146 THE CULTURE OF FARM CROPS. 



which the actual fertility of the soil depends. The compo- 

 sition of a barren and unfruitful soil is given in the follow- 

 ing table. 



COMPOSITION OF A BARREN SOIL EXHAUSTED OF SOME 



ELEMENTS, 9 INCHES DEEP OVER ONE ACRE. 



Silica and sand 2.333.400 pounds. 



Alumina 2S4.700 " 



Oxide of iron 174.000 



Oxide of manganese 3.150 " 



Lime 25.980 " 



Magnesia 21.840 " 



Potash trace only 



Boda trace only 



Phosphoric acid 90 " 



Sulphuric acid trace only 



Carbonic acid 6.000 " 



Chlorine trace only 



Organic matter 150.840 " 



3.000.000 



The failure of this soil to produce crops is clearly due to 

 the absence of potash, soda, sulphuric acid and chlorine, 

 and the exceedingly small quantity of phosphoric acid, and 

 which the abundance of other elements has no power to 

 neutralize. 



The actually available amount of plant food of any fer- 

 tile soil is exceedingly small. No more of any quantity, 

 however large it may be, then is soluble in water can 

 be absorbed by the roots of a plant; and it is rarely that 

 any quantity that is soluble in water, can be discovered by 

 the most delicate analysis, existing in 100 pounds of any 

 soil. In analyzing soils the chemist uses acid solvents, but 

 plants have the aid only of water, with a very small quan- 

 tity of carbonic acid, to prepare their food for them. Con- 

 sequently the exhaustion of the available plant food from 

 an apparently inexhaustible soil is the work of only a very 

 few years. Twenty years at the most, is the period during 

 which the fertile virgin soils of our forests or prairies will 

 bear satisfactory crops unless they are manured and brought 

 under a judicious rotation. 



The amount of organic and inorganic matter which is re- 

 moved by the ordinary farm crops from an acre of soil is 

 shown in the following table. 



