170 . THE ALTERNATION OF CROPS. 



It follows from what has preceded, that the ad- 

 vantage of the alternation of crops is owing to two 

 causes. 



A fertile soil ought to afford to a plant all the in- 

 organic bodies indispensable for its existence in suf- 

 ficient quantity and in such condition as allow^s their 

 absorption. 



All plants require alkalies, which are contained in 

 some, in the Graminece for example, in the form of 

 silicates ; in others, in that of tartrates, citrates,, 

 acetates, or oxalates. 



When these alkalies are in combination with silicic 

 acid, the ashes obtained by the incineration of the 

 plant contain no carbonic acid ; but when they are 

 united with organic acids, the addition of a mineral 

 acid to their ashes causes an effervescence. 



A third species of plants requires phosphate of 

 lime, another phosphate of magnesia, and several do 

 not thrive without carbonate of lime. 



Silicic acid * is the first solid substance taken up 

 by plants ; it appears to be the material from which 



* Silica, or siliceous earth, is the most abundant ingredient in the 

 mineral kingdom, being one of the constituents of most rocks, and 

 extensively distributed over the earth in the form of sand, quartz, 

 carnelian, flint, &c., Slc. It is also held in solution by the water 

 of hot springs, as in the Geysers of Iceland, and the Azores, from 

 which it is deposited, forming what is called siliceous sinter, and often 

 incrusting the stems of plants and other bodies. The vegetable mat- 

 ter in some instances has entirely disappeared, and the silica having 

 taken its place we have silicified or petrified wood, &c. See Web- 

 ster's Description of the Island of St. Michael^ p. 208. From sihca a 

 substance is obtained which is considered as its base and called silicon 

 and siliclum. This base, combined with oxygen, constitutes silica, 

 which is capable of combining with other bases ; from this and other 

 properties it is called silicic acid. By combination with other sub- 

 stances, as potash, soda, &c., silica becomes soluble in water. These 

 compounds are called silicates. A white, earthy substance is found be- 

 neath peat and in swampy lands and ponds, which has long been mis- 

 taken for calcareous marl. It has been proved to consist of the siliceous 

 skeletons of" infusorial vegetables, if they may be so called, or of those 

 equivocal beings, which occupy the borders of the two kingdoms, and 

 render it difficult, not to say impossible, to draw the line between 

 them." This siliceous deposite has been found under nearly every peat 

 bog in this country which has been examined. See Professor Bailey's 

 paper in American Journal of Science. Vol. XXXV. p. 118, and Vol. 

 XL. p. 174. 



