Both these salts are employed as sources of nitrogen; but
nitrate of potash owes also a certain proportion of its value to the
potash it contains. Nitrate of soda, on the other hand, must be
considered to owe its entire value to its nitric acid, as soda is of
little value to the plant; and, moreover, can be obtained in common salt
at a price so low, as to make it a matter of no moment in the valuation
of the nitrate. In its ordinary state, as imported from Peru, nitrate of
soda contains from 5 to 10 per cent of impurities, and it bears a price
proportionate to the quantity of the pure salt present in it. When of
good quality it contains about 15 per cent of nitrogen, equivalent to 18
of ammonia, and is, therefore, richer in that constituent of plants than
Peruvian guano. It is essentially a rapidly acting manure, and produces
a marked effect within a very few days after its application; but owing
to the fact that nitric acid cannot be absorbed and retained by the soil
in the same manner as ammonia, it is liable to be lost unless it can be
at once assimilated by the plant. For this reason it acts best when
applied in small quantity as a top-dressing to grass-land, and to young
corn. A large application has no advantages, and there can be no doubt
that the best effect would be produced by several very small quantities,
applied at intervals. In one experiment, Mr. Pusey found 42 lb. per acre
to increase the produce of barley by 7 bushels, and very favourable
results have been obtained by other experimenters. The beneficial
effects of nitrate of soda appear to be almost entirely confined to the
grasses and cereals. At least experience here has shewn that it produces
little or no effect on clover; and one farmer has stated, that having
recently adopted the practice of sowing clover with a very small
proportion of ryegrass only, he has been led to abandon the use of
nitrate of soda, which he formerly employed abundantly, when ryegrass
formed a principal part of his crop. The action of nitrate of soda is
very remarkable, not only in this respect, but also because a given
quantity of nitrogen in it _appears_ to produce a greater effect than
the same quantity in sulphate of ammonia or guano. At the same time this
statement must be taken as very general, definite experiments being
still too few to admit of its being stated as an absolute fact. The
probability is, that the same quantity of nitrogen, in the form either
of ammonia or