e fact of the great
solubility of nitrates, as well as to the fact, as already mentioned, of
the incapacity of the soil-particles to fix them. To this one exception
must be made. According to Knop, small quantities of nitric acid are
held in the _insoluble_ condition in soils in the form of highly _basic
nitrates of iron and alumina_. The quantity, however, of these insoluble
compounds probably amounts to a very minute trace indeed.

_Permanent Pasture and "Catch-cropping" prevents Loss._

The amount of loss varies, and will depend on a number of different
circumstances--thus the nature of the soil, climate, and season of the
year will all influence its quantity. The way in which the soil is
cultivated is also another important factor. Where it is constantly
covered with vegetation, as in the case of permanent pasture, the loss
will be at a minimum. Under such conditions, plant-roots are always
there ready to fix, in the insoluble organic form, the soluble nitrates
as they are formed. A consideration of this fact forms one of the
strongest arguments in favour of the practice of what is known as
"catch-cropping." The practice consists in sowing some quickly-growing
green crop--_e.g._, _mustard_, _vetches_, &c.--so as to occupy the soil
immediately after harvest, and subsequently to plough it in. The
nitrates, which it is known are most abundantly formed towards the end
of summer,[84] and which are allowed to accumulate in the soil from the
period at which the active growth of, and consequently assimilation of
nitrates by, the cereal crop have ceased, are thus fixed in the organic
matter of the plant, and removed from danger of loss by drainage
incidental to autumn rains.

_Other Conditions diminishing Loss of Nitrates._

The nature of the soil is another important condition regulating this
loss. Some soils are very much opener and more porous than others; in
such soils, of course, the loss by drainage will be greatest. We are apt
at first sight, however, knowing the great solubility of nitrates, to
overrate this source of loss. We have to remember that while nitrates
are constantly being washed down to the lower layers of the soil, there
is likewise an upward compensating movement of the soil-water constantly
taking place. This is due to the evaporation of water from the surface
of the soil, which induces an upward capillary movement of water from
its lower to its higher layers.[85] This upward movement of water i