Page 147 - Biosystems Engineering

126 Cha pte r F o u r

growth, three are ubiquitous: soil temperature, moisture, and mechan-
ical impedance (Finch-Savage et al. 2001).
Besides emergence, plant development is affected by temperature
in later growth phases. Thus, although most crop yield simulations
are based on air temperature for thermal time calculations, it has been
demonstrated that using temperature at a soil depth of 3 to 5 cm
instead of air temperature to simulate maize development increases
simulation accuracy (Vinocur and Ritchie 2001). Similarly, Awal and
Ikeda (2002) obtained positive correlations between soil temperature
and rates of development of phenophases different from germination
(leaf appearance, branching, flowering, pegging, and podding).
Respiration and other plant metabolic processes, such as symbi-
otic nitrogen fixation, photosynthesis, or transpiration, are also
affected by temperature. Results reported by some authors who ana-
lyzed variations in air and soil temperature and their effects on plants
have evidenced that synchronous variation of air and soil tempera-
ture benefits nitrate metabolism (Gent and Ma 2000).
Similarly, soil temperature affects water availability to the plant.
Water availability is lower in soils at very low temperatures, which
affects nutrient uptake and, therefore, the spatial distribution of roots
(Villalobos et al. 2002). Moreover, temperature enhances or inhibits
soil microbial activity; for instance, nitrification is inhibited at low
temperatures, whereas organic matter decomposition increases.
Other important processes, such as the biodegradation of pesticides
or other chemical and organic compounds, are dependent on soil
temperature (Porta et al. 1999).

4.1.2 Factors Affecting Soil Temperature
Factors affecting soil temperature can be classified into environmen-
tal factors and factors inherent to the soil. Environmental factors
comprise solar radiation, condensation, evaporation, rainfall, and
vegetation, while factors inherent to the soil comprise specific heat and
thermal conductivity, biological activity, soil texture, soil structure,
radiation from the soil into the atmosphere, and soil moisture (Jackson
and Taylor 1965). Another classification divides the factors affecting
soil temperature into factors that contribute to heat availability and
factors that contribute to dissipation of heat (Hanks 1992).
Solar radiation transmits a certain amount of heat to the soil. The
amount of heat transmitted is dependent on atmospheric conditions,
season, time of day, and latitude. When solar radiation strikes the
soil, the soil is heated, reflecting a fraction of solar radiation and
transmitting another fraction toward deeper layers. The proportion
of these three fractions depends on the following factors:
1. Albedo: The fraction of incoming radiation that is reflected at
the crop or soil surface. The albedo depends on the nature of
the surface, the angle of the sun, and the latitude.