RESIDUES OF NITROGENOUS MANURES 227 



acre ; the four plots which had received nitrogen as either 

 nitrate of soda or ammonium -salts gave somewhat less, but 

 still over 60 bushels ; while the four plots without nitrogen in 



TABLE LXXX. Total produce without Manure, following Potatoes 

 manured for 26 years. 



previous years gave between 25 and 35 bushels. In the 

 following season these differences were still to be seen, and the 

 leading position of the previously dunged plots was naturally 

 more manifest than ever ; while in the third season (oats) after 

 the manuring had been discontinued, the order of the plots 

 remained the same, although only the dunged plots now grew 

 a large crop. 



That the increase on the plots which had previously 

 received nitrate of soda or ammonium-salts was due to 

 crop residues rather than to the return of nitrates derived 

 from the manure and stored in the subsoil, is probable from 

 the superiority of the crops on Plots 7 and 8, where minerals 

 had accompanied the nitrogenous manures, over Plots 5 and 

 6, where the same nitrogenous manures had been used alone. 

 When residues are being cropped out, the size of the cereal 

 crop grown is almost wholly determined by the available 

 nitrogenous supply, and Plots 7 and 8, which formerly grew the 

 larger crops of potatoes, give better yields than Plots 5 and 6, 

 although they had initially the same supply of nitrogen. On 



