The soil thus contributed 521 pounds of the total yield. It 

 is evident that the exhaustion of the fertility of the soil comes 

 from that portion of the crop which the plant takes from the 

 soil and not the elements taken from the air, and this question 

 at once arises : 



In the particular case under consideration was it necessary 

 to supply the whole of the five hundred and ten pounds in or- 

 . der to produce the crop. To answer this it will again be neces- 

 sary to ask the chemist to tell us what the ash of the crop was 

 made up of. There was 



Nitrogen in the albuminoids, 



Phosphoric acid in the ash, 



Potash, 



Soda, 



Lime, 



Silica, iron, etc., 



Magnesia, 



Total, 521 



(Commencing at the bottom of the list we may strike out 

 the seventeen pounds of magnesia, for soils, as a rule, contain 

 all of this substance that is needed ; there are excepfional 

 cases where the application of magnesia is beneficial. The one 

 hundred and fifty pounds of silica may be set aside at once ; 

 plants get this substance in abundance, for as a matter of fact 

 it is not essential to the growth, but seems an accidental con- 

 stituent of no special use. 



Of the sixty pounds of lime it may be said, that on most 

 soils it is unnecessary to apply it, but even if a soil is deficient 

 in lime, we shall more than make good this deficiency in ilmost 

 any form of fertilizer we may use, for manures and fertilizers all 

 contain a good percentage of lime. 



The seventeen pounds of soda is of no use to the plant, and. 

 even if essential it is abundantly supplied by the soil. 



But here our work of setting aside must end. Potash is one 

 of the substances that becomes exhaused in soils that have been 

 cropped for a considerable time. 



Phosphoric acid is another substance that must be used to 

 restore fertility to worn soils. 



5 



