INCREASED FERTILITY OF DUNGED PLOT 39 



quantity of plant food has been supplied, the annual applica- 

 tion is estimated to contain about 200 Ib. of nitrogen, 78 Ib. of 

 phosphoric acid, and 235 Ib. of potash ; whereas the average 

 crop has only removed about 52 Ib. of nitrogen, 27 Ib. of 

 phosphoric acid, and 53 Ib. of potash per acre. There should 

 be an accumulation of fertility on this plot, and an examination 

 of the curve shows that after a rapid rise during the first eight 

 years of the experiments, when the land was recovering from 

 a state of comparative exhaustion, the yield of grain has been 

 slowly increasing, despite the depression during the decade 

 1872-81. The increase was particularly manifest during the 

 eighties and nineties, on the whole a period of dry seasons, 

 when the moisture retained by the accumulation of humus 

 from the dung also had its effect. The increased fertility 

 of this plot would doubtless have been more manifest were it 

 not for the tendency of the crop to be laid in the heavier 

 yielding seasons. The analyses show that enormous reserves 

 of plant food have been accumulated in the soil of this plot, the 

 amount of nitrogen in the surface soil being more than double 

 that of the unmanured plot, the phosphoric acid being also 

 almost doubled, and the potash showing a very considerable 

 increase. While some of these reserves are in a readily 

 available form, there is evidence from the other experiments at 

 Eothamsted that even in fifty years it would be impossible 

 to crop them entirely out, if a course of growing corn without 

 further manuring were now entered on. 



Regarding now Plots 6 and 7, receiving artificial manures 

 which supply nitrogen, potash, and phosphoric acid, but no 

 organic matter to form humus, we see that Plot 7 for 

 forty years yielded a crop very little inferior to that grown 

 on the dunged plot, and shows no evidence of a decline in 

 fertility. The manure on this plot supplies 86 Ib. of nitrogen 

 per acre, whereas the average crop has taken away not more 

 than 50 Ib. The phosphoric acid and potash supplied are also 

 in excess of the requirements of the crop. On Plot 6 only 

 43 Ib. of nitrogen per acre are supplied, little more than the 



