444 On the Continuous Growth of Wheat on the 
nitrogen, than in the soil where artificial manures have been 
used. 
The evidence which we possess on this point is both interest- 
ing and instructive. 
On the Underground Fertility of Plots 2, 3, and 7. 
Although the nitrogen supplied to the land in the 14 tons of 
dung was considerably in excess of the amount supplied to 
plot 7 in the salts of ammonia, still the produce of the wheat 
on both plots was nearly the same ; while the produce on plot 8 
— which received a larger amount of salts of ammonia — was 
much higher than that on plot 2, although the nitrogen supplied 
was still considerably less than that contained in the farmyard- 
manure. 
The great distinction between the two manures is, however, 
due to the nitrogen in the dung being chiefly in combination 
with carbon, and very little is yet known in regard to the 
various compounds of carbon existing in the soil. 
In agriculture, we know that by the continual application of 
dung to the soil, as is the case in a garden, the soil increases 
in fertility, however much we may crop it. Some compounds 
of carbon and nitrogen are exceedingly durable, a familiar 
instance of which is to be found in coal ; while in coke, nearly 
one-half of the nitrogen contained in the coal appears to be 
retained, although the latter has been exposed to a red heat. 
As regards dung, therefore — which is made up of urine, the 
indigestible portions of food, and straw — we might expect to 
find that the process of its decay and parting with its carbon 
would extend over very different periods of time. 
The soil of the dunged plot has been sampled and analysed 
on four different occasions. These analyses form a portion of 
an immense series of soil analyses which have been made from 
time to time on the various experimental and other fields at 
Rothamsted. 
Some of the analyses have been made upon subsoils taken to 
a depth of 9 feet from the surface, but the greater portion do 
not extend below 27 inches, each sample representing 9 inches 
of soil in depth. 
The great advantage to be derived from taking a large 
number of analyses is, that we are thus enabled to form a better 
judgment in regard to the accuracy of each separate result. In 
the present instance, however, our object is rather to point out 
the circumstances under which fertility accumulates in the soil, 
than to attempt to measure its exact amount. 
We estimate that within 27 inches from the surface, the 
