80 EXPERIMENTS UPON BAELEY 



continuously dunged plot, 7-2, which has varied very little 

 during the period. 



Diagram 12 expresses these results in a graphic form for 

 thirty years, and shows that, though the crop grown with the 

 residues of dung is continually falling, it will only reach the level 

 of that on the continuously unmanured plot after a long time. 



C. Effect of Mineral Manures. 



The Rothamsted barley field affords a more thorough series 

 of comparisons of the effect of the various mineral manures than 

 does the wheat field, for in conjunction with each of the nitro- 

 genous manures we get plots receiving no minerals (1), phos- 

 phoric acid alone (2), potash and the other alkaline salts, but no 

 phosphoric acid (3) ; and the complete mineral manure, con- 

 taining both phosphoric acid and the alkaline salts (4). In the 

 absence of nitrogen the mineral manures have but little effect, 

 though they produce a much greater increase of crop over 

 that of the unmanured plot with barley than with wheat. 

 Ammonium-salts and nitrate of soda used alone are not so 

 effective as with wheat, but the rape cake used without 

 minerals gives almost as big a crop as when supplemented 

 with a complete mineral manure. Of course rape cake is not 

 a purely nitrogenous manure, but itself supplies about 24 Ib. of 

 phosphoric acid and 17 Ib. of potash per acre per annum. 



The diagram * Fig. 13 shows in a graphic form the effects of 

 the various mineral manures, the nitrogen supply being the 

 same in all cases. 



The great importance of phosphoric acid to the barley crop 

 is seen on comparing Plots 3 and 4, which only differ from 

 one another in the omission of phosphoric acid on Plots 3. 

 It will be seen that Plots 3 give but little more crop than Plots 

 1, which receive nitrogen alone only 31*6 bushels per acre 

 against 31*0, taking the average of the three series, A, A A, and 

 C but that a very marked increase to 41 '6 bushels per acre is 



* Figures to 1911 are given in Table XXIX. : they are very similar to 

 those in the diagram. 



