128 



THE SOIL. 



Supposing wheat, potatoes, oats, and clover to be 

 cultivated in a field for four years in succession, each of 

 these plants will absorb from the soil the proportion of 

 mineral constituents which it requires ; and the sum total 

 divided by the number of years, viz. four, shows the 

 average relative proportion of all the nutritive substances 

 which the soil has lost. 

 • If, in the formula, 



Phosphoric acid 



2-5 



Lime and magnesia 



: 1-5 



Silicic acid 



3-0) 



we determine the value of ?2, which is meant here to 

 designate the number of kilogrammes of phosphoric acid 

 which the four crops have received from the soil, we find 

 for the wheat crop 26 kilogrammes of phosphoric acid, 

 for the potato crop 25 kilogrammes, for the oat crop 

 27 kilogrammes, and for the clover crop 36 kilogrammes 

 — altogether, 114 kilogrammes; multiplying the above 

 proportional numbers by this number, we obtain the sum 

 total of all the nutritive substances extracted from the 

 soil by the four crops. 



With the help of these proportional numbers, we are 

 better able than before to give some more accurate 

 explanations. 



Suppose that the soil of a certain field contains, in an 

 available state, the requisite quantities of phosphoric acid, 



