Page 149 - Biosystems Engineering

128 Cha pte r F o u r

there can be additional heat supply from warm air or substrate-
heating systems. When greenhouse substrates are heated by a heat-
ing system, temperature variations are greater than in the soil out-
doors and temperature distribution is fully conditioned by the
geometry of the heating system. In heated greenhouse substrates,
high temperature gradients occur that affect not only temperature
and heat flow, but also the moisture profile, with lower moisture val-
ues in high-temperature zones (Rodriguez et al. 2006).

4.2 Soil Thermal Properties
Seed germination, crop emergence, and subsequent establishment
are affected by the microclimate, which is largely influenced by soil
thermal properties (Ghuman and Lal 1985). For this reason, during
the last few years, considerable efforts have been devoted to develop-
ing techniques that are useful in the determination of soil thermal
properties.

4.2.1 Specific Heat or Heat Capacity
The specific heat per volume or volumetric heat capacity of a soil, C ,
V
is the ability of the soil to store heat per unit volume and unit tem-
perature, and can be expressed as the amount of heat required to raise
a unit volume of a soil by 1°C of temperature under isobaric condi-
–3
tions, measured in J m °C :
–1
C = 1 ⎛ ⎜ dQ⎞ ⎟ (4.1)
⎝
V V dT⎠
–3
where V is volume (m ), Q is heat (J), and T is temperature (°C).
Similarly, the heat capacity per unit mass, measured in J kg °C ,
–1
–1
is defined as
1 ⎛ dQ⎞
C = (4.2)
⎜
⎝
M M dT ⎠ ⎟
where M is mass (kg).

4.2.2 Effective Thermal Conductivity
The thermal conductivity k is defined as the ability of the soil to trans-
–1
–2
fer heat by molecular conduction and is measured in W m s °C –1
(Porta et al. 1999). However, in porous media, we must consider the
apparent or effective thermal conductivity k , which includes con-
e
vection of latent heat and conduction. Because the soil is a granular
medium consisting of solid, liquid, and gaseous phases, thermal