80 EXPERIMENTS UPON BARLEY 



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 alkahne 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 tlie 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 lb. of 

 phosphoric acid and 17 lb. 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 -1, 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 32-9 bushels per acre 

 against 32, taking the average of the three series A, N, and C 

 — but that a very marked increase to 42*2 bushels per acre is 

 found on Plots 4 for' the addition of phosphoric acid. The 

 straw shows just as marked an increase of crop brought about 

 by phosphoric acid as does the grain, rising from 197 cwt. to 

 25"6 cwt. per acre. In the field the most striking effect is 

 seen in the hastened matm^ity brought about by the phosphoric 

 acid. Not only are Plots 2 and 4, which receive phosphoric 



