34 The Rothamsted Wheat Experiments. 



The large crop grown in 1865, the first year after the 

 manures were stopped, is attributed to some of the nitrogen 

 of the salts of ammonia not having been washed out of the 

 soil. The year of the last application, 1864, was one of 

 exceptional drought the driest experienced at Rothamsted 

 during the whole forty years ; it was probably owing to this 

 that, at all events, a portion of the produce of the first year 

 without manure was due. The excess of produce obtained 

 on plot 16, as compared with plot 5, during the next eleven 

 years, probably arose from the slow decay and nitrification 

 of the stubble and roots of the very large crops which had 

 been grown on this plot for so many years. During the 

 thirteen years of the annual application of 8001b. of 

 ammonia-salts, considerably more than lOOOlb. of the 

 nitrogen applied to the soil was not recovered in the crops. 

 It is therefore hardly possible to suppose that the nitrogen 

 (about 601b.) contained in the 66631b. of total produce 

 obtained in excess of that grown on plot 5, during the next 

 twelve years, could have been derived directly from that 

 supplied as manure, excepting so far as a portion of that of 

 the first crop was concerned. All the evidence, indeed, 

 points to the residue of the crop itself as the source of the 

 unexhausted manure ; and it is quite certain that the very 

 large amount of roots and other residue, possessing consider- 

 able fertilising influence, which some of our rotation crops 

 leave in the ground, has much to do with their value as 

 restorative crops. In 1865 samples of soil from various 

 plots in the experimental wheat field were analysed, and the 

 percentage of nitrogen in the first 9in. of soil was higher on 

 plot 16 than on any other plot receiving artificial manures ; 

 whilst, in 1881, after seventeen unmanured crops had been 

 taken, when the soil was again analysed, it was found that 

 the percentage of nitrogen was considerably reduced, and 

 it was in fact then not much higher than that on the 

 continuously unmanured plot. 



