A SOIL WHEN SAID TO BE FERTILE. G5 



growth, and that the latter are not. Further, if we find 

 the same soil gradually acquiring the power of producing 

 these latter plants also, we can assign no other reason 

 than this, that by the combined action of air, water, and 

 carbonic acid, aided by mechanical operations, the chemi- 

 cal resistances have been overcome, and the alimentary 

 substances have been reduced to a form in which they are 

 available for absorption even by plants endowed with the 

 feeblest powers of vegetation. 



A soil can only then be said to be perfectly fertile for 

 a given species of plant, e.g. wheat, when every part of 

 its horizontal section which is in contact with the roots 

 contains the amount of food requu'ed by the plant, in a 

 form allowing the roots to absorb such food at the proper 

 time, and in the proper quantity, during every stage of 

 its developement. 



In a former section mention has been made of a 

 property possessed by arable soil, viz. that when 

 brought into contact with solutions of the articles of 

 food most essential for plants in pure water or in water 

 containing carbonic acid, it can withdraw these elements 

 of food from such solutions. This power throws hght 

 upon the form and condition in which these materials are 

 contained or combined in the soil. 



To estimate this property correctly in its bearing upon 

 the life of plants, we must call to mind a similar property 

 in charcoal, which, like arable soil, withdraws from 

 many fluids colouring matters, salts and gases. 



This power in charcoal depends upon a chemical 

 attraction proceeding from its surface, and the materials 

 withdrawn from the fluid adliere to the charcoal in 

 exactly the same way that the colouring matter adheres 

 to the fibre of coloured stuffs coated over witli it. 



The property of decolorising coloured fluids, wliidi 

 F 



