180 SOILS AND MANUEES 



crops. The quantity of iron contained in ordinary crops 

 is infinitesimal and it is hardly conceivable that any 

 natural soil should be deficient in that respect. Cases 

 in which deficiency of iron has been suspected from the 

 appearance of the crops have come under the writer's 

 notice, but in none of them was it confirmed by examina- 

 tion of the soil. The application of compounds of iron 

 to the soil has occasionally produced a good effect on the 

 growth of potatoes and some other crops, but it is difficult 

 to believe that the result was due to the direct action of 

 the iron as a plant food. 



Soils may be occasionally deficient in magnesia, and 

 cases are on record in which the application of mag- 

 nesium compounds is said to have produced a good effect. 

 Such cases are, however, extremely rare. Magnesium 

 compounds are usually fairly plentiful in the soil, and 

 it will be seen on reference to the table (p. 169), that, of 

 the ordinary farm crops, red clover and mangolds alone 

 contain any considerable quantity. 



Apart from lime and the elements deemed to be non- 

 essential, only nitrogen, phosphoric acid and potash re- 

 main to be considered. Soils are usually more or less 

 deficient in these constituents. Considerable quantities 

 of nitrogen and phosphoric acid are lost by the sale of 

 produce, and a certain amount of nitrogen is unavoidably 

 lost in other ways. These therefore are the most impor- 

 tant constituents of manures. It is chiefly these sub- 

 stances that must be restored to and added to the soil. 

 In estimating the value of manures as sources of plant 

 foods, only the nitrogen, phosphoric acid and potash are 

 considered. They are sometimes referred to as the 

 manurial elements or the fertilising ingredients of 

 manures. 



Functions of Manures. The application of manures 

 to the soil may be merely an act of restitution, i.e., a 



