SOURCE OF SODA. 173 



potash to the soil, in the vegetable, animal and saline ma- 

 nures. If no vegetable and animal matters are added, a 

 constant course of cropping will extract the free alkalies, 

 and however rich the soil may be in humus, plants will not 

 flourish. 



The form in which potash enters the organs of plants, 

 and the mode of assimilation, is a matter of theory. It ex- 

 ists in the form of a silicate, in ashes, in feldspar and in 

 manures. It is possible that it is dissolved in water and thus 

 conveyed to the roots of plants. It is also found in com- 

 bination with organic acids. If it were introduced then, in 

 the form of an oxide dissolved in water, it would combine 

 with the organic acids in the plant, and form the vegetable 

 compounds in which it is found. It may be introduced in 

 combination with humic, crenic and apocrenic acids. But 

 the most probable theory is, that potash is combined with 

 nitric, or some of the inorganic acids, and introduced as a 

 salt ; that it is decomposed by the organic acids in the plant, 

 and the acid, either sent out to act upon the silicates and 

 obtain more alkali, or decomposed and its elements assimi- 

 lated. 



2. Soda. The source and the assimilation of soda is sim- 

 ilar to that of potash. The rocks which supply soda to 

 plants are very few. It is obtained from the sea or salt wa- 

 ter; hence, plants containing this alkali in the greatest 

 abundance, are found near the sea or salt springs. Com- 

 mon salt is a chloride of sodium, and forms soda in the form 

 ofhydrochlorate of soda by the decomposition of the salt and 

 the water. Now a small quantity of salt is evaporated from 

 salt water,* is carried inland, and becomes one source of 

 the soda of plants ; hence the useful effects of salt upon some 

 soils, and for particular crops. 



3. Magnesia. Magnesia is also derived from the rocks, 



* Liebig, p. 192. 



15 



