42 EXPERIMENTS UPON WHEAT 



from the condition of virgin soil with its accumulation of 

 fertility, and yet by cultivation alone it has been able to grow 

 for sixty years a crop averaging 13 bushels to the acre. This 

 is almost the average crop produced in the United States, and 

 is very similar to the general average production of the great 

 wheat-growing areas of the world. Nor is there, as far as 

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

 reason to expect that this crop cannot be maintained in the 

 future, provided 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 Pothamsted 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 increas- 

 ing quantities of ammonium-salts, supplying 43 lb. 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 lb. of 

 nitrogen per acre.] 



The diagram Fig. 3 shows the crops on these j^lots over 

 the whole period since 1852. 



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 lb. of nitrogen, to 33 

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

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



