Agriculture Reference
In-Depth Information
, and the
air-filled porosity
is defined as (Porosity
umetric water content
). The
minimum value of air-filled porosity for adequate soil aeration is 10%.
■
The drainage and aeration of soil depend mainly on the distribution of pore sizes.
Pores and cracks between large aggregates that are drained at the field capacity are
called
macropores
; they include many of the pores created by burrowing soil fauna
and plant roots (the biopores). Pores that still hold water at the field capacity are
called
micropores
.
■
Respiration by roots and soil organisms consumes O
2
and produces CO
2
. In a
well-aerated soil, the O
2
partial pressure in macropores is approximately the same
as in the atmosphere (ca. 20 kPa), and that of CO
2
is 0.1-1 kPa. However, in
the interior of aggregates 3-4 cm in diameter, which remain saturated, the O
2
partial pressure can fall to zero. The soil is then a mosaic of aerobic and anaero-
bic zones.
■
The development of anaerobic zones depends on the soil's respiration rate, which
is markedly influenced by soil temperature. Soil temperature is determined by the
net absorption of the sun's radiant energy, which is influenced by the surface cover
and soil color.
■
Dark-colored soil surfaces absorb more heat than light-colored surfaces during the
day, but radiate it more rapidly at night. Mulches reduce the soil heat flux either
upward or downward, thereby moderating the diurnal soil temperature fluctua-
tions, which are minimal below a depth of 20 cm. Soil temperature also fluctu-
ates in a sinusoidal fashion from summer to winter. The temperature change is
greatest at the surface and is attenuated with depth.
