PRINCIPLES OF MANURING 107 



magnesium, iron, and calcium. But although 

 every one of these ten elements is necessary 

 for the growth of plants, we do not require 

 to supply them all in manure, because, in 

 the atmosphere or in the soil, several are 

 naturally present in abundance. Except 

 under unusual circumstances, the only three 

 substances that the farmer requires to con- 

 sider, from the point of view of manuring, 

 are nitrogen, phosphorus, and potassium, 

 the two latter being usually designated under 

 the name of their oxides, and called, re- 

 spectively, phosphoric acid and potash. No 

 doubt there are exceptional cases where lime 

 (calcium) is so deficient in the soil that crops 

 cannot obtain all their requirements ; but, 

 as a rule, lime is employed rather as an 

 ameliorative than as a directly nutritive 

 substance. ' Again, magnesia (magnesium) 

 may occasionally fall below the necessary 

 quantity, but it is rare that actual experi- 

 ment has proved this to be the case. 



Confining our attention, therefore, to the 

 three elements that are of most importance 

 in manuring, we find numerous illustrations 

 of the working of the Law of Minimum in the 

 published results of the station at Rothamsted. 

 Thus, the average yield of hay for upwards 

 of forty years was 23*2 cwts. per acre without 



