Agriculture Reference
In-Depth Information
Box 3.7
(continued)
where
is the air-filled porosity and
f
is an impedance factor. The value of
f
,
which is between 0 and 1, changes with
in a complex way that depends on a
soil's structure, texture, and water content. As water fills the small pores, the
pathway for diffusion in the air becomes more tortuous and
f
decreases. Once the
macropores between aggregates begin to fill with water,
f
decreases rapidly as
0.1 m
3
/m
3
. This is why the lower limit for
an acceptable air capacity in many soils is 0.1 m
3
/m
3
or 10% (box 3.6). At
decreases, falling close to zero at
0.1 m
3
/m
3
, many of the remaining air-filled pores are discontinuous or “dead end.”
usually associated with a disproportionately large decrease in the macroporosity.
Consequently, although the value of the impedance factor
f
may be as high as
0.6-0.7 in a well-structured soil, it may fall as low as 0.1 in an overcultivated, de-
graded soil at the same water content.
Even in a well-structured soil with adequate air capacity, the interiors of large
aggregates may become anoxic under wet conditions (fig. 3.7) because O
2
has a
low solubility in water. Although the O
2
partial pressure in the air-filled macro-
pores may be 20 kPa, because of its low solubility and very slow rate of diffusion
in water, the partial pressure of O
2
within aggregates more than 3-4 cm in di-
ameter may be close to zero. CO
2
, on the other hand, is relatively soluble in wa-
ter (27 times more soluble than O
2
at the same temperature and pressure). When
the O
2
pressure falls to zero in the center of an aggregate, the CO
2
partial pres-
sure due to respiration at the center will only rise to about 1 kPa. The critical ag-
gregate size to cause such anoxic conditions depends on the soil respiration rate,
which, in addition to depending on the supply of C substrate, is highly depen-
dent on soil temperature.
Air-filled macropores
Anoxic zones within
large wet aggregates in
a structured soil
(redrawn from White
1997).
Figure 3.7
O
2
free zones inside
Saturated soil
the wet aggregates
