DEFICIENCY OR EXCESS OF SOLUBLE SILICIC ACID. 81 



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 well for horse- 

 tail or common reed (Arundo jyhragmites, plants abound- 

 ing in sihca) 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. 



Hydrated silicic acid loses its solubility in water by 

 simple diying, and it frequently happens that the drainage 

 of a marshy field will cause the siliceous plants (reeds and 

 horsetail) to disappear. The action exerted upon the 

 soil by hydrate of lime, or by hme slaked or fallen to 

 powder in the air, is twofold. On a soil rich in humus 

 constituents the Hme combines, in the first place, with 

 the organic compounds present, which have an acid 

 reaction ; it neutralises the acid of the soil, thereby 

 causing the speedy disappearance of many weeds, such as 

 bog-moss (Sphagnum) and reed-grasses, which flourish 

 in a sour soil of this kind. Simple contact with acids 

 powerfidly promotes the oxidation of metals (copper, 

 lead, iron), wiiile contact with an alkali prevents it 

 (iron coated with a dilute solution of carbonate of soda 

 will not rust). Upon organic substances, the action is 

 the very reverse : acids prevent, and alkalis promote, 



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