Agriculture Reference
In-Depth Information
Box 3.6
Calculating Air Capacities for Vineyard Soils
The air capacity is a good indicator of soil aeration for vines (section 3.4).
Many compacted soils have low total porosity, poor drainage, and a low air
capacity. For example, suppose the
BD
of a compacted soil is 1.6 Mg/m
3
. From
equation B3.4.3, this corresponds to a porosity of 0.40 m
3
/m
3
. If the
FC
value is
0.35 m
3
/m
3
, we have
C
a
0.40
0.35
0.05 m
3
/m
3
(B3.6.1)
An air capacity of 0.05 m
3
/m
3
or 5% is too low for adequate soil aeration for vines.
However, suppose we have a well-structured loamy soil of
BD
1.2 Mg/m
3
.
The porosity will be 0.55 m
3
/m
3
, and if the
FC
value is 0.40 m
3
/m
3
, we have
C
a
0.55
0.40 m
3
/m
3
0.15 m
3
/m
3
(B3.6.2)
An air capacity of 0.15 m
3
/m
3
or 15% should provide good soil aeration and
healthy root activity. The minimum air capacity for adequate aeration is 10% (see
box 3.7).
slows. The soil water content
at this point (in the absence of evaporation) de-
fines the
field capacity
(
FC
). The pores drained at field capacity are sometimes
called
macropores
, and correspond to the pores and cracks between the larger soil
aggregates. Macropores are mainly created by soil fauna and roots (collectively
called
biopores
) and by the shrinkage on drying of large aggregates in clay soils.
The smaller pores within aggregates that remain water-filled at the
FC
are called
micropores
. The distinction between macropores and micropores is useful for defin-
ing the soil's
air capacity,
(
C
a
), which is obtained from equation 3.6 by substi-
tuting the value for
at the
FC
. The calculation of air capacity is illustrated in
box 3.6.
The subdivision of porosity into macropores and micropores, although use-
ful, is a simplification of reality because the distribution of pore sizes in soil is
continuous from large to very small. This distribution can be estimated from the
soil water retention curve (section 6.2.1). It is also important to understand the
empirical relationship between pore size and function, as outlined in table 3.3.
Relationship Between Pore Size and Function in Soil
Table 3.3
Pore Diameter (
m)
Associated Biotic Agent and Function
5000-500
Earthworm channels and main plant roots; pores for rapid drainage
and aeration
500-30
Grass roots and small mesofauna; normally draining pores; aeration
30-10
Fine lateral grass roots and fungal hyphae; very slowly draining pores
30-0.2
Root hairs, fungal hyphae, and bacteria; “available water” storage
0.2
Swell-shrink water in clays; residual or “nonavailable” water storage
Source:
After Cass et al. (1993) and White (1997)
