361 



a very different state : large quantities of manure are usually 

 applied, which contains much potash, and the soil is constantly 

 stirred so as to facilitate the decay of vegetable matter, and 

 promote disintegration of the silicates. We should undoubt- 

 edly have much more potash liberated in this case, than we 

 have when the land remains at rest, as it does during the 

 growth of clover ; at this time also, be it remembered, evapo- 

 ration from the surface goes on very rapidly, fresh portions 

 of soluble potash can be brought from below, and none wastes 

 down by drainage. 



With regard to the other alkali, soda, the same remarks 

 may be applied as to potash; it exists in combination with 

 silica, and is separate in a soluble state by the action of 

 carbonic acid and general atmospheric agencies ; — when so 

 separated, it is absorbed by the roots of plants, and applied 

 to various purposes in their organisms. It is a question 

 whether soda may not replace potash in many of its combina- 

 tions in plants ; indeed analysis proves such to be the case, 

 for we are constantly finding these two substances, (which 

 are exceedingly similar in all their properties,) differing in 

 their proportions to each other in the same variety of plant ; 

 in clover, for instance, grown on soils containing much soda 

 and less potash, we find the ashes of the clover contain less 

 potash and more soda than the analysis here given ; while in 

 soils containing much potash and little soda, the alkali con- 

 sists entirely of potash, which indeed appears to be its most 

 natural base. The presence of either of these bases, con- 

 sequently, in a free state, as far as theory goes, might be 

 considered sufficient to insure a crop when all the other 

 materials are present. There is, however, another compound 

 of soda present in all soils, which is important to the growth 

 of plants, — it is common salt ; this does not only supply them 

 with a certain amount of soda, but at the same time gives 

 them their necessary quantity of chlorine, another element 



