PARING AND BURNING
“You see that man over there on the hillside,” said Uncle Paul,
pointing to a laborer who, with a large hoe, was paring the ground, so
to speak, by shaving off great squares of earth covered with grass and
weeds and shrubs. “You see how he stands those pieces up, either in
pairs, back to back, or one at a time, so bent or vaulted that they
will stay upright by themselves. Thus the air is allowed to circulate
and dry them rapidly. If we come back in a few days, after sun and air
have done their work and the drying process is complete, we shall find
our man there again at his work; and we shall see how he piles up the
turf with the earthy side upward and outward. In the middle of the pile
he leaves a cavity which he fills with brushwood and dry leaves. Then
he sets fire to the whole. A second pile is constructed in the same
manner and likewise set on fire. Soon the entire hillside is covered
with a great number of these small furnaces, burning slowly and sending
out long trails of smoke. In a few days, three or four at most, the
fires burn themselves out, and then, as soon as all the piles are cold,
the mixture of ashes and calcined earth is spread over the ground with
a shovel. This agricultural operation is known as paring and burning,
and is carried out for the purpose of rendering arable a tract of land
not yet under cultivation and still covered with wild vegetation.
“The operation of paring and burning produces two effects, one with
reference to the clay in the soil, the other having to do with the
ashes left from the burning of the weeds. Clay, as you know, is a
tenacious, binding substance, impervious to both air and water.
Consequently a soil that is too clayey is unfavorable for vegetation,
furnishing the roots with insufficient air and moisture. Now, when clay
is heated to a high temperature, it acquires very different properties:
it no longer makes paste by the addition of water, but is porous,
permeable, and readily admits air and water. The paring-and-burning
process, therefore, improves an argillaceous soil by calcining the clay
and rendering it permeable. That is as much as to say that if paring
and burning are beneficial to heavy or clayey soils, they are, on the
other hand, harmful to those that are light or sandy.
“Finally, the operation just described affects the soil through the
ashes of the burnt weeds. After the combustion of all vegetable matter
there remains an earthy powder or ash comprising the mineral substances
contained in that vegetable matter, substances unchanged by combustion
because of their great resistance to heat. The most important of these
is potash. All the ingredients that once belonged to the burnt plants
are evidently adapted to the formation of new plants. The ashes, then,
of the weeds consumed in the process of paring and burning will be very
useful to the plants about to be raised on the land that has been burnt
over. By the burning, however, it is impossible to turn to account all
that the weeds contained: what escapes in the form of smoke is so much
lost. Hence care should be taken not to carry combustion too far. In
this connection the calcined clay renders still another service. By
becoming porous through calcination its nature is altered so that it
can absorb and retain the gaseous products of combustion and thus save
just so much waste. But if a soil lacks clay, paring and burning are
harmful, and it is better simply to turn the weeds under, whereupon
they will be converted into mold instead of being dissipated in the
atmosphere as smoke.
“Ashes other than those resulting from paring and burning are also used
as an agricultural fertilizer, though they are rarely put to this use
just as they are, because the contained potash, a highly valuable
substance, is first extracted by leaching. After this process the ashes
are called buck-ashes. They contain silica and also carbonate and
phosphate of lime, all in a condition most favorable for assimilation
by plants. Of less strength than ordinary ashes, leached ashes
nevertheless produce good results, especially on clayey soil. Coal
ashes, too, it should be added, serve to lighten a heavy soil since
they contain a large proportion of calcined clay.
“The subject of ashes leads us naturally to that of soot, a substance
composed of vegetable matter incompletely decomposed by heat and
containing ammonia, which renders it highly efficacious as a
fertilizer. It is applied to young plants, giving them an increased
vigor of growth. By its acrid quality, moreover, it is excellent as a
protection against insects that attack vegetation.”