Glossary of Soils 4

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Factors that efffect soil heating from solar energy
Aspect, vegetative cover, color/albedo
Which soil warms faster - wet or dry?
Dry. Wet soil such as in the spring warms more slowly
cal/gram of energy need to raise the temperature of water 1 degree, and mineral soil?
1 cal/gram H20

.2 cal/gram soil
Methods of providing energy for soil cooling through evaporation
Solar energy
Soil heat
Long wave radiation from earth
Amount of energy needed to evaporate 1 g of water
540 g
How does heat move through soil?
By conduction. From areas of heat(energy) to cooler areas (less energy).
Most conductive to least conductive elements of soil
Water - Mineral soil - air
Paradox of the role water plays in the movement of heat through soil.
Water enhances heat transfer through high conductivity but requires more energy to get hot in the first place
Describe soil variability
Varies by season, depth, location but in predictable patterns. More variable at the surface, lag time for max and min temps.
How do humans impact soil temperature?
Remove vegetation which broadens extremes. Mixing which removes insulating organic horizons. Buildings and pavement heat up nearby soils. Irrigaion adds H20.
Properties of organic and plastic mulches
Organic mulches tend to cool the soil and reduce temperature fluctuations. Plastic mulches heat the soil, clear more than black, but black keeps light from causing seed germination.
Typical percentages of soil air compared to the atmosphere.
Atmosphere is 21% O2 and .03% CO2. Soil air variers from 5-20% O2 and .35 to 10% CO2.
Other gases besides O2 and CO2 that are found in soil.
N20, H2S, methane - products of organic decomposition - especially anerobic.
Why is soil O2 lower?
Plant root respiration and limitation on soil air movement
The process of soil aeration is
the exchange of soil gases with gases in the atmosphere. Release CO2, absorb O2.
Two methods of soil aeration
Mass flow and Diffusion
Mass Flow
Bulk movement of air, no gas exchange. Happens mostly in macropores (.08mm or bigger). Rapid.
Gas exchange from areas of high concentration to low concentration. Happens in both macro and micro pores. Slow. Only method of aeration in micropores
Why is soil aeration important?
Oxygen is needed by roots and microorganisms to respire and make energy to take up nutrients. As O2 goes down, switch to more anerobic, lower energy reactions, slows decomposition, plant ability to take up nutrients.
Key to good soil aeration is
Maintaining soil aggregates (macropores) and reducing barriers.
Polarity of water
Water is slightly positive on H side and slightly negative on O side.
Hydrogen bonding
Bonding of H side of water to other cations or other water molecules.
Desire of water molecules to stick together - results from polarity and hydrogen bonding.
Tendancy of water to stick to surfaces because of polarity - surface tension.
How water is drawn up a small tube or through small spaces in soil because of cohesion and adhesion. Inverse relationship between height of water and radius of the capilary.
Factors effecting soil water movement
capilarity and gravity
Soil water potential energy
Refers to the relative ability of water to do work in soil relative to a reference state of pure water
What is the reference state for measuring soil energy potential?
Pure water
Components of soil water potential
Elevation - gravity
Adhesion and cohesion - matric
Dissolved salts - Osmotic potential
Describe gravity as a component of soil water potential
Moves downhill (high to lower energy). Always positive value.
Describe matric potential as a component of soil water potential
Water bound by cohesion or adhesion has less energy - is a negative value compared to the reference state. Water moves from saturated to unsaturated areas of soil.
Types of water flow in soil
Saturated and unsaturated. Also vapor -rare.
Saturated flow
Fast. Happens when all pores filled. Moves with gravity.
Unsaturated flow.
Slow. Happens when all pores are not filled. Moves laterally and up with capilarity - wet to dry.
Saturated vs unsaturated flows in course and fine soil.
Course soil quickly gets saturated and flows quickly down. Fine soil takes flows through unsaturated flow for a long time before becoming saturated.
Field capacity (FC)
When all free water is drained from a soil over a short period time (48 hours). Macropores are empty.
Permenant wilting point (PWP)
Point where plants are unable to get water from soil. What water remains it too tightly held by adhesion/cohesion.
Available water
The difference between field capacity and permenant wilting point.

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