90 DEPARTMENT OF AGRICULTURE GEORGIA. [376] 



2nd. They show the different degrees to which these 

 plants exhaust the mineral elements of the soil, if only the 

 marketable products are removed. Thus, if the cotton 

 seed and stalks are returned to the soil, only the lint being 

 removed, and the whole tobacco plant be removed, 1,000 

 pounds of the latter will extract from the soil 15J times as 

 much potash as 1,000 pounds of the former, 8 times as 

 much soda, 22 times as much magnesia, more than 29 

 times as much lime, 6 times as much phosphoric acid, 15 

 times as much sulphuric acid, 72 times as much silica, and 

 12 times as much chlorine. This explains the rapid ex- 

 haustion of lands by the removal of successive crops of 

 tobacco, without compensative returns of manure. Com- 

 parisons similar to the above may be made by the reader 

 from the Tables of the Appendix. 



3rd. They indicate to the farmer the elements which 

 should be combined, both in kind and relative quantity, 

 for the production of particular plants. The advantage of 

 such knowledge is forcibly illustrated by the remarkable 

 results of the experiments of Prof. Stockbridge on plant 

 fertilization, which have already been cited. 



It should be remembered, however, that most soils, not 

 absolutely barren, are store-houses of mineral substances 

 which, under the influence of natural agencies, are con- 

 stantly undergoing mechanical and chemical changes, 

 which gradually convert inert substances into available, 

 assimilable forms. In these transformations, the alkalies, 

 carbonic and sulphuric acids, and vegetation, both dead 

 and living, perform an active part. 



A soil may be rich in all the mineral elements of plant- 

 food, but if they are not in a soluble condition, or readily 

 rendered so, they are absolutely worthless to plants. 

 Natural agencies, however, are constantly at work, both 

 mechanically and chemically, upon the mineral compounds 

 of the soil, pulverizing them, forming new compounds, or 

 destroying those already formed. 



