78 THE rKlNC'lPLES OF AGRICULTURE. 



Per Cent. 



Organic matter (containing some nitrogen) . . !).;i 



Pliusplioric acid 0..3 



Potassium oxide 0.2 



Sodium oxide 0.4 



Lime (I.O 



SuljAuric acid 0.2 



Carbonic acid 4.2 



Silica CG.l 



Oxide of Iron 6.5 



Magnesia 0.9 



Alumina 5.5 



Chlorine 0.2 



1 00.0 



The percentage of many of these seems small, but it 

 amounts to a large quantity per acre. One tenth of 

 one per cent, of the dry soil of an acre would generally 

 amount to two or three tons. 



The quantity of many of these substances required by 

 crops is so small that they are practically inexhaustible. 

 Of those substances which crops require in larger quanti- 

 ties there would be enough to last many years, provided 

 they could be changed and made available as fast as 

 required. 



The ordinary crop of wheat raised upon an acre might 

 require, among other elements, fifteen pounds of phos- 

 phoric acid and eighteen pounds of potash. If the en- 

 tire amount of these substan(;es natui'ally contained in 

 good soil could be made useful as fast as needed, there 

 would l)e no lack for many years. 



The Effeot of Agriculture. — In a state of natiu'c fer- 

 tility is naturally maintained. Plants that grow upon 

 the soil die and decay upon it. Thus, those elements 

 of fertility which have been withdrawn from the soil by 

 plants in their growth are returned to it by their death. 



