226 GEOLOGY AND CHEMISTRY OF SOILS. 



becomes of this alkali 1 We know that it enters into all 

 vegetables, as it is found in their ashes, and with them has 

 been removed from the soil. It will be found that the alka- 

 line substajices, limejand magnesia, are also abstracted in a 

 similar way, and hence, as a practical deduction, soils gene- 

 rally need to have these alkalies added, that their fertility 

 may be kept up. It is rarely the case, that the potash be- 

 comes wholly taken from the soil, but it is locked up in the 

 minerals, and is not exhausted until they are all decomposed. 

 Liebig asserts, that the soils of Virginia, from which harvests 

 of wheat and tobacco were obtained for a century, became 

 exhausted, because their alkalies were all removed, or so 

 large a quantity of the free alkali, that the annual disintegra- 

 tion did not furnish a sufficient quantity to supply the wants 

 of the crop. The amount of alkalies, as estimated per Hes- 

 sian acre,* removed in the space of 100 years, is 1,200 lbs. 

 mostly of lime and potash. But generally, the soil contains 

 enough potash, locked up in the minerals, to answer all the 

 wants of vegetation. " A thousandth part of loam," says 

 Liebig, "mixed with the quartz, in new red sandstone, or 

 •with the lime in the different limestone formations, affords as 

 much potash to a soil only twenty inches in depth, as is suffi- 

 cient to supply a forest of pines, growing on it for a century. 

 A single cubic foot of feldspar is sufficient to supply a wood, 

 covering a space of 40,000 square feet, with the potash re- 

 quired for five years." 



It would be easy to show, that our forest granitic and 

 gneiss soils, and even our pine plain land, contain sufficient 

 potash and lime for all the wants of vegetation. But they 

 are not in a free state, hence, although growing plants, by 

 galvanic force, eliminate them from the minerals, still they 

 are not returned to the soil because they are removed 

 with the crop. If the plants were all turned into the soil, 



* A little less than an English acre. 



