42 EXPERIMENTS UPON WHEAT 



be judged from the records of the last forty years, any reason 

 to expect that this crop cannot be maintained in the future, pro- 

 vided that the cultivation and cleaning of the land be continued. 



B. Effect of Nitrogenous Manures. 



It will be remembered that one of the main objects in 

 starting the Rothamsted Experiments was to ascertain the 

 value of nitrogenous manures, and test the truth of Liebig's 

 opinions that the crop could obtain a sufficiency of nitrogen 

 from the atmosphere provided the ash constituents were 

 supplied. Plots 5, 6, 7, and 8 all receive the same dressings of 

 mineral manures, i.e., phosphoric acid, potash, magnesia, and 

 soda, in greater quantities than are removed in the crops. Plot 

 5 receives no nitrogen, Plots 6, 7, and 8 receive increasing 

 quantities of ammonium -salts, supplying 43 Ib. of nitrogen per 

 acre on Plot 6, double that quantity on Plot 7, and treble the 

 quantity on Plot 8. [An average crop of 30 bushels of grain, and 

 28 cwt. of straw, will remove about 50 Ib. of nitrogen per acre.] 



The diagram Fig. 3 shows the crops on these plots over the 

 period from 1852 to 1902. Table XV. gives the figures to 1912.* 



Plot 5, which receives the minerals but no nitrogen, grows 

 very little more than the continuously unmanured plot ; its 

 average over the whole period is only 14*9 bushels, as against 

 13'1 without manure of any description. The other three plots 

 yield crops which increase with each addition of nitrogen ; the 

 grain increases from 24 bushels with 43 Ib. of nitrogen, to 33 

 bushels with 86 Ib. of nitrogen, and to 37 bushels with 129 Ib. 

 of nitrogen ; the straw is even more affected by a free supply of 

 nitrogen, rising from 21|- cwt. to 33 and 41 cwt. as the nitrogen 

 is doubled and trebled. It is thus seen that the wheat crop is 

 very specially dependent upon the supply of nitrogen in the 

 manure. With nitrogen alone (e.g., ammonium- salts alone on 

 Plot 10, nitrate of soda alone on part of Plot 9, and rape cake 

 alone on Plot 19), even over a long period of years, the crop 



* For the sake of clearness the figures in the diagram (1852-1902) are 

 quoted. The differences between these figures and those in Table XV. are 

 very small. 



