DEFICIENCY OR EXCESS OF SOLUBLE SILICIC ACID. 89 



potash, leave a certain amount of silicic acid unabsorbed, 

 whereas an equal bulk of soil poor in organic remains 

 will take up the whole of the silicic acid in the solution. 

 The incorporation of decaying vegetable and animal 

 matter will, therefore, in a soil containing disintegrable 

 silicates, first of all accelerate the decomposition of the 

 silicates, by the action of the carbonic acid generated in 

 the process of de.cay, and then as these substances 

 diminish the absorptive power of the soil for silicic acid, 

 as soon as this acid has passed into solution, it is dis- 

 tributed through the soil more widely than would have 

 been the case had these substances not been present. 



On many fields poor in clay, the growth of grass 

 for several years will, in consequence of the organic 

 matters collecting in the soil, which serve to promote 

 the distribution of the silicic acid, act more favourably 

 on a succeeding crop of a cereal plant than a plentiful 

 application of farm-yard manure, whose organic con- 

 stituents, quite irrespective of the silicate of potash in 

 the straw, are always in operation to effect the same 

 object. On many other fields, especially on those 

 abounding in lime, where there is no actual deficiency 

 of silicic acid, but the quantity present is not properly 

 distributed through the soil, a dressing of pulverised 

 turf-waste often produces an equally favourable effect 

 on a succeeding cereal crop as a plentiful application 

 of farm-yard manure. 



Deficiency or excess of soluble silicic acid in the 

 ground is equally injurious to the growth of cereal 

 plants. A soil which would answer very w r ell for 

 horse-tail or common reed (Arundo phragmites, plants 

 abounding in silica) is not on that account equally well 

 suited for the superior kinds of meadow grass, or for 

 cereals, although these demand a rich supply of silicic 

 acid. Such a soil may be improved by drainage, 

 which, by giving free access to air, decomposes and 

 destroys the organic substances present in excessive 

 quantity ; or it may derive benefit from a dressing of 

 marl, or of burnt lime, slaked, or fallen to powder by 

 moist air. 



