to its inorganic constituents and to its mechanical
effect on the soil.

_Saw-dust_ has little value as a manure, as it undergoes decomposition
with extreme slowness. It is a good _mechanical_ addition to heavy
soils, and diminishes their tenacity; and though its manurial effects
are small, it sooner or later undergoes decomposition, and yields what
valuable matters it contains. The saw-dust of hard wood is to be
preferred, both because it contains more valuable matters than that of
soft wood, and because the absence of resinous matters permits its more
rapid decomposition. It is a useful absorbent of liquid manure, and may
be advantageously added to the dung-heap for that purpose.

_Manuring with Fresh Vegetable Matter--Green Manuring._--The term green
manuring is applied to the system of sowing some rapidly growing plant,
and ploughing it in when it has attained a certain size, and the success
attending it, especially on soils poor in organic matters, is very
marked. It is obvious that this mode of manuring can add nothing to the
mineral matters contained in the soil, and its utility must therefore be
due to the plant gathering organic matters from the air, which, by their
decomposition, yield nitrogen and carbonic acid--the former to be
directly made use of by subsequent crops, the latter, in all
probability, acting also on the soil, and setting free its useful
constituents. Hence those plants which obtain the largest quantity of
their organic elements from the air ought to be most advantageous for
green manuring. The plants used for this purpose act also as a means of
bringing up from the lower parts of the soil the valuable matters which
exist in it out of reach of ordinary crops, and mixing them again with
the surface part. Many of the plants found most useful for green
manuring send down their roots to a considerable depth; and when they
are ploughed in, all the substances which they have brought up are of
course deposited in the upper few inches of the soil. Vegetable matter
when ploughed in in the fresh state, also decomposes rapidly, and is
therefore able immediately to improve the subsequent crop; and as this
decomposition takes place in the soil without the loss of ammonia and
other valuable matters, which is liable to occur to a greater or less
extent when they are fermented on the dung-heap, it will be obvious that
in no other mode can equally good results be obtained by its use.

Many plants have been e