the ability of the different crops to obtain
the ingredients from the soil. If we take into account these two
considerations in estimating the value of farmyard manure as a general
manure, we shall find that they accentuate the inadequacy of the ratio
existing between the nitrogen and the mineral ingredients. Messrs Lawes
and Gilbert have found in the Rothamsted experiments with farmyard
manure, that while it restored the mineral ingredients, it was
inadequate as a sufficient source of nitrogen. Nitrogen is, of all
manurial ingredients, in least abundance in soils. It is consequently
found that the ingredient in which farmyard manure requires to be
reinforced is nitrogen. With regard to phosphoric acid and potash, it
has already been shown that the ratio between them is probably greater
than that in a good average manure. We should, arguing from this alone,
be inclined to think that farmyard manure would be best reinforced with
potash. The reverse is the case, however, as every farmer knows. This is
due, first, to the fact that the potash, unlike the phosphoric acid, is
entirely of a soluble nature, and therefore immediately available for
the plant's needs; and secondly, to the fact that the necessity for the
application of potash as a manure is generally not nearly so great as in
the case of phosphoric acid. The result is, that farmyard manure will
be, as a rule, more valuably supplemented by phosphoric acid than by
potash.

Another point of great importance, in estimating the value of farmyard
manure as a chemical manure, is the inferior value possessed by much of
the nitrogen it contains, as compared with the nitrogen in such
artificial manures as nitrate of soda and sulphate of ammonia. According
to the Rothamsted experiments, weight for weight, the nitrogen in
farmyard manure is not half so valuable as it is in sulphate of ammonia.
Much of the nitrogen becomes only very slowly available; not a little of
it perhaps actually takes years to be converted into nitrates.[176]

Thus, with regard to the direct value of farmyard manure as a manure, we
have seen--

1. That it contains a very small quantity of the three fertilising
ingredients.

2. That the proportion in which these three ingredients are present is
not the best proportion for the requirements of crops.

3. That the form in which a portion of these ingredients--nitrogen and
phosphoric acid--is present is not of the most valuable kind.

It is consequent