48 EXPERIMENTS UPON WHEAT 



third addition of manure only returns 14s. for an expenditure 

 of 30s., and the fourth application only produces an increase of 

 €rop worth 8s. 



C. Effect of the Mineral Constituents. 



The series of Plots 7, 10, 11, 12, 13, and 14 all receive 

 the same amount of nitrogen — 86 lb., in form of 400 lb. 

 of ammonium-salts per acre— but differ in regard to their 

 mineral manuring. Plot 10 receives nothing beyond the 

 nitrogen, Plot 11 has superphosphate also, while 12, 13, 

 und 14 receive a further addition of sulphate of soda, 

 sulphate of potash, or sulphate of magnesia respectively, all 

 three of which are combined to form a complete mineral 

 manure on Plot 7. It should be remembered that soda, 

 magnesia, and potash are always found in the ash of plants, 

 and at the time the experiments were started little was known 

 about the part they played in the nutrition of the plant. And 

 although we know to-day that for practical purposes potash 

 iilone of the three need be supplied in a manure, we are still 

 uncertain what is the function of the other two, which being 

 present in every plant can hardly be without some action. 

 Fig. 6 shows the crops upon these plots in successive ten- 

 yearly periods. It will be seen that Plot 11, receiving super- 

 phosphate, has always given a better crop than Plot 10, without 

 it. This superiority was more marked in the early years of 

 the experiment, when the reserves of potash, etc., were 

 abundant in the soil, and when in consequence the nitrogen 

 and phosphoric acid together had practically the effect of a 

 complete manure. Latterly, as the potash has become ex- 

 hausted by the continual cropping, the yield with nitrogen and 

 phosphoric acid has been but little superior to that produced 

 by nitrogen alone. Similarly, in the earlier years of the experi- 

 ment the crop on Plots 12 and 14, where soda and magnesia 

 .are added to the superphosphate and ammonium- salts, was 

 but Httle inferior to that of Plot 13, which receives 



