INTRODUCTION. xxv 



WASHINGTON. CLARKE. 



Silica SiO. 57-?8 59-9 



Alumina Al.,0, 15.67 15.45 



Peroxid of Iron Fe2Og 3.31 2.64 



Protoxid of Iron FeO 3.84 3.53 



Magnesia MgO 3.81 4.37 



Lime CaO 5.18 4.91 



Soda Na 2 Oi 3.88 3.56 



Potash K 2 O 3.13 2.81 



Water, basic H2O-f- 1.42 1.52 



Water, acid H 2 O .36 .40 



Ferric Sulphid FeSg 1.03 .60 



Phosphoric acid PsOs .37 .22 



Manganese Protoxid MnO .22 .10 



The salient point which at once attracts attention in these 

 tables is the great predominance of the oxid of silicon silica, 

 silicic acid, quartz, etc., over all other substances. While 

 quartz occurs alone in enormous masses, as will be shown 

 later, probably the greater proportion is found in combina- 

 tion with other oxids, notably those of aluminum, calcium, 

 iron, magnesium, and the alkali metals potassium and sodium. 

 Chlorin and fluorin, however, do not occur as oxids. 1 



The Chemical Elements Important to Agriculture. Of the 

 numerous elements known to chemists, only eighteen require 

 mention in connection with either soil formation or plant 

 growth ; and of these only thirteen or fourteen participate in 

 normal plant growth. They are the following: 



METALLIC ELEMENTS. NON-METALLIC ELEMENTS. 



Potassium Carbon 



Sodium Hydrogen 



Calcium Oxygen 



Magnesium Nitrogen 



Iron Phosphorus 



Manganese Sulphur 



Aluminum Chlorin 



Titanium Fluorin 



lodin 

 Silicon. 



Of this list, titanium, though a very constant ingredient of 



1 A trifling amount of chlorin is found oxidized in the form of sodium perchlor- 

 ate, in the nitre deposits of Chile. 



