Soil Physics: Properties, Texture, Structure, and Water Potential - Prof. Jac Varco, Study notes of Plant Taxonomy and Evolution

Download Soil Physics: Properties, Texture, Structure, and Water Potential - Prof. Jac Varco and more Study notes Plant Taxonomy and Evolution in PDF only on Docsity! **Test 2** Ch.4- Soil Architecture and Physical Properties Physical Properties  Soil productivity  H2O holding capacity & drainage  Construction suitability Soil Texture  Fine earth fraction, excludes pebbles, rocks and etc.  Soils are approx.. 45% by volume mineral matter, relative proportions of sand, silt, & clay (soil seperates) comprise of determine the texture of a soil.  ↓ particle size; surface area ↑  Absortion of H2O, gases, & nutrients & a greater attraction of particles to each other greater. (also faster weathering, ↑ swelling, plasticity, cohesion). Soil Texture Generalization  Clay soils -hold > H2O than coarser textures -hold > nutrients that silt and sand -absorb > gases such as NH3 -develop> structure  Sandy soils -> permeability to air & H2O -lower H2O & nutrients -low OM -less dev. Structure -Class. Of structure- A. Grade 1. Weak- Aggregates too weak to remain intact upon removal 2. Moderate- Remained intact; can be crushed 3. Strong- Visible in undistributed soil, break into whole peds when removed from profile. Difficulty in crushing. 4. Structure less- No observable aggregation, no natural planes of weakness. -Single grain in non-coherent - Massive if coherent B. Class *Size of ped or aggregate A. Fine B. Med C. Coarse C. Type- Shape of peds, natural planes of weakness. 1. Spherical-Round in shape A. Granular-Mainly nonporous. B. Crumb- Porous *good aeration, desirable plant growth. A horizons especially under grassland conditions. 2. Platy- Peds w/ greater horizontal dimensions than verticle. * Usually found in A-E horizons. Related to PM, frost action, water table fluctuations and horizontal water movement. 3. Prism-like- Dimensions greater in vertical than horizontal. A. Prismatic- Square tops B. Columnar- Rounded tops * B horizons well-developed soils. Most common in aridisols. Fragipans of MS. 4. Block-like- Equal sides in 3-D A. Angular blocky B. Subangular blocky * Common in B horizon * Strong (G), med. (C), subangular (T) Genesis of Soil Structure ***Density of water=1.0 Solid Porosity  % solid space= Db/Dp x 100  % total pore space= 1- Db/Dp x 100 *The ↓ the Db ,the ↑ TPS * Compaction cause ↓ in TPS (soils particles are more closely packed together) *Fine textured soils tend to have a > % TPS than coarse-textured soils  Why sandy soils drain better than clay soils if % TPS can be > for clay soil? -Sand can’t hold it; Larger pores; Can’t hold it against the force of gravity  Pore spores= large pores + small pores  Large pose-Water drains freely out of; small pores hold water against gravity.  Macropores > 0.08 mm diameter Micropores < 0.08 mm diameter  Pore-size distribution (PSD) - Porportion of macro & micropores - Influenced by texture & structure - Sandy soils- > proportion of macropores to micro compared to finer textured soils. Optimum near 50/50.  Compaction/ loss of structure = ↓ in % TPS, primarily macropores are lost.  Cultivation/tillage initial ↑ in pore space, but overall ↓ due to soil setting & a ↓ in aggregation  Surface crusting-Loss in porosity (disproportionately > macropores lost than micropores)  Excess Na- ↓ in soil structure w/ a loss in porosity. Typically, affected areas puddle.  Continuity of pores-Drainage & aeration -NT vs CT -Earthworm channels -Root channels etc. *Fig. 4.25; pg. 122  Compaction: cart paths, trails, campgrounds, sheepsfoot, skidder trails, and traffic pans Soil Color  Easily identified and inferences can be drawn, especially in reference to drainage/ permeability soil genesis & land use. *Redox, hydromorphic 1. Dark colors- Browns to black usually cause by OM. Darker the soils ↑ the OM. 2. Reds- Unhydrated & oxidized from good drainage & ↑ degree of weathering. 3. Yellows- Hydrated iron oxides, usually intermediated drainage. 4. Grays- blues- ↓ iron & manganese; lack of oxygen due to poor drainage. **Btg (greying) 5. White or bleached- Lack of OM, quartz usually the predominant mineral. In arid climate possibly accumulation of salts of calcium carbonate. 6. Molted-Red-yellows indicate alt. drainage between wet and dry and better aerated zones vs. poorly aerated zones.  Color- Indication of soil productivity and drainage. -Dark-Highly productive -Poorly drained soils= hydric or wetland soil; poor drainage= septic tank efficiency/suitability, cropping, tree action, foundation plantings, etc.  Munsell Color Book -Hue- Dominant wavelength of color. -Value- Brillance or lightness and darkness of color. **low value- dark **high value-light -Chroma- Color purity, higher the Chroma, the purer the color Ch.5- Soil Water: Characteristics & Behavior **1972-Clean Water Act Importance  ↓ aquifers as a result of irrigation and urban usage.  Competition for water between farmers- cities increasing  Crop usage -Corn= 430 # H2O/ # D. M. -Wheat= 700 -Hay= 1100+  Transpiration, turgidity, and nutrient availability (universal solvent).  Soil solution- site of chem. React. And microbial growth  Water quality- Filter for drinking water, sediments, ag-chemicals, and industrial pollutants.  Wetlands  Soil-hydrologic cycle Properties of H2O 1. Structure-Assymmetrical 105 degrees 2. Polarity-Dipolar (electrically/neutral), polymerization 3. High surface tension, cohesion of H2O 4. Density of H2O= 1 g/cm 3 @ 25℃, max @ 4℃ then ↓ to zero. 5. Dielectric constant (e) water= 80 *F= q+ x q-/ e 12.6 d2 *F-force of attraction *D-distance between two charges  Hydrated ions-Greater attraction to H2O than to each other. Forces of H2O A. Adhesion-Attraction of H2O to a surface. B. Cohesion- Attraction of H2O molecules to each other. ~Hygroscopic H2O~ (clay soils)  Adhesion water, ↑ w/ ↑ surface area.  Hygroscopic water= air-dry soil  Layer or 2 of water molecules absorb to clay surfaces, more or less crystalline in nature.  Absorbed water has lower energy than non-absorbed water. Hygroscopic water is held by strong electrical forces and removal requires oven-drying @ 105 ℃. ~Cohesion H2O~  > energy than adhesion water  A portion of it available to plants  Solution phase- plants nutrients dissolved in it. Capillarity  Adhesive and cohesive forces hold water in micropores against forces of gravity.  Water can move in all directions. 