in
agriculture, viz. that it is a wasteful process to put nitrates and manure
together on the land. Fresh manure abounds in de-nitrifying bacteria, and
these organisms not only reduce the nitrates to nitrites, even setting free
nitrogen and ammonia, but their effect extends to the undoing of the work
of what nitrifying bacteria may be present also, with great loss. The
combined nitrogen of dead organisms, broken down to ammonia by putrefactive
bacteria, the ammonia of urea and the results of the fixation of free
nitrogen, together with traces of nitrogen salts due to meteoric activity,
are thus seen to undergo various vicissitudes in the soil, rivers and
surface of the globe generally. The ammonia may be oxidized to nitrites and
nitrates, and then pass into the higher plants and be worked up into
proteids, and so be handed on to animals, eventually to be broken down by
bacterial action again to ammonia; or the nitrates may be degraded to
nitrites and even to free nitrogen or ammonia, which escapes.

[Sidenote: Bacteria and Leguminosae.]

That the Leguminosae (a group of plants including peas, beans, vetches,
lupins, &c.) play a special part in agriculture was known even to the
ancients and was mentioned by Pliny (_Historia Naturalis_, viii). These
plants will not only grow on poor sandy soil without any addition of
nitrogenous manure, but they actually enrich the soil on which they are
grown. Hence leguminous plants are essential in all rotation of crops. By
analysis it was shown by Schulz-Lupitz in 1881 that the way in which these
plants enrich the soil is by increasing the nitrogen-content. Soil which
had been cultivated for many years as pasture was sown with lupins for
fifteen years in succession; an analysis then showed that the soil
contained more than three times as much nitrogen as at the beginning of the
experiment. The only possible source for this increase was the atmospheric
nitrogen. It had been, however, an axiom with botanists that the green
plants were unable to use the nitrogen of the air. The apparent
contradiction was explained by the experiments of H. Hellriegel and
Wilfarth in 1888. They showed that, when grown on sterilized sand with the
addition of mineral salts, the Leguminosae were no more able to use the
atmospheric nitrogen than other plants such as oats and barley. Both kinds
of plants required the addition of nitrates to the soil. But if a little
water in which arable soil had been shaken u