Chapter 5 Pages 132-160 Outline

Chapter 5 Pages 132-160 Outline - AGRO 301 1/31/08 Chapter...

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AGRO 301 1/31/08 Chapter 5 Soil Water: Characteristics and Behavior I) Structure and Related properties of water A The ability of water to influence so many soil processes is determined primarily by the structure of the water molecule. B Polarity 1) The arrangement of the three atoms in a water molecule is not symmetrical and are polar C Hydrogen Bonding 1) A hydrogen atom may be shared between 2 electronegative atoms forming a relatively low-energy link 2) The type of bonding accounts for the polymerization of water D Cohesion Versus Adhesion 1) Cohesion- the attraction of water molecules for each other. 2) Adhesion- the attraction of water molecules for solid surfaces. 3) Both make possible the property of plasticity possessed by clays E Surface Tension 1) Property of water that markedly influence its behavior in soils 2) Evidenced at liquid-air interfaces and result from the greater attraction of water molecules for each other than for the air above. II) Capillary Fundamentals and Soil Water A The movement of water up a wick typifies the phenomenon of Capillarity B Two forces cause capillarity 1) The attraction of water for the solid(adhesion or adsorption) 2) The surface tension of water, which is due largely to the attraction of water molecule for each other (cohesion) C Capillarity is demonstrated by water in a clean glass tube 1) The water rises in the tube (a) The smaller the tube bore the higher the water will rise 2) Water molecules are attracted to the sides of the tube 3) Cohesive forces hold the water molecules together and create surface tension causing a curved surface (called a Meniscus ) to form at the interface between water and air in the tube 4) Lower pressure under the meniscus in the glass tube (P2) allows the higher pressure (P1) on the free water to push water up the tube 5) The height (h)of rise in a capillary tube is inversely proportional to the tube radius (r) 6) h=.15/r (“h” and “r” are in centimeters) 7) Happens in soils too. 8) Can move and takes place in any direction. III) Soil Water Energy Concepts A Forces affecting Potential Energy
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1) Adhesion, or the attraction of water to the soils solids (matrix), provides a Matric force (responsible for adsorption and capillarity) that markedly reduces the energy state of water near particle surfaces. 2) The attraction of water to ions and other solutes, resulting in Osmotic forces, tends to reduce energy state of water in the soil solution. 3) Gravity pulls the water downward B Soil Water Potential 1) The difference in energy level of water from one site or one condition (wet or dry) to another determines the direction and rate of water movement in soils and in plants. 2) Soil Water Potential -a measure of the difference between the free energy state of soil water and that of pure water, technically it is defined as ”That amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermically an infinitesimal quantity of water from
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Chapter 5 Pages 132-160 Outline - AGRO 301 1/31/08 Chapter...

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