no doubt that all the component parts of 

 arable soil have a share in these properties, but only when 

 they possess a certain meclianical condition, like wood or 

 animal charcoal ; and that this power of absorption 



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