n the average but 1/2500 of its
bulk of this gas, whereas plants flourish in air containing a larger
quantity, and, in fact, their other wants being supplied, they grow
better as the quantity is increased to 1/12 the bulk of the air. These
considerations make sufficiently obvious how important it is that the
soil have in itself a constant and abundant source of carbonic acid gas.
As before said, _organic matter, in a state of decay_, is the single
material which the farmer can incorporate with his soil in order to make
the latter a supply of this most indispensable form of plant-food.

When organic matters decay in the soil, their carbon ultimately assumes
the form of Carbonic acid. This gas, constantly exhaling from the soil,
is taken up by the foliage of the crops, and to some extent is absorbed
likewise by their roots.

Boussingault & Lewy have examined the air inclosed in the interstices of
various soils, and invariably found it much richer (10 to 400 times)
than that of the atmosphere above. Here follow some of their results:

CARBONIC ACID IN SOILS.
--------------------------------------------------------------------------
Key:
A - _Volumes of Carbonic acid in 100 of air in pores of Soil._
B - _Cubic feet of air in acre to depth of 14 inches._
C - _Cubic feet of Carbonic acid in acre to depth of 14 inches._
D - _Volumes of Carbonic acid to 100 of air above the soil._
E - _Cubic feet of air over one acre to height of 14 inches._
F - _Cubic feet of Carbonic acid over one acre to a height of 14 inches._

--------------------------------------------------------+-----+------+----
_Designation and Condition of Soil._ | A | B | C
--------------------------------------------------------+-----+------+----
Sandy subsoil of forest |0.24 | 4,326| 14
Loamy " " " |0.82 | 3,458| 28
Surface soil " " |0.86 | 5,768| 56
Clayey soil of artichoke field |0.66 |10,094| 71
Soil of asparagus bed, unmanured for one year |0.79 |10,948| 86
" " " " newly manured |1.54 |10,948| 172
Sandy soil, six days after manuring, and three | | |
days of rain.|2.21 |11,536| 257
" " ten " " " " " | | |