Experimental Plots at Rothamsted, from 1864 to 1883. 439 
Plots 16 a and b. 
The Unexhausted Residue of Manures. 
From 1852 to 1864, inclusive, these two plots received a mixed 
mineral manure and 800 lbs. of salts of ammonia, an amount 
which furnished to the crop annually 172 lbs. of nitrogen. It is 
verv rarely that a sufficiently large crop of wheat is grown to 
remove one-half of this quantity of nitrogen. 
In 1863 and 1864 the seasons were highly favourable for the 
growth of wheat ; and as the size of the crop is regulated very 
much bv the amount of nitrogen at its disposal, the two seasons 
were well adapted for ascertaining how much of this very large 
application could be employed in the growth of the crop. 
In 1863 the yield of plot 16 was 56 bushels per acre, and in 
1864, 51 bushels per acre. But plots 8 A and B produced 
respectively 56 bushels and 50 bushels in the same years, by 
means of an application of 600 lbs. of ammonia ; and as the 
additional 200 lbs. only added 1 bushel to the crop, it was quite 
evident that we had reached the possible limits of growth, even 
in seasons so favorable. It did not indeed seem likely — and 
the result afterwards confirmed the view — that such favourable 
seasons would occur again in a period of 20 years. Under 
these circumstances we therefore decided to stop all further 
manuring after 1864, with the view of obtaining information, 
which might prove to be very valuable, in regard to the un- 
exhausted residue of the manure employed. 
The following table (p. 440) gi"\es the produce of plot 16 for 
2 years during which it received manure ; and for 19 years 
during which it was unmanured : also for comparison is given 
the produce of plot 5, which received an annual dressing of 
minerals. 
It will be seen that the first year after the manures were 
stopped, the produce of plot 16 was 32 bushels, or 18 bushels 
in excess of that grown on plot 5. In the next year, plot 16 gave 
bushels, and plot 5 gave 13 bushels, a difference of only 
4 bushels in favour of plot 16. In the two years that succeeded, 
plot 16 yielded 5 bushels in excess of plot 5 for each year ; 
but in the years that followed them, the produce on the two 
plots was almost identical, in fact, the total produce of straw 
and corn on plot 16 was 6 lbs. per acre less than the total 
produce on plot 5. During the next 6 years, however, the 
total produce of plot 16 exceeded that of plot 5 to the extent 
of from 116 lbs. to 221 lbs. each year ; it was, therefore, not until 
12 years after the last application, that all influence due to 
previous manuring ceased. 
VOL. XX. — S. S. 2 G 
