258 
BOYS DEPARTMENT. 
Bogs’ ^Department. 
AGRICULTURAL CHEMISTRY, No. 5. 
Boys, in my last letter, I explained the manner 
m which plants obtain their carbon, and told you 
that this element was derived chiefly from the car¬ 
bonic acid contained in the atmosphere. It will now 
be my object to show in what manner the other con¬ 
stituents of air and what contribute to the suste¬ 
nance of vegetable life. 
1. Oxygen .—I have already described this gas, 
and in treating on the absorption of carbonic acid, 
I explained one method by which it is obtained. 
It noAv only remains to say there is but one other 
source for its supply; namely, the moisture or 
water, which the roots of plants absorb. These 
two sources furnish really more oxygen than the 
plant requires; yet the hydrogen of the water is 
needed, as well as the carbon of the carbonic acid; 
consequently, when these two compounds (water 
and carbonic acid), are decomposed, there will re¬ 
main a surplus of oxygen to be returned to the air. 
In this method the growth of vegetation' tends to 
restore to the atmosphere a portion of the oxygen 
consumed by animal respiration. 
2. Hydrogen. —-The principal source of this ele¬ 
ment is undoubtedly the water which is taken up 
by the roots -of plants. The constituents of the 
water (oxygen and hydrogen), being separated, 
each performs its proper office. The peculiar con¬ 
nexion which the several elements have with va¬ 
rious parts of the vegetable structure will be ex¬ 
plained when treating hereafter of vegetable organ¬ 
ization. Another source of hydrogen is ammo¬ 
nia, though this probably furnishes but a limited 
supply . 
3. Nitrogen. —Different opinions have been and 
still are entertained, as to the manner in wdiich 
plants obtain their nitrogen, and it is a matter of no 
little moment to have correct views on this subject. 
Carbonic acid, oxygen, and hydrogen.are supplied 
in such a manner as to be almost independent of 
artificial means, and it is therefore of less practical 
importance to determine and understand the precise 
mode of their operation than that of nitrogen. 
The analysis of plants shows that this element is 
alwajrs present in their organization, and the expe¬ 
riments of chemists have proved that they will not 
grow without it; yet, owing to its inert character, 
or to an inadequate supply, there may not always 
be so much furnished by nature as is needed to en¬ 
sure a luxuriant growth, or abundant yield of grain. 
Indeed, we are indebted to this element for much of 
the benefit resulting from the use of some of our 
best fertilizing agents. The mode of supplying 
nitrogen by artificial means will be considered when 
we come to the subject of momures . 
Among the theories that have been advanced to 
explain the manner by which plants obtain their 
nitrogen, one is, that it is derived from the uncom¬ 
bined nitrogen of the air. But the experiment has 
been made of confining a plant in a limited quantity 
of air, and it has been found that the nitrogen con¬ 
tained in this air was not diminished by the growth 
of the plant. Again, it seems quite improbable that 
an element so inert in its nature, and so difficult 
even to force into combination with other substan¬ 
ces, should voluntarily forsake its connexion with 
the atmosphere, and form a new one in the vegeta¬ 
ble constitution. Another opinion is, that the nitric 
acid found in the air during a thunder shower, and 
brought down with the rain, imparts its nitrogen to 
plants; and another that decaying vegetable mat¬ 
ter in the soil is the principal source of nitrogen. 
These several opinions have been discarded by 
some of our best modern chemists, since the disco¬ 
very of ammonia in the atmosphere, and this last 
agent (ammonia), they believe to be the only one 
for transmitting the element under consideration. 
Yet it seems quite probable that a portion of am¬ 
monia may be supplied as well by decaying vege¬ 
table as by the atmosphere. Still, we must regard 
the atmosphere as the great reservoir from which 
most of this compound is obtained. Ammonia, 
you know, is composed of hydrogen and nitrogen, 
and I have told you that it is very soluble in water, 
and is always found in rain water. It is always 
present as a constituent of the air, and consequent¬ 
ly, like carbonic acid, is brought to the earth with 
every shower, and thus brought in immediate con¬ 
tact with the roots of plants. It is absorbed with 
the water which the roots imbibe; it is decompos¬ 
ed, like water and carbonic acid, and during the 
life of the plant is not returned to the air. 
It has been ascertained by chemists that the 
quantity of nitrogen necessary to sustain an ordi¬ 
nary crop does not exceed thirty pounds to the acre, 
though cabbages, turnips, and some other crops 
carry off upwards of a hundred pounds. All plants 
evolve nitrogen from their leaves, and this nitro¬ 
gen is undoubtedly furnished by ammonia. 
Production of Ammonia .— We now come to the 
inquiry, by what means is the ammonia thus ab¬ 
stracted from the air, restored. Although there 
are several channels through which it may pass 
into the air, yet the principal one, and the only 
one now deserving attention, is, the putrification of 
substances containing nitrogen. As animal sub¬ 
stances always abound in this element, their decay, 
or putrefaction, is always attended with the forma¬ 
tion and emission of ammonia. When animal or 
vegetable substances are in a state of decay, as all 
the elements they contain forsake their former con¬ 
nexions, they must either form new ones, or remain 
in the earth or air uncombined. They most fre¬ 
quently form new connexions, and it has been dis¬ 
covered that when a gas is first liberated from any of 
its combinations, it has a stronger tendency to unite 
with other bodies than under any other circum¬ 
stances. This point of time , or moment of liberation, 
is called the nascent state. Now, although nitrogen 
in its natural state may show little inclination to 
form a union with hydrogen, yet in their nascent 
state these two gases eagerly combine, and the re¬ 
sult is the formation of ammonia. This gas is thus 
continually forming, so that the atmosphere can 
never become exhausted. 
I have now explained (as plainly and concisely 
as possible), the manner in which the several essen¬ 
tial constituents of the atmosphere are made to 
contribute to the growth of vegetation. My next 
business will be to point out the constituents of the 
soil, and show in what manner their influence is 
exerted. J. McKinstry. 
Greenport , Columbia Co., N. Y. 
