probably amounts to very little.
That, however, it takes place is beyond a doubt; and that it may--as we
shall see by-and-by--under certain circumstances amount to something
very considerable is also proved. Where large quantities of nitrogenous
organic matter decay, and where, consequently, the supply of atmospheric
oxygen is insufficient to effect complete oxidation, "free" nitrogen may
be evolved in considerable quantities. Similarly, it may be evolved in
the case of vegetable matter decaying under water. In soils rich in
organic matter the reduction of even nitrates may take place,
accompanied with the evolution of free nitrogen, which is thus lost.

_Total Amount of Loss of Nitrogen._

What the rate of total loss of nitrogen is from these different sources
does not admit of easy calculation. Sir John Lawes, in dealing with the
question of soil-fertility, estimated some years ago, by comparing the
soil of old pasture at Rothamsted with that which had been under arable
culture for 250 years, that during that period some 3000 lb. of nitrogen
per acre had disappeared from the arable land. Examples of decrease of
nitrogen in Rothamsted soils, under various conditions of culture, will
be found in the Appendix.[87]

_Loss of Nitrogen by Retrogression._

A source of loss of nitrogen may be here mentioned which has to do with
diminution of amount of available nitrogen, rather than absolute loss of
nitrogen to the soil, and which we may term _loss by retrogression_.
Nitrogen in an available form, such as nitrates, has been found to be
converted into a less available form. This retrogression may be
effected, as in the case of nitrates, by reduction--_i.e._, by removal
of the oxygen in combination with the nitrogen, which in many cases may
be set free, and thus partially although not necessarily entirely lost.
Such reduction is due to the action of bacteria of the denitrifying
order.[88] Or, on the other hand, nitrogen may be converted into some
kind of insoluble form which seems to resist decomposition and lies in
an inert condition in the soil utterly unavailable for the plants'
needs. A striking example of this retrogression of nitrogen seems to be
afforded in the case of farmyard manure. It has been found in the
Rothamsted experiments, as has been pointed out in the preceding pages,
that when farmyard manure is applied, year after year, to the same land
in large quantities, a very considerable percentage of its nitr