mployed as green manure, and different opinions
have been expressed as to their relative values. In the selection of any
one for the purpose, that should of course be taken which grows most
rapidly, and produces within a given time the largest quantity of
valuable matters, but no general rule can be given for the selection, as
the plant which fulfils those conditions best will differ in different
soils and climates. The plants most commonly employed in this country
are spurry, white mustard, and turnips. Rye, clover, buckwheat, white
lupins, rape, borage, and some others, have been largely employed
abroad. Some of these are obviously unfitted for the climate of the
British Islands; and the others, although they have been tried
occasionally, do not appear to have been very extensively employed. The
turnip is sown broadcast at the end of harvest, and ploughed in after
two months. White mustard and spurry are employed in the same way as a
preparation for winter wheat, and with the best results. The latter is
sometimes sown as a spring crop in March, ploughed in in May, and
another crop sown which is ploughed in in June, and immediately
followed by a third. The effect of this treatment is such that the worst
sands may be made to bear a remunerative crop of rye.

It is not easy to estimate the addition made by green manuring to the
valuable matters contained in the soil, but it is probably far from
inconsiderable. A crop of turnips, cultivated on the ordinary
agricultural system, after two months' growth, weighs between five and
seven tons per acre, and contains nitrogen equivalent to about 48 lbs.
of ammonia, and half a ton of organic matters; but nothing is known as
to the quantity produced when it is sown broadcast, and is not thinned,
although it must materially exceed this. Neither is it possible to
determine the relative proportions derived from the soil and the air,
although it is, in all probability, dependent on the resources of the
soil itself,--plants grown on a rich soil obtaining their chief supplies
from it, while, on poorer soils, a larger proportion is drawn from the
atmosphere. Hence light and sandy soils are most benefited by green
manuring, partly on this account, and partly also, no doubt, because the
valuable inorganic matters, which are so liable to be washed out of
these soils, are accumulated by the plants and retained in them in a
state in which they are readily available for the subsequent crop.

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