ch is the most important constituent of the
potato. Mineral manures have a greater effect in increasing the
percentage of starch than purely nitrogenous manures; but when used
together, a still greater increase is obtained than when used singly.
The effect of nitrogenous manures on the composition of roots and
potatoes is thus seen to be similar. In the case of both crops the
effect is to increase the proportion of the characteristic carbohydrate
constituent, which in roots is sugar, and in potatoes starch. Potatoes,
like roots, are also much influenced by the season. The effect of season
and manuring on the potato disease is worthy of notice. Wet seasons are
favourable to the development of the disease. It has been found that in
a highly nitrogenous manured crop the proportion of diseased tubers is
greater than in a non-manured crop.

LEGUMINOUS CROPS.

We have already referred to the manuring of crops of the leguminous
class in discussing the manuring of meadows and permanent pasture. It
was there pointed out that the tendency of certain manures was to
encourage the growth of the leguminous plants of the herbage, while
other manures had the effect of encouraging those of the gramineous
class. It was pointed out that a manure which had this effect was
potash, or any manure which owed its characteristic action to the fact
that it supplied potash to the soil or set it free in the soil.

_Leguminous Plants benefit by Potash._

This is one of the most important points to notice in manuring
leguminous plants. Just as we can say that nitrogenous manures are
specially beneficial to cereals, and phosphatic manures to roots, so
potash is the special manure for leguminous crops.

_Nitrogenous Manures may actually be hurtful._

But we have, further, an even more striking characteristic of leguminous
crops to notice. We have seen that, with regard to the crops already
discussed, while there are cases in which a fertilising ingredient may
be of no value, or may positively exert a hurtful action on the crops,
such cases are only exceptional. With regard to leguminous crops,
however, we find that almost invariably they derive little or no benefit
from the use of artificial nitrogenous manures. And this is all the more
striking since they contain large quantities of nitrogen in their
composition--twice as much as the cereals. The fact, which has long been
noticed with regard to certain members of this class of plants, such