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T9+notes - Soil Water (Topic 9) Soil Water Content Mass (or...

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(Topic 9) Soil Water Content Mass (or Weight) Basis V o l u m e B a s i s P W % = (Mass of H 2 O/Mass of dry soil) 100 P V % = (Volume of H 2 O/Volume of Soil) 100 P W % = Mass of wet soil - Mass of dry soil 100 Since one cm 3 of H 2 O weighs one gram, Mass of dry soil (105 o C for 24 hr) we can use the mass loss of water as the volume of water contained in the sample. Conversion between P W and P V can be obtained by using the D b : P V % = P W % D b Structure and Properties of Water Structure of water (H 2 O) Positively charged H + ions form a V-shaped arrangement with negatively charged O 2- . The angle between the H ions is 104.5 o . This arrangement results in an asymmetrical distribution of charge in which the O end is electronegative and the H end is electropositive. Molecules that display this uneven charge distribution are termed polar molecules. Hydrogen bonding A basic principle of chemistry and physics is: Like charges repel each other Opposite charges attract! Thus, the electropositive end of one water molecule is weakly attracted to the electronegative end of an adjacent molecule. This type of water-water attraction involving hydrogen bonding can be termed cohesion . Hydrogen bonding and other types of weak bonds can also form between water and solid surfaces such as the soil particles. This type of water-solid surface attraction is termed adhesion . Water bonded to surfaces varies from weak to strong attraction depending on proximity to the surface. The less water there is in a soil, the closer the water will be to the soil particles and the more strongly it will be held. Capillary rise in a glass tube involves adhesion of water to the glass which pulls water molecules upward in the tube. Cohesion between water molecules allows water molecules away from the glass surface to be pulled up along with the water at the glass surface. These same principles (adhesion & cohesion) that result in capillary rise in a glass tube are active in the soil and result in water being held on surfaces and in pores. Soil Water Energy Concepts Soil Water is held on SURFACES (adsorption) and in PORES (capillarity) Forces affecting the energy level of soil water: 1. Matric forces forces responsible for adsorption and capillarity (adhesion and cohesion). 2. Osmotic forces the attraction of water by ions and other compounds dissolved in the water. (sodium [Na
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This note was uploaded on 01/27/2011 for the course SSO 10 taught by Professor Randydahlgren during the Fall '10 term at UC Davis.

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T9+notes - Soil Water (Topic 9) Soil Water Content Mass (or...

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