WEATHERING OF SILICATES. 155 



for a given quantity of silica rendered soluble, a much larger 

 amount of alkali is liberated than corresponds to the proportion in 

 which both are taken up into the straw of the cereals. 



During the time of fallow, which in this case must elapse be- 

 fore two crops of wheat can be obtained, we may employ the ex- 

 cess of alkalies in the culture of other plants requiring salts with 

 alkaline bases without silica. Between these crops, therefore, we 

 may grow mangel wurzel, or even potatoes, if we remove only 

 the tubers of the latter, and allow the plant itself, which contains 

 much silica, to remain on the field. 



In the preceding remarks, we have considered the changes in 

 the nature and composition of a field on which a rotation of culti- 

 vated plants has been carried on for a series of years. If this 

 field contain an ordinary proportion of alkaline silicates, clay, 

 lime, and magnesia, it will possess an almost inexhaustible pro- 

 vision of alkalies, alkaline earths, and silica ; with this difference, 

 however, that they are not all in a fit state to be used by the plant 

 at the same time. By the mechanical operations of the farm, and 

 by chemical means (by the use of lime, &c), we may shorten 

 the time in which these obtain a form fitted for the vital functions 

 of the plant ; but these matters do not suffice for its complete 

 maturity. 



When phosphates and sulphates are absent from the soil, the 

 plants growing on it cannot form seeds, because all seeds, without 

 exception, contain compounds in which phosphoric acid and sul- 

 phur are invariable constituents. Although all the other ingre- 

 dients of plants be present in superabundance, the soil will become 

 completely sterile, when the period arrives at which it can no 

 longer furnish phosphates or sulphates to a new vegetation. 



We must suppose that, for the formation of the stem and herb, 

 for the fixation of carbon, and for the production of sugar, starch, 

 and woody fibre, a certain amount of alkalies (in the case of the 

 potash -plants), or an equivalent of lime (in the case of the lime- 

 plants), is necessary. But we must bear in mind, at the same 

 time, that the constituents of blood can be formed in the organism 



of wheat, oats, and rye, for 10 eq. of silica, there is only 1, or at the mott, 



2eq in combination with alkalies. 







