LEAF-MOULD FOR EXOTICS. 
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ammonia, as one prime source of its fixation in vegetable tissue, we see no reason 
to believe that the air itself may not be made to yield a supply from the strong 
affinities brought into play by the living principle. At present, while our 
ignorance remains so profound, we must be content to know facts that exist, and 
that nitrogen is frequently a necessary constituent of vegetable bodies. 
LEAF-MOULD FOR EXOTICS. 
One of the most prominent characteristics of horticultural literature in the 
present day, is the examination and discussion of those agents in soils which affect 
the growth of plants, either chemically or mechanically. The fact that no plant 
can flourish unless its roots have a free development, and that no condition, however 
suitable, in which the branches and leaves may be placed, could, of themselves, 
suffice to keep the roots vigorous, and the whole specimen healthy, ought long 
ago to have been more distinctly recognised and acted upon by cultivators. Now 
that attention has been drawn towards the matter, however, it is desirable to meet 
this freshened inquiry, by pointing out a few principles which apply to the 
subject ; and likewise to furnisli some suggestions as to the best materials for 
improving the texture and properties of earths, whether for delicate plants that will 
thrive out of doors, or for those that require more or less of artificial protection 
and warmth. 
To deal first, then, with principles,— since all peculiarities of practice ought to 
have a sound basis of that kind, — we would refer to the true office of soils in the 
economy of vegetation. Earths are of value to plants in many ways. They 
afford a medium, by the spreading of the roots through them, for sustaining 
plants in their right position. They shelter the roots from light, which would 
injuriously alter their texture and nature ; and from the full influence of the air, 
which would generally prevent the exercise of their functions. At the same time, 
they are so permeable to air, and so acted upon by light, that the fluids they 
contain are continually being changed and modified. 
But the chief circumstances in soils which affect plants are the extent of their 
porosity, in admitting a circulation of airs and fluids ; their yielding character, in 
respect of allowing the roots to shoot readily through them ; and their chemical 
constituents, with reference to the supply of proper nourishment. These must of 
course be varied, according to existing peculiarities of situation, or of the species 
cultivated ; but as a common rule, especially in regard to potted plants, it may be 
observed, that an open soil, through which air and fluids can freely pass without 
stagnating, and which the roots can easily pierce, is as essential to superior culture 
as it is to have a due amount of light, &c. for the leaves and branches. 
The texture of soils has been hitherto far too much disregarded in the culture 
