7 S SCIENTIFIC AGRICULTURE. 



Its value as a manure is denied by some experiment- 

 ers. It acts, no doubt, in an indirect manner, releas- 

 ing nitrogen from some of its compounds and aiding 

 in the solution of calcium phosphate and other salts. 

 According to some writers, salt may be used to ad- 

 vantage for checking growth on lands which pro- 

 duce such an excess of straw as to cause wheat to 

 fall before the seed is matured. 



170. In chemical language, salts are compounds 

 in which the hydrogen of an acid is replaced by a 

 metal. The union of a non-metallic element with a 

 metal forms a salt like sodium chloride. Another 

 class of salts have oxygen in them in addition to the 

 other non-metallic element, like potassium nitrate or 

 nitre, sodium nitrate or Chili saltpetre. These are 

 used as fertilizers, as are also sulphates of various 

 kinds, and the refuse salts from salt-works and 

 chemical manufactories. The utilization of what 

 were formerly waste products is carried to such 

 an extent in Europe that scarcely anything is lost. 

 Every product has a marketable value, and what 

 cannot be consumed by animals, or put to some 

 useful purpose, is returned to the soil as food for 

 plants. 



171. Among mineral fertilizers may be classed 

 the ash of vegetable substances whether of wood, 

 coal, or plants. The following result of the analysis 

 of the ash of oak and beech wood shows the nature 

 of this fertilizer : 



