SOIL SCIENCE 322

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SOIL SCIENCE 322 PHYSICAL PRINCIPLES OF SOIL AND
WATER MANAGEMENT
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INTRODUCTION Read Chapters 1, 3 and 5
(Introduction to Environmental Soil
Physics) Landowners, scientists, engineers, and
others resource managers should have knowledge of
the physical properties and behavior of soil, and
soil water to best manage and conserve these, our
most valuable resources. Every aspect of soil
and water management and conservation (i.e.,
irrigation, drainage, tillage, and erosion
control, etc.) encompasses soil physical
properties. In this class we will assess the
physical realm of soil science in a piece-wise
fashion starting with the physical phases of
soil, that is, a single grain concept --
progressing to complex flow dynamics and other
application process. For the first half of each
topic, we will discuss the related physical
properties on an introductory/rudimentary basis.
In the second half of each topic, we will
demonstrate the application of these principles
toward soil and water management, conservation,
and protection.
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OVERVIEW Soil is the product of mechanical,
chemical and biological weathering of rocks and
other parent materials. Soil is a Three-phase
System Solid, Liquid, Gas
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• OVERVIEW
• A. Soil is a Three-phase System (solid, liquid,
  gas)
• 1. Solid phase (mineral and organic matter)
• a. Mineral fraction
• 1) Particles of various shapes, sizes and
  chemical composition.
• 2) Mineralogical composition determines many
  chemical and physical properties.
• b. Organic fraction
• Organic matter consists of plant and animal
  residues, and living organisms. --It sometimes
  appears as a coating on grains (e.g. Sparta
  sand).
• The organic fraction contributes to optimum soil
  physical conditions such as adequate
  infiltration of water into the soil, stable soil
  aggregates, etc.
• 2. Pore space (gas and liquid phase)

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a) Liquid Phase 1) Liquid phase (also called
soil solution, is an aqueous solution of various
electrolytes). Soil solution may fill soil pores
either completely or partly. If soil pores are
completely filled by soil solution, the soil is
said to be "saturated." If it is partly filled,
the soil is "unsaturated." In this case, the
liquid exists as thin films along particle
surfaces, as annular wedges around contact points
of particles and as isolated bodies in small pore
spaces.
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a) Liquid Phase (soil solution) 1) Porosity
volume of voids /total volume 2) Soil water
content volume of water / total volume 3) Air
filled porosity
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Soil is a major reservoir for water. Assume that
half of the total porosity (50 in an ideal soil)
of a soil that is 1 m deep is full of water. This
would represent a 25-cm depth of water. A hectare
of this soil would contain 2.5 x 103 m3 of water.
Soil water content at saturation is an indication
of the soils total porosity. A 1 m3 volume of
soil with 50 pores has 0.5 m3 of pores. When
this soil is saturated, it will contain 0.5 m3 of
water.
2.5x103 m3660,502 gallons m3galx3.785x10-3 ha2.
47 ac
1 m
0.5 m
1 m
0.5 m3 132 gal
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LIQUID PHASE

• Soil solution is an aqueous solution containing
  dissolved inorganic and organic solutes.
• The make-up of these solutes affect plant growth
  and potential for environmental pollution.
• Wetness (water content) varies from, at or near,
  0 to 100 (saturation) pores filled.
• Solute movement (by diffusion and mass flow) is
  directly related to soil wetness (water content).
• As a dry soil wets, the water first adsorbs as
  films on particle surfaces, then forms annular
  wedges around contact points or aggregates then
  it starts to fill pores.

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2) The electrolyte or solute content and
composition affect plant growth and other
biological activities.
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• b) Gas Phase (soil air)
• Mainly composed of N2, O2, water vapor, and CO2,
  with traces of other gases.
• Proportion of gas-filled pores is inversely
  related to proportion of water-filled pores.
  Thus, as the number of pores filled with gas
  increases, the number of pores filled with water
  decreases.

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Mobility of gas in soil is determined by
proportion and continuity of air-filled pores and
the partial pressure of the gas. Chemical
composition of the gas phase determines
oxidation-reduction status. In the absence of
aeration, chemical reduction takes place,
producing gases like methane, hydrogen sulfide
and nitrous oxide.
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• Gas Phase - Soil Air
• O2 is consumed and CO2 is generated by the
  metabolic activity of microorganisms and plant
  roots in the soil.
• O2, CO2, and water vapor diffuse into and out
  the soil
• 79 N2
• 21 CO2 and O2
• Well aerated soil, CO2 0.25, O2 20.75
• Poorly aerated soil O2 can be near zero.
• O2 lt 10-15 can inhibit plant growth (Taylor and
  Ashcroft, 1972).
• Rate of respiration in the soil depends
  primarily on availability of O2 and C, soil
  temperature, and water content.
• Mobility of a gas depends on the amount
  (gas-filled pore space and partial pressure).

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B. Soil is a Disperse System Within the
three-phase system are generally two interfaces
across which most of the important reactions
occur. a. Solidliquid reactions 1)
Dissolution of ions from solid to solution and
precipitation from solution to solid. 2)
Adsorption and desorption of ions or molecules
between solution and solid surfaces. b.
Liquidgas reactions 1) Evaporation of liquid
water to water vapor and condensation of water
from the vapor to the liquid phase. 2)
Solution and dissolution of gases in water is
necessary for oxygen to reach plant roots and
CO2 to leave the system.
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C. Soil is a Dynamic System 1. Dynamic chemical
reactions include a. Soil weathering 2.
Dynamic physical reactions a. Shrink-swell on
drying and wetting or freezing and
thawing b. Dispersionflocculationaggregation
of colloids c. Shear or compaction under
applied force d. Movement of water in response
to water potential gradients 3. Dynamic
biological reaction a. Absorption of water and
nutrients b. Soil compression because of root
expansion c. Release of compounds from
microorganisms or plant roots d. Pores
created/enlarged by organisms