Agriculture Reference
In-Depth Information
Factors Affecting Soil Temperature
Color
affects the albedo of a soil surface. For example, black and dark red surfaces
reflect less radiation and therefore warm up more than light-colored surfaces. But
at night, dark surfaces radiate energy (by longwave radiation) faster than light-
colored surfaces. This radiation loss adds to the transfer of sensible heat by the
process of convection. Bare soil surfaces with no vegetative cover lose heat rapidly
at night, especially if there is no low cloud cover. However, the vine canopy traps
heat emanating from the soil after dark and reduces heat loss at night, so that the
surface soil temperature and near-surface air temperature remain higher than oth-
erwise expected. This is important for frost avoidance.
Because of their low thermal conductivity,
mulches
of organic material such
as straw or bark pieces reduce
J
H
(either downward during the day or upward at
night). The presence of a mulch therefore moderates the temperature fluctuations
between day and night (section 3.5.3), which can be particularly important in
warm climate vineyards where high temperatures in the vine rows promote high
soil evaporation rates. Lower daytime soil temperatures also favor earthworm ac-
tivity and slower rates of organic matter decomposition. However, the insulating
effect of a mulch may increase the chance of early morning frost in vineyards
where this is a risk. Further, because both cover crops and mulches have a higher
albedo than bare soil, the energy absorbed by the soil, and hence soil warming, is
less under these covers, thereby increasing the risk of frost.
The effect of a given value of
J
H
on soil temperature depends on the soil's
heat capacity per unit volume (
C
h
), and its thermal conductivity. Water has a
much higher
C
h
than dry soil, so wet soil must absorb much more heat than dry
soil to change the temperature by 1°C. In vineyards in cool climates, soil wetness
can therefore be an important factor determining when root growth will occur.
Vine roots start to grow at temperatures around 6°C (43°F), but they have an op-
timum close to 30°C (86°F).
3.5.2
Diurnal and Annual Temperature Changes
The change in the direction of soil heat flux downward during the day to out-
ward at night causes the soil temperature to change in a sinusoidal way over a 24-
hour period. This is called the diurnal temperature variation. The longer term
variation in heat flux between summer and winter causes a similar sinusoidal vari-
ation of much longer period—the annual temperature variation. These diurnal
and annual variations in temperature
T
can be described by the equation
2
3.5.3
t
T
T
A
A
sin
(3.8)
where
t
is time,
T
A
is the average temperature (daily or annual),
(tau) is the pe-
riod of the sine wave (either 1 day or 1 year), and
A
is the size of the tempera-
ture change.
An example of diurnal variation in soil temperature is shown in figure 3.9.
The size of the variation is greatest at the surface and decreases with depth. Within
a 24-hour period, either the maximum or minimum temperature is attained at a
progressively later time as the depth increases. In addition to heat being trans-
ferred in a vertical direction across the soil surface, heat is transferred by lateral
