and Price of Wheat, &fc. 
341 
Plot 9. Mixed mineral manure, and 550 lbs. nitrate of soda, 
each year, twenty-five years, 1854-5, and since. 
In forming the estimate of the average produce per acre of 
the country at large, the plan adopted has been to take the 
mean produce of the unmanured plot, of the farmyard-manure 
plot, and of the three artificially manured plots reckoned as one, 
and to reduce the result so obtained to bushels of the standard 
weight of 61 lbs. per bushel. As will be shown further on, 
experience has proved that this mode of estimate leaves but little 
to be desired as a means of computation of the average yield of 
the country over a number of years ; but it has not been found to 
be equally applicable for each individual year. Careful compa- 
rison leads to the conclusion that the so-calculated average pro- 
duce per acre on the selected plots gives somewhat too high a 
result for the country at large in seasons of great abundance, 
and too low a result in unfavourable seasons. Accordingly, as 
above referred to, in some seasons, instead of the actual average 
indicated by the experimental plots, a higher or a lower figure 
has been adopted ; and, especially in the case of some of the 
recent bad seasons, a higher one has been taken. 
Independently of any such admitted differences between the 
so directly calculated, and the actually adopted, estimates for 
individual years, the question arises — whether the average result 
indicated by the several selected plots remains as applicable as 
heretofore ? or whether the produce of some is annually declining, 
or that of others annually increasing, irrespectively of the in- 
fluence of season, so as to vitiate the continued applicability 
of such results for the purposes of such an estimate ? 
Tlie Unmanured Plot. — -There can be no doubt that the produce 
on this plot is gradually declining from exhaustion ; and, inde- 
pendently of the evidence of diminishing produce, analyses of 
the soil, at different periods, show that there is a gradual dimi- 
nution in the amount of nitrogen in it. Owing, however, to 
the great fluctuations in the amount of produce from year to 
year, dependent on the season, it is by no means easy to estimate 
the rate of decline due to exhaustion of the soil, as distinguished 
from that due to the seasons. In the first place, it is difficult to- 
say what figure should be adopted as the standard produce of 
the plot, by which to compare the yield from year to year. The 
whole field was manured with farmyard-dung in 1839, and then 
grew turnips, barley, peas, wheat, and oats, before the commence- 
ment of the experiments in 1843-4. The plot then grew eight 
crops of wheat, to 1850-1, without manure, before the commence- 
ment of the period to which our present estimates refer. No doubt 
the land would suffer more or less exhaustion during those firs4 
eight years ; but, as serving to counteract the tendency to decline 
