AMOUNT OF PLANT FOOD IN THE SOIL. 145 



production of crops. Nothing is or can be taken from the 

 soil unless from the materials which are stored in it, or are 

 added to it from time to time as the store is drawn upon. 

 The store consists of those elements of plant growth which 

 have been previously described, and which, as niay be seen 

 by the figures which represent their various proportions,, 

 (given below) exist in definite and known quantities. They 

 are therefore far from inexhaustible, and all the more so r 

 that only a very small quantity of the most valuable of them 

 exists in an available condition. If a farmer should be led 

 to think otherwise and hope to grow crops until the entire 

 stores are used up, he will be quickly undeceived by the ear- 

 ly and rapid lessening of the yield, until in time only puny 

 weak plants are grown, having not enough of vigor and' 

 strength to produce a seed. Then the soil is exhausted, or as 

 he says it is worn out ; run down and impoverished ; and the 

 supply of raw material having given out, the manufactory is 

 obliged to stop until the stock is replenished. 



The amount of plant food in an acre of good arable 

 soil 9 inches deep, which is equal to about 3,000,000 Ibs. 

 when dry, is shown in the following table : 



COMPOSITION OF A FERTILE SOIL, 9 INCHES DEEP OVER 

 ONE ACRE. 



Silica 2.308.700 pounds. 



Alumina 255.000 



Oxide of iron 132.000 



Lime 60.900 



Magnesia 79.800 



Potash 34.200 



Soda 36.200 



Phosphoric acid 19.500 



Sulphuric acid 1.830 



Chlorine 1.800 



Organic matter 70.000 



There are fertile soils which do not contain more than a 

 tenth part of the above quantities of lime, soda, potash, and 

 phosphoric acid; and it must be remembered that it is not 

 so much the total absolute quantities of these elements, but 

 their condition of availability ; their solubility in fact; upon. 



