Agriculture Reference
In-Depth Information
the energy lost by evaporation‚ is the energy radiated from the surface and Q is
the energy gained or lost by the soil through conduction and convection of the air‚
and conduction by the soil. If Q is positive‚ the soil surface heats up and will transmit
energy to the lower layers of the soil‚ if they are cooler. Where Q is negative‚ the soil
surface cools and gains energy from the subsurface layers‚ providing they are warmer.
Transmission of heat to the lower soil layers depends on the thermal capacity and
the thermal conductivity of the soil. Thermal conductivity is dependent on the particle
size distribution‚ the water content‚ the bulk density and‚ in organic soils‚ the organic
matter concentration. Due to the high thermal capacity of water in comparison with
that of the dry soil‚ the thermal capacity of the soil is much higher when it is wet.
Additionally‚ the high thermal conductivity of water and its intimate contact with the soil
ensures that wet soils transmit heat readily. Because of rapid heat transmission to greater
soil depths‚ wet and water-logged soils take much longer to warm in Spring‚ a matter
of considerable importance to agriculture in colder climates. Soil water content thus
plays a major part in controlling the absorption and transmission of heat.
From the foregoing‚ it is clear that both thermal capacity and conductivity will
normally vary substantially down the profile and between soils. Additionally‚ regular
temporal variation may be expected with seasonal and other changes in soil water
status and with surface radiation.
2.1.2
DAILY AND SEASONAL VARIATION
Because of latitudinal differences‚ the radiation incident on the soil surface increases
systematically from the poles to the equator‚ although similar patterns of daily and
seasonal variation may be expected. Additional local variation results from changes
in vegetation cover‚ aspect‚ slope and exposure.
Daily temperature variation at the soil surface follows an approximately sinusoidal
pattern and produces a wave that is propagated into the soil. This quickly attenuates
with depth and the amplitude of daily variation is slight below approximately fifty
centimetres. The rate of penetration of the wave of heat energy through the soil is
relatively slow leading to the situation where daily maxima at depth lag substantially
behind surface temperatures.
The above points are illustrated with data from two very different locations. Figure 1.2la
presents the march of temperatures over a 24-hour period during summer at a site near
the Shackleton Glacier‚ Antarctica (84°30'S‚ 174°W) (Wise and Shoup‚ 1971). This area
is near the southern limit of animal existence and the site supports a sparse population
of Acari based on a simple microbial food chain (Wise and Gressitt‚ 1965). Soils at
the site remain frozen for much of the year but‚ during summer‚ thaw daily to depths of
at least 15 cm for sufficient periods to permit the animals to complete their life cycles.
The lag in temperature change at depth is illustrated by data from the different depths;
the maximum occurred at the surface at 1400 h while that at 15 cm did not occur until
1600 h. Air temperature during the 24 hour period of observations ranged between
-6 °C to 4.5 °C. Surface temperature varied more widely than that at depth; the rapid rise
and fall of temperature at 0600 h was due to a brief period of insolation. The biota of this
