138 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



Straw manure, after decay, leaves silicate of potash 

 behind, and in the process of putrefaction evolves carbonic 

 acid, which by its action upon the sihcates sets free silicic 

 acid ; hence by using this manure the diffusion of sihcic 

 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 solutions ; 

 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 

 disintegi'ation. 



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 sihcic acid is not less but greater than that of the 

 latter. Hence it follows that there are other circum- 

 stances which influence the absorptive power of the soil 

 and consequently the diffusibility of silicic acid. A soil 

 abounding in hydrated sihcic acid will, under any circum- 

 stances, absorb less sihcic acid than one deficient in that 

 acid, even though the latter soil should contain a much 

 larger amount of organic substances. 



The ' absorption numbers ' of two different arable soils 

 afford no criterion for determining the quahty of the soil 

 or the amount of nutritive substances which it contains ; 

 they merely tell us that, in the one soil, the elements of the 

 food of plants wiU spread beyond certain places, further 

 than in the other ; that the one soil opposes greater ob- 

 stacles to their diffusion than the other. The farmer, in 



