OF THE FERTILITY OF SOILS. 153 



plants require for their growth. Upon the same 

 field, which will yield only one harvest of wheat, 

 two crops of barley and three of oats may be 

 raised. 



All plants of the grass kind require silicate of 

 potash. Now this is conveyed to the soil, or ren- 

 dered soluble in it by the irrigation of meadows. 

 The equisetacece, the reeds and species of cane, for 

 example, which contain such large quantities of 

 siliceous earth, or silicate of potash, thrive luxuri- 

 antly in marshes, in argillaceous soils, and in ditches, 

 streamlets, and other places, where the change of 

 water renews constantly the supply of dissolved si- 

 lica. The amount of silicate of potash removed from 

 a meadow, in the form of hay, is very considerable. 

 We need only call to mind the melted vitreous mass 

 found on a meadow between Manheim and Heidel- 

 berg after a thunder-storm. This mass was at first 

 supposed to be a meteor, but was found on exami- 

 nation (by Gmeliri) to consist of silicate of potash ; 

 a flash of lightning had struck a stack of hay, and 

 nothing was found in its place except 'the melted 

 ashes of the hay. 



Potash is not the only substance necessary for 

 the existence of most plants, indeed it has been 

 already shown that the potash may be replaced, in 

 many cases, by soda, magnesia, or lime ; but other 

 substances, besides alkalies, are required to sustain 

 the life of plants. 



Phosphoric acid has been found in the ashes of 



