618 



SCIENCE 



[N. S. Vol. XXVIII. No. 723 



the proper fertilizer for any particular 

 crop must contain the amounts of nitrogen, 

 phosphoric acid, potash and other constitu- 

 ents which are withdrawn from the soil 

 by a typical good yield of the plant in 

 question. 



In this form the opinion, that the com- 

 position of the crop affords the necessary 

 guide to its manuring, prevailed for some 

 time and still survives in horticultural pub- 

 lications, but the course of field experi- 

 ments, particularly those at Rothamsted, 

 and the accumulation of farming experi- 

 ence 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 con- 

 tained. Water-culture experiments dem- 

 onstrated that certain elements, e. g., so- 

 dium 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 fertilizer became re- 

 duced 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 ex- 

 ample — normal crops of barley and wheat 

 would withdraw from the soil approxi- 

 mately the following fertilizing materials: 



Now the results of field experiments, 

 which are abundantly confirmed by ordi- 

 nary farming experience, go to show that 

 the yield of wheat is chiefly determined by 

 the supply of nitrogen; phosphoric acid is 



of secondary importance and only on ex- 

 ceptional soils will there be any return for 

 the application of potash. 



With barley, though its composition is 

 very similar to that of wheat, the results 

 are very different ; nitrogen is still the most 

 important element in nutrition, but phos- 

 phoric acid has equally marked effects, 

 whilst in ordinary soils potash counts for 

 little or nothing. 



This may be illustrated from the Roth- 

 amsted experiments, and the part played 

 by the reserves in the soil will be made evi- 

 dent by comparing the results obtained in 

 the first and the fifth series of ten years. 



AVERAGE YIELD OF BAELEY GRAIN, HOGS FIELD, 

 ROTHAMSTED 



The analysis of the barley plant would 

 indicate that it requires nitrogen in the 

 largest amounts, then potash and least of 

 aU phosphoric acid, but if the results for 

 the first ten years of the experiment are 

 considered it will be seen that the omission 

 of either nitrogen or phosphoric acid from 

 the fertilizer causes a big decline in yield 

 in comparison with that of the completely 

 fertilized plot. The omission of potash, 

 however, is of little or no moment, since it 

 only causes the yield to faU from 46.1 to 

 45.6 bushels per acre. Evidently the soil 

 was able to supply all the requirements of 

 the plant for potash despite the large 



