COMPOSITION OF SOILS. 155 



15*5 parts of ashes (H. Davy) ; the same quantity 

 of the dry stalks of barley, 8.54 parts (Schrader) ; 

 and one hundred parts of the stalks of oats, only 

 4*42 ; — the ashes of all these are of the same com- 

 position. 



We have in these facts a clear proof of what 

 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 pot- 

 ash. Now this is conveyed to the soil, or rendered 

 soluble in it, by the irrigation of meadow^s. The 

 equisetacece, the reeds and species of cane, for ex- 

 ample, which contain such large quantities of silice- 

 ous earth, or silicate of potash, thrive luxuriantly in 

 marshes, in argillaceous soils, and in ditches, stream- 

 lets, and other places where the change of water 

 renews constantly the supply of dissolved silica. 

 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 examina- 

 tion (by Gmelin) 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 sub- 

 stances besides alkalies are required to sustain the 

 life of plants. 



Phosphoric acid has been found in the ashes of all 

 plants hitherto examined, and always in combination 

 with alkalies or alkaline earths.* Most seeds con- 



* Professor Connall was lately kind enough to show me about half 

 an ounce of a saline powder, which had been taken from an interstice 

 in the body of a piece of teak timber. It consisted essentially of phos- 



