Soil Temperature and Atmosphere
To observe how soil temperature is affected by soil moisture, soil texture, and organic residue
To calculate the total energy adsorbed by soils in a given period of time
To observe how flooding changes the soil atmosphere and the redox potential of a soil INTRODUCTION Soil Temperature
Soil temperature has significant impact on many soil properties, such as chemical reactions, microbial
growth, and plant growth. Chemical reaction rates increase as soil temperature increases. The Q10 is a
rule-of-thumb that says that the rate of a chemical reaction will double for every I 0°C increase in
temperature. Therefore, soil temperature is important in determining the chemical dynamics of a soil and
ultimately, the nutrient supply for plants. Soil temperature also affects which members of the microbial
population are active at any given time and the rate of their biochemical reactions. In addition, soil
temperature affects the rate of plant root growth and biochemical activity. In the spring, many seeds do not
germinate until the soil warms to a certain temperature, and if the seeds do germinate their root systems
will tend to grow more slowly in cool soils. ) The primary source of heat energy for soils is from solar radiation. Pathways of energy adsorption within
the soil are direct and indirect, with most soil heat energy coming from the direct adsorption of solar
radiation and diffuse scatter radiation.
Once solar radiation is adsorbed by the soil, the increase in soil temperature is a function of several factors
including soil water content, the amount of organic residue on the surface, soil texture and slope aspect. Of
these factors, soil water content has the most affect on the rate at which soils warm and cool. The large
specific heat of water requires large quantities of energy must be acquired in order to increase the
temperature of the water. Therefore, wetter soils are buffered against temperature changes, they will warm
up and cool down slower than drier soils. Specific heat is defined as the amou~t of energy (cal) needed to
raise 1 g of material 1°C (Eq. 9 .1 ). Soil solids and water have different specific heats. The specific heat for
soil minerals and water are 0.2 cal g· 1 0 C- 1 and 1.0 cal g· 1 0 c· 1, respectively. Spec1'fi1c Heat =-cal- go C Eq. [9.1) Therefore, the energy needed to raise 24 g of soil minerals and 5.4 g of water 1°C is given in the following
example.
Energy= (mass soil *Specific Heat minerals)+ (mass water *Specific Heat water) * l[ 0.2 cal
0
+ 5.4 gwater * I.0 cal
* 0 ]- 10 •2 ca1
gsoil C
gwater
C
There are many pathways for heat loss from soils, including reradiation, evaporation transpiration, and
reflection. -[ 24 gsoil
cal- 75 Soil Atmosphere
The amount of air in the soil has a major effect on biological and chemical reactions in the soil. Soil air
contains the same constituents present the atmosphere, but in different concentrations (Table 9 .1 ). In
general, soils have lower 0 2 and higher C02 concentrations than the atmosphere due in part to respiration of
plant roots and soil microbes. Respiration can be summarized by the reaction: Table 9.1. Average amount of compounds found
in the soil and in the atmosphere
Air
Soil
Compound
(%)
78
78
N2
20
21
02
0.035
0.5
C02
20-90
95-99
H20
Respiration is the conversion of reduced C to oxidized C resulting in the release of energy. During
respiration, 0 2 is used as the terminal electron acceptor thereby allowing organisms to carry out aerobic
respiration. When 0 2 is not present, some organisms are capable of using other compounds as an electron
acceptor for alternate respiration pathways. To determine if a molecule is being used as an electron
acceptor, one can measure the redox potential (Eh) of a soil in voltage (V). Compounds that undergo redox
reactions do so at characteristic Eh values. Therefore, by knowing the Eh of a soil we can establish which
form (i.e. oxidized or reduced) of an element is pr~sent (Table 9.2). If the Eh measurement is more positive
than the characteristic Eh value, then the oxidized form of that compound or element is present. If 1
h
measurement is more negative than the characteristic Eh value, then the reduced form is present. • or
example a soil with a measured Eh of 0.220V would indicate that most of the manganese would be in the
Mn2+ state but the iron would be in the Fe3+ oxidation state.
Table 9.2. Characteristic redox potentials (Eh) for selected
compounds which are often present in the soil
Element
Oxidized Form Reduced Form
Eh (V)
0.400
0
02
H 20
N
N0 3
NH 4
0.300
Mn
Mn4+
Mnz+
0.250
Fe
Fe3+
Fez+
0.200 s sot H2s -o.soo C C02 CH4 -0.100 Review Questions
1.
2.
3.
4. Why does it take longer to heat a wet soil versus a dry soil?
Why is soil temperature important for plant growth?
What are the ways heat energy is lost from the soil? Which one is most important?
Why does the composition of soil air differ from the atmosphere? REFERENCES Jacobs, H.S. and R.M. Reed.
Madison, WI. 1964. Soils Labo~atory 76 Exercise Source Book. American Society of Agronomy.
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