8 On the Valuation of Unexhausted Manures. 
plot received 14 tons of farmyard-manure per acre per annum, 
for 8 years, 1856-'63, and gave an average produce of 43 cwts. 
of hay, against 23;| cwts. on the unmanured plot over the same 
period. During the subsequent 11 years, l864-'74, there has 
been no further application of dung or of any other manure on 
the previously dunged plot, and the average produce over the 
11 years has been 33^ cwts. of hay, against 19^ over the same 
period on the plot unmanured from the commencement. The 
total increase during the 8 years of the application of the dung 
was 7 tons 12^ cwts. of hay ; and the total increase during 
the next 1 1 years, due to the residue of the dung previously ap- 
plied, was 7 tons 13^ cwts. ; but it has fallen off very much 
during the later years, averaging considerably less than one-half 
as much over the last 5, as over the first 6 of the 11 years. It is 
probable, that during the whole 19 years, not more than two- 
thirds as much nitrogen has been removed in the total produce 
of hay as was supplied in the manure, and the increase of nitro- 
gen over that contained in the permanently unmanured produce 
has probably been not one-fourth as much as was supplied. 
Again, for twenty years in succession 14 tons of dung were 
applied per acre on one plot in the experimental barley-field. 
Calculation showed that a much smaller proportion of the nitro- 
gen of the dung was taken up by the increase of crop, than of 
that supplied in ammonia-salts or nitrate of soda ; and, judging 
from other experiments, it is concluded that the percentage of 
nitrogen in the surface-soil has been increased by the residue of 
the dung to nearly double that of any other plot in the field. 
Yet when, after twenty years, the application of dung was 
stopped on one-half of the plot, and continued on the other half, 
the average produce over the next three years was, without 
further application, 44 bushels of dressed corn, and 2684 lbs. 
of straw ; but where the application was continued, it was, over 
the same three years, bushels of dressed corn, and 3502 lbs. 
of straw ; or there was an average per acre per annum of 8 J 
bushels more of dressed corn, and 818 lbs. more straw, where 
the dung was applied afresh, than where the application had 
been discontinued. It is true that the produce without further 
application was large, and no doubt largely due to the residue 
from the previous applications of dung ; but, notwithstanding 
the very great accumulation within the soil of nitrogen, and, 
doubtless, of all other constituents also, the produce did not 
reach the maximum which the characters of the seasons admitted 
of, but was considerably exceeded on the fresh application of 
dung. 
Dung, however, possesses two very important properties — one 
mechanical and the other chemical. By reason of its bulk, and 
