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Tonight

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Tonight

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

  2. 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.

  3. 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

  4. Fig. 7.2. Descriptive terms for Soil pH ranges

  5. Dark Gray Luvisol Orthic Humo-Ferric Podzol Orthic Black Chernozem Fig. 7.1. Soil have distinct properties Credit: Pedosphere.com

  6. Soil pH vs Soil Type & Depth • Let us study data in Table 7.2 (Section 7.3)

  7. 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)

  8. 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)

  9. 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

  10. 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

  11. pH of Diagnostic Horizons • Let us studyTable 7.4

  12. 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

  13. Fig. 7.3. Soil pH vs cations on the exchange complex (Brady and Weil, 1996)

  14. 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

  15. 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

  16. 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

  17. Classification of Soil Acidity - + - + + - - + + + - - + - + + - Clay surface + - + Bulk solution + + - - + + - + + - + - - + - Fig. 7.4. Hydrogen is part of the crystal lattice,and can be present as an exchangeable cation and in the soil bulk solution

  18. 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

  19. Acidification Use of ammonium-based fertilizers (NH4)SO4 + 4O22HNO3 + H2SO4 + 2H2O Acid Deposition Nitric (HNO3) + Sulfuric (H2SO4) acids

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

  21. Micelle Micelle 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?

  22. Lecture Material • Motivation • Classification of soil water • Soil water potential curves • Water movement • Water properties and texture triangle

  23. 16 r Particle size & pore space Large Particle 2 x 2 x 2 = 8 Pore radius = 4r

  24. 16 r Particle size & pore space Medium Particle 4 x 4 x 4 = 64 Pore radius = 2r

  25. 16 r Particle size & pore space Small Particle Pore 8 x 8 x 8 = 512 radius = r

  26. Fig. 1.9. Pores and particles in soil (Pawluk) Credit: Pedosphere.com

  27. Fig. 3.3. Soil textural classes in the Canadian System of Soil Classification Credit: CSSC & Pedosphere.com

  28. Fig. 8.10. Saturated and Unsaturated Flow

  29. Fig. 8.4. Capillary rise and capillary retention Credit: Brady & Weil, 1996; Kohnke, 1968

  30. Fig. 8.6. Interaction of water molecules with clay surfaces, and cations and anions in soil Credit: Pedosphere.com

  31. Fig. 8.5. Classification of soil water (after Heaney, Crown and Palylyk, 1995). Credit: Pedosphere.com

  32. 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

  33. 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.

  34. 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.

  35. 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

  36. 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

  37. Fig. 8.5. Classification of soil water (after Heaney, Crown and Palylyk, 1995). Credit: Pedosphere.com

  38. Fig. 8.7. Soil water potential curves Credit: Pedosphere.com

  39. 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 Darcy’s equation

  40. Fig. 8.8. Darcy’s equation (q = Ks * DH/DL) Credit: Pedosphere.com

  41. Table 8.2. Hydraulic conductivity in soils with different textures Credit: After Hanks and Ashcroft, 1980

  42. Fig. 8.9. Unsaturated Flow