80 THE SOIL. 



phate of lime in water containing carbonic acid is 

 filtered through a funnel filled with earth, the upper- 

 most layer of the earth first takes up the phosphoric 

 acid or the phosphate of lime from the fluid. Once 

 saturated therewith it no longer stops the free passage 

 of the dissolved phosphate of lime which now reaches 

 the layer beneath ; the latter then again becomes satu- 

 rated in the same way, and thus by degrees the phos- 

 phate of lime is completely diffused throughout the 

 earth in the funnel, so that every particle retains on its 

 surface an equal proportion of this substance. If the 

 phosphate of lime were of the colour of madder and the 

 soil colourless, the latter would now actually present 

 the appearance of a madder lake. Just in the same 

 way potash is diffused through the soil when a solution 

 of carbonate of potash is filtered through it ; the lower 

 layers receive what the upper do not retain. 



There is no need of any special disquisition to show 

 that the phosphate of lime contained in a particle of 

 bone-earth is diffused in exactly the same way through 

 arable soil, with this difference, that the solution of 

 phosphate of lime in rain-water containing carbonic 

 acid is effected at the very spot where the particle lies, 

 and spreads thence downward and in all directions. 



The potash and the silicic acid rendered soluble by 

 disintegration, or by the action of water and carbonic 

 acid upon silicates, are diffused through the soil in the 

 same way, so is ammonia also, which is conveyed in 

 rain-water, or is generated by the putrefaction of the 

 azotised constituents in the decayed roots from the suc- 

 cessive generation of plants grown on a field. 



Every soil must therefore contain potash, silicic acid 

 and phosphoric acid in twp different forms, namely, in 

 chemical and ^physical combination : in the one form, 

 infinitely diffused over all the surface of the porous par- 

 ticles of the soil ; in the other, in the shape of granules 

 of phosphorite, or apatite and felspar, very unequally 

 distributed. 



In a soil abounding in silicate and in phosphate of 

 lime, which has for thousands of years been exposed to 



