Lbs. of Nitrogen.
1. Turnips (13-1/2 tons) 60
2. { Wheat (28 bushels at 60 lbs.) 29
{ Straw 16
3. Hay (2-1/2 tons) 56
4. { Oats (34 bushels at 40 lbs.) 27
{ Straw 14
5. Potatoes (3 tons) 27
6. Wheat and straw as before 45
----
Total 274

The supply is therefore quite sufficient for the requirements of the
crop; and when it is borne in mind that a considerable quantity of
ammonia and nitric acid is annually carried down by the rain, and that
during a long rotation other substances are very generally used in
addition to farm-yard manure, it is obvious that the crop need not
depend to any extent upon what it derives from the air. What is true of
the nitrogenous matters applies with still greater force to the mineral
constituents of the manure. Twenty tons of farm-yard manure contain 32
cwt. of mineral matters, while the average crops of a six course-shift
contain only 1088 lbs., or less than one-third of this quantity. It is
obvious, therefore, that in well manured land there must be a gradual
increase of all the constituents of plants, but that of the mineral
matters is relatively much greater than that of the nitrogenous. If
therefore from any cause the crop produced on a soil to which farm-yard
manure had been applied were greatly to exceed the average, the amount
of produce, so far as the soil is concerned, would be limited not by
deficiency of mineral, but of nitrogenous food. Hence also when
farm-yard manure is liberally applied, there is a gradual accumulation
of valuable matters, and a progressive improvement of the productive
capacity of the soil.

It is far otherwise, however, if a special manure is employed, because
in that case the crop is thrown upon the resources of the soil itself
for all its constituents except those contained in the substance
employed, and by persisting in its exclusive use exhaustion is the
inevitable result. It would be wrong, however, to infer from this, that
special manures are to be avoided. On the contrary, great benefits are
derived from their judicious employment, and the circumstances under
which they are admissible may be readily gathered from what has already
been said. They are agents which bring into useful