Two physical properties, aeration and temperature,
exert a profound effect on soil life. Each soil-reliant organism has an optimal
temperature and both high and low temperatures at which growth ceases. Likewise each
macroorganism and most microorganisms need O2 to complete their life cycle.
Soil color can be a helpful indicator of aeration, mineralogy and humus content of a soil.
- Natural respiration consumes O2, and produces
CO2. Soil air is lower in O2 and higher in CO2 than
the atmosphere. This is due to the difficulty with which gases in the soil exchange with
gases in the atmosphere.
- Dry atmospheric air has a fairly stable makeup. It
contains (by mole fraction):
Plus small amounts of other gases
These contents apply only to dry air, because water vapor (%RH) varies greatly, ranging
from 20 to 100% relative humidity.
- Rate of O2 exchange and rate of O2
consumption determine O2 content in soil. Diffusion can be rapid or slow,
but will most certainly be slower in the soil than in the atmosphere. Because wet soils
have very little air-filled porosity, they can be anaerobic. Also, deep soils, clay soils,
and heavily compacted soils are at risk for being anaerobic.
- Anaerobic conditions affect soil biology and chemistry.
As normal respiration ceases, anaerobic glycolysis proceeds. This is an inefficient and
generally undesirable way to produce useable energy. Some microorganisms have the ability
to reduce substances other than O2, especially NO3-, SO42-,
and others (see Table 3-5 in the textbook). Anaerobic products are often toxic, and are
- Soil color reveals information about the soil. White
specks are usually salts or lime. Dark colors usually indicate the accumulation of
microscopic pieces of organic matter. Mottles indicate periods of poor aeration. Gray,
colorless soils are usually anaerobic.
- Color notation is defined by hue, value, and chroma.
The Munsell color book is the standard from which colors are described. The hue is the
rainbow color; red, yellow, etc. The chroma refers to purity of color, and is analogous to
how much tint is put in a bucket of paint. The value is the relative blackness or
whiteness, in other words, it describes what shade of gray the paint was before the tint
- Soil temperature affects soil biology and chemistry.
Each organism has an optimum temperature for growth and reproduction. Some plants require
heat units or chill units to complete their annual cycle. Temperature also affects
chemical weathering--it proceeds more rapidly at higher temperatures. Temperature affects
phase changes of water (evaporation & freezing).
- Many factors affect soil temperature. The amount of
heat absorbed is controlled by such factors as: reflection (at least 30% of solar
radiation is reflected back into space), slope and aspect (a south-facing slope receives
sunlight at higher intensity than does a north-facing slope), color and cover (affect
reflection and wetness), specific heat (water has a much higher specific heat than does
dry soil), photosynthesis (about 3% of solar energy is trapped by photosynthesis), and
Most energy is used for evaporation. To evaporate 1 g water requires 540 cal @ 20 oC.
This is a very high number. Evaporation absorbs latent heat which can be released upon
Heat is transported by conduction and other mechanisms. Heat conduction is sinusoidal (see
Figure 3-9 in the textbook). Note that heat flow from soil surface to the subsoil
undergoes a time lag and a damping effect.
- Aeration and temperature are somewhat manageable.
Management of temperature is accomplished by mulching (i.e., covering the soil with
plastic, organics, etc.). Both aeration and temperature are affected by soil water
management techniques such as irrigation and drainage.
Students are encouraged to look up the following
vocabulary words in the textbook glossary or elsewhere and to browse the following
The Merriam-Webster page is a convenient place to look-up