RELATIONS EXISTING AMONG FOOD ELEMENTS. 131 



proportion makes the other not a whit more effective, 

 because the additional supply exercises by itself no 

 action. 



An increase of phosphoric acid alone has just as 

 little influence in making the returns greater, as an in- 

 crease of potash alone : this law applies equally to 

 every nutritive substance, potash, magnesia, and silicic 

 acid ; no supply of these substances beyond the re- 

 quirement of the wheat-plant, or its capacity of absorp- 

 tion, will have any effect upon its growth. The relative 

 proportions of the mineral substances, which the plants 

 draw from the soil, are easily determined by analysing 

 the ashes of the produce. It is found by analysis that 

 wheat, potatoes, oats, and clover receive the following 

 proportions of phosphoric acid, potash, lime, magnesia, 

 and silicic acid : 



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 sub- 

 stances which the soil has lost. 



If, in the formula, 



Phosphoric acid. 



Potass. 

 2-5 



Lime and magnesia. 

 1-5 



Silicic acid. 

 3-0) 



we determine the value of n, which is meant here to 

 designate the number of ^kilogrammes of phosphoric 



