NRES 201 Lecture 15 Outline of Last Lecture I What is soil organic matter SOM II Functions of soil organic matter III Soil organic matter management IV Decomposition in soils V Factors affecting soil content and distribution of organic matter VI The global carbon cycle Outline of Current Lecture VII Properties of water VIII Capillarity and soil water IX Energetics of soil water X Classification of soil water Current Lecture Properties of water National occurrence o The only chemical compound that occurs naturally on Earth as a solid liquid and gas The liquid phase is by far the most abundant o The only inorganic compound that occurs naturally on Earth as a liquid Polarity o H2O forms by covalent bonding o V shaped configuration due to greater electron affinity by O than H o The result is a polar molecule consisting of An electronegative O atom Two electropositive H atoms Hydrogen bonding o Arises when a H atom of one water molecule is attracted to the O end of another o The reason why water polymerizes and is unusually high in Boiling point Specific heat Viscosity Hydration o Polar water molecules are attracted to ions or clay particles o Charge structured orientation These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute H end toward negative ions or clay particles O end toward positive ions o Due to polarity water is the universal solvent Better than any other liquid Cohesion versus adhesion Two forces are responsible for water retention and movement in soils o Cohesion the attraction of water molecules for each other o Adhesion the attraction of water molecules for solid surfaces Both are due to H bonding o Also causes flickering H2O clusters to form and dissociate in liquid water Capillarity and soil water Capillary action o Water rises in a tube when Some of the molecules climb up the tube wall by adhesion And they pull others up by cohesion o A curved surface meniscus forms where water meets air o Simple example Dye movement up a celery stalk Capillary rise in soils o Occurs in all moist soils o But limited by Tortuous nature of soil pores Entrapment of air o Height of rise clays loams sands o Rate of rise sands loams clays Classification of soil water Dynamics of soil drying o As an initially saturated soil is dried water is increasingly confined to smaller pores and thinner films o The decrease in water content brings a corresponding increase in the energy of water retention o Four stages are characterized along the continuum of soil wetness
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