140 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



time, we infer that distribution under favourable cir- 

 cumstances will take place in one-half or one-third of 

 the time it would take without the division. 



If, therefore, in a given case the restoration of the 

 productive power in a soil by fallowing or manuring 

 depends upon the earth when drained of phosphoric 

 acid by the roots of plants receiving the needful phos- 

 phoric acid back again from the surrounding earthy 

 particles, it follows that with an equal amount of earthy 

 phosphates the time required to accomplish this end 

 will be shortened in proportion to the division of the 

 earthy phosphates. 



Straw manure, after decay, leaves silicate of potash 

 behind, and in the process of putrefaction evolves car- 

 bonic acid, which by its action upon the silicates sets 

 free silicic acid ; hence by using this manure the diffu- 

 sion of silicic acid must be promoted as the organic 

 matters absorb none of it, and they, when mixed with 

 the earth, must diminish the absorptive powers of the 

 soil. 



The forest soil above mentioned absorbed only very 

 small quantities of silicic acid from its alkaline solu- 

 tions ; and it is evident that the addition of such soil to 

 the Hungarian earth would have the effect of diffusing 

 through a larger volume of earth the silicic acid set free 

 by disintegration. 



It is not, however, the case with every soil, that its 

 absorptive power for silicic acid decreases in equal pro- 

 portion to the quantity of combustible substances 

 which it contains. Thus the Hungarian earth above 

 alluded to contains more (9*8 per cent.) combustible 

 matter than the Bogenhausen loam (8*7 per cent.), yet 

 its absorptive power for silicic acid is not less but 

 greater than that of the latter. Hence it follows that 

 there are other circumstances which influence the 

 absorptive power of the soil, and consequently the 

 diffusibility of silicic acid. A soil abounding in hy- 

 drated silicic acid will, under any circumstances, absorb 

 less silicic acid than one deficient in that acid, even 



