MINERAL CONSTITUENTS OF SOIL. 211 



the matter of the earth, and Daubeny points out its source as 

 proceeding from volcanic action. The exact amount in the 

 atmosphere or the soil has not been accurately determined. 

 It is found in iron-rust, clay, etc., and is retained in the soil 

 in the form of sulphate, carbonate, humate, etc. 



Its relations to vegetation are of the highest importance. 

 According to Liebig, it is the only source of the nitrogen of 

 plants. Others regard it as the solvent of geine, and the con- 

 verter of the vegetable matter into food ; and some add, that 

 it stimulates the functions of plants. Its action has already 

 been considered in the third chapter. 



Potassa or potash. Pure potassa is not found in soils. It 

 is a well known alkali originating from several rocks, in 

 which it exists mostly in combination with silicic acid {sili- 

 cate of potash), but it is also found combined with several 

 other acids. 



The minerals which supply potash to plants are numerous 

 and widely diffused. All the aluminous minerals contain it. 

 Feldspar, a constituent of granite, contains I7:| per cent. 

 Basalt contains from |^ to 3 per cent., clay-slate from 2.75 to 

 3.36, and loam from IJ to 4 per cent. Hence we should 

 expect to find potash in large quantities in the soil ; but owing 

 to the action of growing plants which eliminate the potash, soils 

 which have been cultivated for some time, contain much less 

 than might appear from its abundance in the rocks. This is 

 a case, in which analysis must be resorted to, in order to de- 

 termine the exact amount of an ingredient. In the recent 

 analysis of the soils of New England, we have been unable to 

 find potash as an ingredient, although it must exist in all our 

 soils in a greater or less quantity, locked up in the minerals. 

 Dana estimates its amount in the soil, composed of granitic 

 sand, to be 36 tons per acre, 6 inches in depth. In some 

 soils, it is found to constitute from 5 to 10 per cent. 



The relation of potash to vegetation is similar to all alkaline 

 substances. It is a powerful converter of vegetable matter 



