nsiderable interest is the practical absence of the process
in forest-soils. The absence, or occurrence in the most minute traces,
of nitrates in forest-soils has been accounted for by the lowness of the
normal temperature of such soils and their extreme dryness. This latter
condition is accounted for by the enormous transpiration of water which
takes place through the trees, especially in summer-time, which is such
as to render the soil almost air-dry. Lastly, it may be accounted for by
the want of mineral food ingredients.

_Important Bearing of Nitrification on Agricultural Practice._

Before concluding this chapter, it may be well to draw attention to the
important bearing which nitrification has on agricultural practice. The
light which our present knowledge--imperfect as it is--of this most
interesting process throws on the theory of the rotation of crops is
very striking, for it shows how the adoption of a skilful rotation may
be made to prevent the loss of enormous quantities of the most valuable
of all our soil-constituents,--the one on the presence of which
fertility may be said most to depend--viz., nitrogen.

_Desirable to have Soil covered with Vegetation._

The constant production of nitrates going on in the soil, the inability
of the soil to retain them, and the consequent risk of their being
removed in drainage, furnish a strong argument in favour of keeping our
soils as constantly covered with vegetation as possible.

_Permanent Pasture most Economical Condition of Soil._

From the point of view of conservation of soil-nitrates, permanent
pasture may be said to be the most economical condition for the soil to
be in. In such a case the nitrates are assimilated as they are formed,
and, by being converted in the plant into organic nitrogen, they are at
once removed from all risk of loss. A consideration, therefore, of the
process of nitrification furnishes many arguments in favour of laying
down land in permanent pasture--a practice which of late years has been
increasingly followed in many parts of the country. As, however, it is
not possible or desirable to carry out this practice beyond certain
limits, the rotation which most nearly conforms to the condition of
keeping the soil covered with vegetation, and most approximates in this
respect to permanent pasture, is most to be recommended.

_Nitrification and Rotation of Crops._

The chief risk of loss of nitrates is in connection with a ce