o suppose that
any of the root crops are not dependent on a ready supply of nitrogen;
and the fact that large crops of turnips can often be grown by the
application of superphosphate alone, may be taken as a proof that the
soil contains plenty of nitrogen. Mangels are, from their deeper roots,
more capable of drawing their supply of phosphoric acid from the soil
than turnips. They respond, therefore, as a rule, less freely than
turnips or swedes to an application of superphosphate. Generally
speaking, we may say that the characteristic manure for turnips is
superphosphate, and that for mangels is a nitrogenous manure such as
nitrate of soda or sulphate of ammonia.

A special reason for manuring root crops is the fact that they are more
liable to disease than other crops; and this is especially the case in
the early stages of their growth. One of the great benefits conferred on
the turnip crop by an application of superphosphate, is the help it
gives the crop to pass safely the critical period of its growth. The
superphosphate is best drilled in with the seed, in quantities varying
from 3 to 5 cwt. In Scotland, it may be well to point out, the manure
applied to this crop is very much in excess of the amount customarily
applied in England; for in the former country larger applications of
manure may be profitably employed. Roots generally receive a large
dressing of farmyard manure. Salt has been found in some districts to
have a very good effect on the mangel crop, and potash is often found to
amply repay application.

_Influence of Manure on Composition._

A most interesting point in connection with the manuring of roots is the
effect of manure on their composition. This has been most elaborately
investigated at Rothamsted and elsewhere. Thus it has been found that
the effect of the application of excessive quantities of nitrogenous
manures is to produce too great a development of leaves at the expense
of the roots.

_Nitrogenous Manures increase Sugar in Roots._

Nitrogenous manures also tend to increase the proportion of sugar and
diminish the proportion of nitrogenous matter in roots. This has an
important bearing on the treatment of roots which are cultivated for
their sugar, such as beets, in the growth of which nitrate of soda is
the chief artificial manure applied.[245]

The leaf, it may be pointed out, contains a larger percentage of dry
matter, both in swedes and in turnips, than the root.

_Amoun