October-Feb. | 12.95 | 9.81 | 9.55 | 10.2 | 10.4 | 22.57 | 22.47
+--------+--------+--------+--------+--------+--------+-------
Whole year | 30.21 | 15.45 | 15.05 | 10.7 | 10.5 | 37.29 | 35.64
--------------+--------+--------+--------+--------+--------+--------+-------

CHAPTER IV.

NITRIFICATION.

The most important compound of nitrogen for the plant is _nitric acid_.
It is as nitrates that most plants absorb the nitrogen they require to
build up their tissue. In nature the nitrogen, present in the soil as
ammonia and different organic forms, is constantly being converted into
nitric acid. This conversion of nitrogen into nitrates, known as
_nitrification_, is a process of very great importance, and, as has been
already pointed out in the Introductory Chapter, is effected through the
agency of micro-organisms (ferments).[97] The process of nitrification,
as well as the nature of the other changes taking place in the soil
between the various compounds of nitrogen, are as yet but most
imperfectly understood, but much light has been thrown on this most
interesting department of agricultural research during the last few
years; and it cannot be doubted that the increased attention which it is
receiving from different investigators, both on the Continent and in
this country, will be fraught with most important results for practical
agriculture.

_Occurrence of Nitrates in the Soil._

The occurrence of nitre,[98] or potassium nitrate, in soils has been
long known, although it is only within the last few years that we have
obtained any precise knowledge with regard to the mode of its
production. While its amount in most soils, especially in this
country,[99] is very minute, there are certain parts of the world where
nitrates are found in large quantities. The nitrate fields of Chili and
Peru are the chief natural sources of nitrates, and they are referred to
in the chapter on Nitrate of Soda. We have other parts of the world,
however (in China and India), where soils rich in nitre occur, and which
in the past have formed a source of the commercial article.[100]

_Nitre Soils of India._

The most important of these nitre soils are those found in the
North-west of India, in the province of Bengal. In these districts the
soil is of a light porous texture, rich in lime, and situated at a
considerable height above water-level. They are the sites of old
villages, and the nitr