116 THE ART OF CULTURE. 



All plants of the grass kind require silicate of potash. Now 

 this is conveyed to the soil, or rendered soluble in it by the irri- 

 gation of meadows. The equisetacea, the reeds and species of 

 cane containing such large quantities of siliceous earth, or sili- 

 cate of potash, thrive luxuriantly in marshes, in argillaceous 

 soils rich in potash, and in ditches, streamlets, 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 

 Heidelberg after a thunder-storm. This mass was at first sup- 

 posed to be a meteor, but was found on examination (by Gmelin) 

 to consist of silicate of potash ; a flash of lightning had struck a 

 stack of hay, and nothing was founa in its place except the melted 

 ashes of the hay. 



Alkalies and alkaline earths are not, however, the only sub- 

 stances necessary for the existence of most plants ; but other 

 substances besides alkalies are required tosustain the lifeof plants. 



Phosphoric acid has been found in the ashes of all plants hither- 

 to examined, and always in combination with alkalies or alka- 

 line earths. By incinerating the seeds of wheat, rye, maize, peas, 

 beans, and lentils, ashes are obtained quite free from carbonic 

 acid, and consisting entirely of phosphates, with the exception of 

 very small quantities of sulphates and of chlorides. 



Plants obtain their phosphoric acid from the soil. Tt is a con- 

 stituent of all land capable of cultivation, and even the soil of 

 the heath at Liineburg contains it in appreciable quantity. Phos- 

 phoric acid has been detected also in all mineral waters in which 

 its presence has been tested ; and in those in which it has not 

 been found it has not been sought for. The most superficial 

 strata of the deposits of sulphuret of lead (galena) contain crys- 

 tallized phosphate of lead (green lead ore) ; clay slate, which 

 forms extensive strata, is covered in many places with crystals 

 of phosphate of alumina (Wavellite) ; all its fractured surfaces 

 are overlaid with this mineral. 



Apatite (phosphate of lime of similar composition to bone earth) 



After the burnings of forests of pines in North America poplars grew on 

 the same soil 



