Soils 210: Introduction to Soil Science and Soil Resources

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Tonight

• Review mineralogy and soil colloids
• Soil Reaction
• Soil Water
• Assignment 3 due
• Assignment 4 and Water calculations handed out

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Soil Reaction

• Soil reaction is the degree of acidity or
  alkalinity of a soil, usually expressed as a pH
  value.
• Soil pH -log H
• Soil pH is an indicator of physical, chemical and
  biological properties in soil.
• Soil pH is also related to the cations present on
  the exchange complex.

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pH of Common Materials

• Milk of magnesia 10.5
• Bicarbonate of soda 8.3
• Pure water 7.0
• Milk 6.8
• Natural rain 5 to 6
• Beer/coffee 4
• Lemon Juice 2

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Fig. 7.2. Descriptive terms for Soil pH ranges
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Fig. 7.1. Soil have distinct properties
Credit Pedosphere.com
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Soil pH vs Soil Type Depth

• Let us study data in Table 7.2 (Section 7.3)

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Soil pH vs Cation Composition

• Total cation exchange capacity (TCEC) is a
  function of quantity of clays, organic matter and
  iron and aluminum oxides (Section 6)
• Types of clay are very important! (Section 6)

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Soil pH vs Cation Composition

• Base cations (Ca, Mg, K, Na) concentration
  decreases as soil becomes more acidic (pH
  decreases)
• Let us study data in Table 7.3 (Section 7.3)

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Percent Base Saturation

• Basic cations Ca, Mg, Na, K
• Acidic cations Al, H
• Percent base saturation A measure of the
  proportion of basic cations occupying the
  exchange sites of a soil

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Formula

• Cation exchange capacity is the sum of all
  cations on the exchange complex
• Base saturation ? (Ca, Mg, K, Na) x
  100

Cation Exchange Capacity
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pH of Diagnostic Horizons

• Let us study Table 7.4

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Fig. 6.9. Impact of soil pH on net charge
ofnoncrystalline aluminum oxide. At low pH, H
ions become bound to Al and Fe oxides
Credit Pedosphere.com
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Fig. 7.3. Soil pH vs cations on the exchange
complex (Brady and Weil, 1996)
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Dissolution of amorphous Al(OH)3

• Al(OH)3 H ? Al(OH)2 H2O
• Al(OH)2 H ? Al(OH) H2O
• Al(OH) H ? Al H2O
• The equilibrium reactions result in buffering of
  soil

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Buffering Mechanisms (Table 7.6)

• Oxidation of pyrite and reduced S minerals
  dissolution of minerals pH 2 to 4
• Aluminum compounds pH 4.0 to 5.5
• Cation exchange pH 5.5 to 6.8
• Organic matter and minerals pH 6.8 to 7.2
• Ca and Mg carbonates pH 7.2 to 8.5
• Exchangeable Na Dissolution of solid sodium
  carbonate pH 8.5 to 10.5

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Soil Acidity Types

• Active acidity The activity of hydrogen ions in
  solution
• Reserve acidity The acidity that is associated
  with the exchange complex. It is neutralized by
  lime or other alkaline material

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Classification of Soil Acidity
-

-
-


-



-
-

-


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Clay surface

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Bulk solution


-
-
-




-

-
-

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Fig. 7.4. Hydrogen is part of the crystal
lattice,and can be present as an exchangeable
cation and in the soil bulk solution
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Nutrient Availability

• The availability of nutrients is strongly related
  to its solubility at different pH values
• At extreme pH values, solubility of some
  nutrients increases tremendously, leading to
  toxicity of plants
• Let us study Fig. 7.5 in Section 7.7

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Acidification

• Use of ammonium-based fertilizers
• (NH4)SO4 4O2 ?2HNO3 H2SO4 2H2O
• Acid Deposition
• Nitric (HNO3) Sulfuric (H2SO4 ) acids

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Acidification

• Drainage of some coastal wetlands leads to the
  oxidation of pyrite (FeS2), iron sulfide (FeS)
  and elemental S and formation of sulfuric acid

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Liming soils

• Use liming materials CaCO3 Ca(OH)2, CaO
  MgCO3
• -H CaCO3 -Ca2
  H2O and CO2
• Are CaCl2 or CaSO4 liming materials?If yes, why?
  If not, why not?

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Lecture Material

• Motivation
• Classification of soil water
• Soil water potential curves
• Water movement
• Water properties and texture triangle

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Particle size pore space
Large Particle
2 x 2 x 2 8
Pore
radius 4r
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Particle size pore space
Medium Particle
4 x 4 x 4 64
Pore
radius 2r
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Particle size pore space
Small Particle
Pore
8 x 8 x 8 512
radius r
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Fig. 1.9. Pores and particles in soil (Pawluk)
Credit Pedosphere.com
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Fig. 3.3. Soil textural classes in the Canadian
System of Soil Classification
Credit CSSC Pedosphere.com
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Fig. 8.10. Saturated and Unsaturated Flow
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Fig. 8.4. Capillary rise and capillary retention
Credit Brady Weil, 1996 Kohnke, 1968
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Fig. 8.6. Interaction of water molecules with
clay surfaces, and cations and anions in soil
Credit Pedosphere.com
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Fig. 8.5. Classification of soil water (after
Heaney, Crown and Palylyk, 1995).
Credit Pedosphere.com
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Matric Potential

• Matric Potential Adhesion of water to surfaces
  through adsorption and capillarity markedly
  reduces the energy state of adsorbed water
  molecules
• Matric potential is universally important and is
  used in calculations of water movement

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Osmotic Potential

• Osmotic Potential Attraction of ions and other
  solutes for water reduces the energy level of
  water molecules
• Osmotic potential is attributable to the presence
  of solutes in the soil solution.

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Gravitational Potential

• Yg ghwhere g is the acceleration due to
  gravity and h is the height of soil water above a
  reference elevation.
• Gravity plays an important role of removing
  excess water from the upper rooting zones
  following heavy precipitation or irrigation.

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Soil Water Potential

• The difference in energy levels between pure
  water and soil water is termed soil water
  potential
• Difference in energy level determines the
  direction and rate of water movement in soils and
  plants

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Soil Water Potential

• Soil water potential is made up of matric,
  osmotic and gravitational potentials
• Water flows from a point which has a higher water
  potential to another point which has a lower soil
  water potential

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Fig. 8.5. Classification of soil water (after
Heaney, Crown and Palylyk, 1995).
Credit Pedosphere.com
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Fig. 8.7. Soil water potential curves
Credit Pedosphere.com
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Water Movement

• Saturated flow Vertical movement of water due
  to force of gravity in a soil in which all the
  pores are completely filled with water.
• Movement can be defined by Darcys equation

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Fig. 8.8. Darcys equation (q Ks DH/DL)
Credit Pedosphere.com
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Table 8.2. Hydraulic conductivity in soils with
different textures
Credit After Hanks and Ashcroft, 1980
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Fig. 8.9. Unsaturated Flow