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UI SOIL 206 - Soil Water

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ObjectivesIntroductionSoil Water PotentialSoil Water ContentSoil Moisture-Energy Curves and Plant Available MoistureMovement of Water in Saturated and Unsaturated SoilsSaturated SoilsUnsaturated SoilsExercise 1: Field capacity, permanent wilting point and plant available waterExercise 2: Flow through saturated soilSoil Water SOIL 206 – Soil Ecosystem Lab ObjectivesAfter completing this laboratory the student should be able to: 1. Explain the terms polarity, hydrogen bonding, cohesion and adhesion as they apply to soil water 2. List and describe the three components of the total soil water potential 3. Describe soil water in terms of water potential and water content 4. Interpret a soil moisture-energy curve with respect to water retention, pore space, pore size distribution and plant available water 5. Perform calculations involving gravimetric water content and soil water potential 6. Describe the influence of soil texture on plant available water 7. Describe the movement of water in saturated and unsaturated soils using the concepts of soil water potential IntroductionThis lab continues your investigation of soil physical properties. Prior labs have taken a detailed look at soil texture, porosity and density. In this lab you will examine the characteristics, behavior, measurement,and quantification of soil water. It is virtually impossible to overstate the importance of water in soils. Water is an important consideration in all aspects of soil science including parent material deposition, weathering, horizon development, microbial activity, plant growth, and soil erosion. One of the most important properties of the water molecule is its polarity; the hydrogen atoms in a watermolecule (H2O) exhibit partial positive charges while the oxygen atom exhibits a partial negative charge. Polarity helps explain how water molecules interact with one another and with other atoms through the process of hydrogen bonding. Hydrogen bonding, in turn, accounts for two basic forces responsible for water retention and movement in soils. Cohesion is the force of attraction of water molecules for each other and adhesion is the attraction of water molecules for solid surfaces. Adhesive-cohesive forces give rise to surface tension and capillarity. Capillary rise is defined by the equation: h = (2 * T * cos  / (r *  * g) Where: h = the height of rise (cm) T = surface tension of water cos  = the contact angle of water and capillary wall, assumed to be 1 for water r = radius of the capillary  = density of liquid, assumed to be water at 1g cm-3g = force of gravity, a constant of 981 cm sec-2For water calculations, the surface tension, the contact angle, the density and the force of gravity can be combined and simplified to: h = 0.15/ rFall 20051This indicates that the height of rise of water in a capillary tube or a soil pore is inversely proportional to the tube or pore radius. Capillary forces are at work in all moist soils. The extent of capillary rise depends somewhat on soil texture. Capillary rise is usually greater with fine-textured (small pore size) soils as predicted by the capillary rise equation. It is important to remember that capillarity can cause soil water movement in horizontal as well as vertical directions. The significance of capillarity in controlling water movement should be kept in mind as the topic of soil water potential is discussed. Soil Water PotentialThe fundamental principles controlling the behavior of soil water are its energy relationships. The retention and movement of water in soil, its uptake and translocation in plants, and its loss to the atmosphere through evapotranspiration can be explained in terms of free energy. Soil water, like other bodies in nature, contains free energy in different forms and quantities. Two principal forms of free energy are commonly described: kinetic and potential. Kinetic energy is proportional to the square of the velocity of a moving body. Since the velocity of water moving in soil is quite slow, its kinetic energy is generally considered negligible. Potential energy, on the other hand, is a measure of the amount of work a body can perform by virtue of stored energy. The potential energy of soil water is governed by its relative position within the soil profile, the arrangement of the solid matrix, and the presence of solutes. Potential energy is of primary importance in determining the movement of water in soil. The total soil water potential is the sum of three component potentials: 1. Gravitational potential (g):a function of the height of water above a reference point 2. Matric potential (m): a function of adhesive and cohesive forces, and 3. Osmotic potential (o): a function of the attraction of water molecules for solutes The free energy of water at a given elevation is greater than the free energy of water at a lower elevation. Thus the gravitational potential increases the free energy of water and is always positive in sign. The adhesive and cohesive forces that comprise the matric potential significantly reduce the free energy of soil water making the matric potential negative in sign. The interaction of water molecules withsolutes also lowers the free energy of soil water and hence, the osmotic potential is also negative in sign.The combined effects of gravitational, matric and osmotic potentials cause the free energy of soil water to be less than that of pure water and thus, in most cases, the total soil water potential is negative in sign. Soil water potential, like other forms of energy, may be expressed using several different units. The most common unit of water potential is the bar. One bar equals the pressure exerted by a column of water 1023 cm in height. One bar also equals one atmosphere, 14.7 lbs/sq. in. and 760 mm Hg. The SI unit of water potential is the kilopascal (kPa). One hundred kPa is equal to 1 bar. Soil Water Content Another means of describing soil moisture is the water content. Soil water content is simply the weight or volume of water present in a given weight or volume of soil. The water content of a soil is most Fall 20052commonly expressed as a percent of the oven-dry weight of the soil. This value, known as the mass water content (m) can be determined gravimetrically by drying a moist soil sample at 105 ºC to a constant weight. The weight lost upon drying represents the water content and thus, the mass water content (v) can be calculated


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