Chapter 5 Soils Notes
Chapter 5 Soils Notes PSS 3303
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This 7 page Class Notes was uploaded by Payton Gilmore on Wednesday September 28, 2016. The Class Notes belongs to PSS 3303 at Mississippi State University taught by Dr. Jac J. Varco in Fall 2016. Since its upload, it has received 47 views. For similar materials see Soils in plant and soil sciences at Mississippi State University.
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Date Created: 09/28/16
Chapter 5 (Notes for Quiz on 9-30-16) Soil Water: Characteristic and Behavior Importance: Declining aquifers as a result of irrigation and urban usage Competition for water between farmers-cities increasing Crop usage rates: o Corn=430# H2O/#d.m. o Wheat=700#H2O/#d.m. o Hay 1100+#H2O/#d.m. #=pound Plant Importance: Transpiration, turgidity, and nutrient availability (universal solvent) Soil solution-site of chemical reactions and microbial growth Global Importance: Water Quality-filter for drinking water, sediments, ag- chemicals, and industrial pollutants Wetlands (this is why we have laws against destroying them…very important) Soil-hydrologic cycle California 3-y Drought o images Delta o Alluvial aquifer o More and more permits are being given each year for well o Has to reach greater depths o Now to get water the ground is shrinking, drought Properties of Water 1. Structure-H2O - Asymmetrical, 105 * - Hydrogen Bonding - Covalent Bonding 2. Polarity- dipolar (electrically neutral), polymerization 3. High surface tension, cohesion of water molecules. (This is why water bugs can walk on water.) 4. Density=1g/cm3 @ 25*C, max @ 4*C then decreases down to zero. - Density increases down to 4*C and then decreases below 4*C - Ice floats because it is less dense 5. Dielectric constant () water=80 - F=q+ X q- / 12.6 d^2 - F= force of attraction - d= distance between two charges - This is why water is the universal solvent Hydrated ions- greater attraction to water than to each other Forces of Water A. Adhesion-attraction of water molecules to a surface B.Cohesion- attraction of water molecules to each other - Fig. 5.1 pg. 134 in book 2 Hygroscopic Water - Hygroscopic water= adhesion water, increases with increasing surface area. - Hygroscopic water= air-dry soil - Heat soil to 105*C to get rid of the hygroscopic water (oven dry soil) - Plant material 60-70*C - Layer of two of water molecules adsorbed to clay surfaces, more or less crystalline in nature. - Adsorbed water has lower energy than non-adsorbed water. Hygroscopic water is held by strong electrical forces and removal requires oven-drying @ 105*C Cohesion Water - > energy than adhesion water - a portion of it is available to plants - solution phase: plant nutrients dissolved in it Capillarity - Adhesive and cohesive forces hold water in micropores against forces of gravity. - Water can move in all directions. - h = 2T/ rdg o h= height of capillarity o T= surface tension o R=capillary radius o d= density o g= gravitational constant - h= 0.15/r simplified - Fig 5.3, pg. 136 3 - Fine sand has smaller capillaries, more micropores, more water intake Energy of Soil Water - Energy status of water determines movement - Free energy of water reflects different forces acting on soil water. Water moves from a greater energy level to a lower one. Soil water moves from wetter to drier zones. - Moves wetter areas to drier area Energy levels of water - Water vapor> liquid water> adsorbed water A. Gravitational potential- water moving in response to gravity - 1 atm=14.7 #/ in^2 - 1 atm – 1 bar - column of water 1023 cm (33.56 ft) - 760 mm Hg: standard of 1 atm (not used much anymore) - Pressure-force/area - 2 liters of water w/a bottom area of 100 cm^2. Pressure at bottom= 2000g/100 cm^2 or 20 g/cm^3 - g= gravitational potential - water is present in macropores - > energy level than hygroscopic and capillary H2O - moves in response to gravity B.Matric potential- arises from capillary forces, can be expressed as a tension or suction or negative pressure. - Sponge example 4 - m= matric potential, water held against forces of gravity o micropore water o moves in response to tension differences - Note: also movement of water into seeds, roots, and microbes due to matric forces C.o= osmotic potential, negative potential solutes in soil solution. Attraction of water to solutes, free energy of water is lowered - water adsorbed by ions or solutes - influences water uptake by roots and seeds - saline soils and fertilizer bands ex. Osmosis of water through a semi-permeable membrane-water moves from a higher free energy level to a lower free energy level. High salt | Low salt (H2O) total soil water potential= T=g+m+o T is potential to slightly negative when gravitational water is present, otherwise is negative Fig 5.5 Pg. 138 Water Content vs. SWP, Fig. 5.12 pg. 143 Fig 5.7 pg. 141 (homework) -0.1-0.33 bars is where the soils macropores are completely dry and micro starts to dry/micropores are completely full Field Capacity maximum retentive capacity: all pores are full 5 At 20% volume soil water content what soil is the wettest- sand How do these forces apply to the ability of soils to supply plants with water? - Hygroscopic water: -31 bars not available to plants, held too tightly - Capillary water: micropore- 31 bars to -0.1 bar - Plant available: -15 to -0.1 bars. All is capillary water. -15 bars=permanent wilting point (PWP) - Field capacity- maximum amount of water a soil can hold against gravitational forces; -0.1 to -0.33 bars. (Macropores drained micropores full) - Maximum retentive capacity: maximum amount of water a soil will hold when saturated (most macropores filled with water) - Fig. 5.21 pg. 155 Calculation of Soil Soil H2O content expressed as a % of the oven-dry weight= gravimetric H20 content Pm= (weight of water/oven dry wt. soil (105*c)) x 100 By volume= (vol. water/vol. soil) x 100 where vol. water=wt. water/density of water and volume of soil=wt. soil/bulk density therefore, (Wt water/(Dw)/OD wt soil/Db) X 100= Pv or Pv=DbXPm Fig. 5.23 pg. 157 Fig. 5.24 pg. 158 Soil Water Movement Water flow is proportional to r^4, the size of the pore determines conductivity 6 Water flow V=Kf Where, - V= volume of water flow per unit time - K= hydraulic conductivity- depends on pore size distribution (PSD) and water content - f= water potential difference or gradient Fig. 5.17, pg. 151 Types of water movement in soil A. Saturated- hydraulic gradient, depends on gravitational forces - Sand soils have a high K- large % of macropores; finer textured soils have a lower K under saturated conditions. - Saturated flow movement mainly in macropores downward, rapid, and water is under low tension T0 - Saturated flow- influences percolation (through soil) and infiltration (into soil) rates, leaching, translocation of clays, ions, and organic matter B.Unsaturated- liquid water and vapor movement. Movement in micropores, all directions. Slower, movement in response to tension differences - Unsaturated has to be dryer than field capacity Effect of Soil texture on conductivity, Fig. 5.15 pg. 149 7