BOYS DEPARTMENT. 
65 
Uqjjs' department. 
AGRICULTURAL CHEMISTRY, No. 10. 
The subject next in order, and to which I shall 
devote the whole of this letter, is one of much in- 
terest, viz., humus. 
When straw, leaves, pieces of wood, or any other 
species of vegetable matter is exposed to the action 
of air and moisture, with a moderately-warm tem- 
perature, a gradual change takes place. The 
elements which combined to form the solid struc- 
ture of the vegetable matter are disunited, some of 
them returning to the air, the primitive source from 
whence they were drawn, and others forming new 
combinations, until finally all traces of the original 
texture are destroyed, and nothing but a black mass 
of the decayed substance is left. This is called 
humus, or vegetable mould, and you are, no doubt, 
familiar, from frequent observation, with its appear- 
ance. Humus, then, may be briefly defined as 
vegetable matter in a state of decay. 
Now, every practical agriculturist knows, that 
when this substance (humus), is mingled with the 
soil, it is productive of much advantage to the suc- 
ceeding crops; but if you ask an unenlightened 
farmer in what way his vegetable manure acts, or 
how his crops are benefitted by it, he will either 
candidly tell you he does not know, or he will 
manifest his ignorance still more plainly by a very 
absurd, and unscientific explanation. If the same 
question be put to others who have given attention 
to the subject, with such aid. as science affords, 
there will be found, among them a diversity of 
opinion ; for the question is one which science has 
not yet solved to the satisfaction of all minds. I 
would here observe that if science sometimes fails 
to grasp every truth within her sphere, this con- 
sideration should not lead you to distrust her use- 
fulness, but rather stimulate you to more thorough 
investigations with the hope of bringing to light 
such truths as still lie concealed. When we reflect 
on what she has done and is still doing, we feel 
that we have no right to set limits to her powers, 
and that we may be constantly looking forward to 
still more brilliant achievements. 
Humus has been frequently analysed, and nu- 
merous experiments have been made to test its 
various properties. As regards its chemical con- 
stituents, they will always be found to vary with 
the vegetables from which it is formed. 
The volatile, or organic constituents of plants, 
viz., oxygen, hydrogen, carbon, and nitrogen, 
compose by far the greater part of their bulk or 
weight — usually over 90 per cent. These are 
called organic constituents, because they enter 
largely into the formation of all organic bodies, 
both animal and vegetable. All the other con- 
stituents of plants, including salts, and earthy or 
mineral substances, are called inorganic consti- 
tuents. As the process of decay proceeds in the 
humus, the organic elements pass again into the 
air. The hydrogen of the vegetable matter unites 
with a portion of its oxygen, and passes off in the 
form of vapor; the remainder of the oxygen unites 
with a part of the carbon, forming carbonic acid, 
which also mingles with the atmosphere. What 
is left of the carbon (for there is not in vegetable 
fibre sufficient oxygen to consume it all), combines 
with a portion of the oxygen of the air ; and now 
the only organic ingredient left is nitrogen. This 
at the moment of its liberation, or in its nascent 
state, unites with a portion of hydrogen forming 
ammonia. 
Such is the process which takes place when 
vegetable matter is decomposed at a temperature 
above 65°, and with free exposure to air and 
moisture ; but as all these circumstances do not 
always combine, the process is subject to many 
variations, all of which, however, produce the 
same result, viz., the separation of the organic 
from the inorganic constituents. Now this process 
corresponds with combustion, or the change which 
vegetable matter undergoes when burned — the 
only difference being the time required for its com- 
pletion — and what is left after the four volatile 
ingredients have been returned to the air, is pre- 
cisely what would have remained in the form of 
ashes, had the vegetable matter been consumed by 
fire. This remainder, or ashes, rarely constitutes 
more than from 4 to 8 per cent, of the whole, and 
consists entirely of inorganic constituents, which 
are most of them variously combined in the form 
of salts. The ashes of rye straw contain the fol- 
lowing salts : silicates of lime, potash and mag- 
nesia ; sulphate of potash ; chlorides of potassium 
and sodium ; and phosphates of lime, magnesia 
and iron. In the humus, then, formed from rye 
straw, all these ingredients will be found, and 
will remain upon the land, or in the soil, where 
the decomposition of the humus takes place. 
It has been supposed by many that nearly all the 
benefit derived from humus must be attributed to 
the carbon it contains. But the fact that plants 
requiring an enormous amount of carbon (as the 
trees of our forests), grow and thrive in soils en- 
tirely deficient in humus, and the facility with 
which all plants extract carbonic acid from the 
atmosphere, seem to prove that humus is not an 
essential source of carbon. These considerations, 
and others which might be mentioned, have led 
others to suppose that humus does not supply any 
carbon to plants, and some have gone so far as to 
oppose theory to the teachings of experience, and 
to maintain that humus is of little or no value in 
supplying food to plants. There are, however, 
few at the present day who deny the beneficial 
effects of humus ; the only difficulty seems to be 
to determine in what way it acts. In giving my 
own views on this subject, I make no claim to infal- 
libility, and will willingly discard them when far- 
ther investigations and discoveries shall prove them 
to be erroneous. 
As regards the' source of carbon, it has been 
proved beyond question, that the leaves of plants 
absorb a large amount of carbonic acid from the 
atmosphere. True, the carbonic acid escaping from 
the humus in the soil, may be absorbed by the 
moisture in the earth, and imbibed with it by the 
roots of plants — still, this quantity must be very 
small. But small as it undoubtedly is, it may 
contribute very materially to the nourishment of 
plants, when growth has just commenced, and 
previous to the development of leaves. Plants 
may also be slightly benefitted by the carbonic acid 
rising from the humus and mingling with the air 
which comes in contact with the leaves, though 
