CHAP. X.] LIEBIG'S THEOR Y OF PLANT NUTRITION 277 



fertiliser is to supply to the soil the materials removed 

 therefrom by the crop, and the fertiliser required can 

 be ascertained beforehand by the analysis of a similar 

 crop, so that the soil can be supplied with the exact 

 amounts of potash, soda, magnesia, lime, phosphoric 

 acid, etc., which would be removed by a normal yield 

 of that particular crop. Neglecting Liebig's miscon- 

 ception of the source of the plant's nitrogen and the 

 long controversy which arose as to the necessity of its 

 artificial supply, we can restate the theory as assuming 

 that the proper fertiliser for any particular crop must 

 contain the amounts of nitrogen, phosphoric acid, potash, 

 and other constituents which are withdrawn from the 

 soil by a typical good yield of the plant in question. 



In this form the opinion that the composition of the 

 crop affords the necessary guide to its manuring pre- 

 vailed for some time and still survives in horticultural 

 publications, but the course of field experiments, 

 particularly those at Rothamsted, and the accumula- 

 tion of farming experience soon demonstrated that it 

 was a very imperfect approximation to the truth. 

 Liebig's theory fails because it takes no account of the 

 soil and of the enormous accumulation of plant food 

 therein contained. Water culture experiments demon- 

 strated that certain elements, e.g., sodium and silica, 

 though universally present in the plant's ash, are 

 unessential to its nutrition. Field experiments also 

 showed that other elements — magnesium, calcium, 

 chlorine, sulphur, iron — though essential, are always 

 supplied in sufficient quantities by all normal soils. 

 Thus the elements to be supplied by the fertiliser 

 became reduced to three — nitrogen, phosphorus, and 

 potassium — and even the amounts required of each of 

 these are not indicated by the composition of the crop. 

 To take an example — normal crops of barley and wheat 



