OF AGRICULTURE. 211 



There is no speculation, and we may say no 

 mystery, as to the manner in which these ingredients 

 of the soil act. The effect produced by such wonder- 

 fully small quantities of the mineral fertilizers is no 

 longer astonishing. It is no more wonderful than 

 that leaven should make dough rise, it is even less 

 mysterious. 



CHAPTEE VI. 



MINERAL FERTILIZERS THEIR ACTION IN THE SOIL. 



Silex or sand, lime, potash, soda, magnesia, iron, 

 sulphate, phosphorus, Chilian saltpetre and salt are 

 the mineral parts of plants. It is mainly with these 

 substances we have to deal in connection with 

 fertility in agriculture. 



" SILEX .' ' 



Silica or silex is a very abundant natural product. 

 It forms a large part of all granite or primitive rocks, 

 and is the chief ingredient in sand stones earthly 

 formations, rock crystal or quartz. Flint and sub- 

 stances of this kind are composed almost entirely of 

 silex or sand. Silex, in the form of sand, is the 

 principal article used in the manufacture of glass. 

 Oxygen with silicon, forms silicic acid (as before 

 explained). This acid uniting with bases forms 

 silicates, as the silicate of potash, silicate of lime and 

 of soda. Silex in the form of sand is the principal 

 part of all soils. 



