NRES 201 1nd EditionExam # 2 Study Guide Lectures: 14 - 26Soil Organic MatterWhat is soil organic matter and what are its functions? Soil Organic Matter is the organic component of soil, consisting of living organisms (biomass), organismal remains (residues), and decay products (humus) Functions - -Nutritional o Energy and C source for heterotrophic microbeso Supplies N, P, and S to plants and microbeso Increases nutrient holding capacity, Binds micronutrients, Contributes buffer capacity -Physicalo Improves soil structure, aggregation, and macroporosity- critical for temperate soilso Enhances water infiltration and aerationo Increases water - holding capacity - important for sandy soilso Reduces erosion and compactiono Promotes root growtho Decreases evaporation o Reduces crustingImportant for clayey soils-Biological o Essential for all heterotrophs including most microbes, macro and micro fauna -Ecologicalo SOM is the Earth's major surface C reservoir How does residue decomposition vary with C/N ratio?-SOM decreases when residue decomposition is greater than residue input-SOM increases when residue input is greater than residue decomposition -Residue inputs are increased by - o High crop yields with:Improved varieties, High planting rates, Optimal fertilizer useo Decomposition decreased by:Reduced tillage Less decomposable residues having:-A higher C/N ratio-Or a higher lignin contentAvoiding excessive N or P fertilization -For residues with a high C/N ratio (corn stalks)o Decomposition is prolonged o N is tied up (immobilized) o Plant N availability is depressed-For residues with a low C/N ratio (alfalfa tops)o Decomposition is rapido N is liberated (mineralized)o Plant N availability increases-Rate of decompositiono Faster with a low C/N ratioo Slower with a high C/N ratioHow do soil orders vary in organic matter content?More organic matter in fine textured soils because:- A higher water content reduces aeration- Clay protects organic matterTillage and drainage promote the loss of soil organic matter- By stimulating microbial decomposition What effect has synthetic N fertilization had on soil organic matter levels in the Morrow Plots?Synthetic N fertilizers also promote organic matter depletion - as documented for the Morrow Plots-N fertilization increased residue C inputs but not SOC storage - in five decades of corn-Synthetic N fertilization does not replace crop rotationSoil WaterHow do polarity and H bonding affect the properties of water?Polarity - -H2O forms by covalent bonding-V - shaped configuration due to greater electron affinity by O than H-The result is a polar molecule consisting of:o An electronegative O atomo Two electropositive H atomsHydrogen Bonding - -Arises when an H atom of one water molecule is attracted to the O end of another-The reason why water polymerizes and is unusually high in: boiling point, specific heat, viscosityWhat causes capillarity?Water rises in a tube when:-Some of the molecules climb up the tube wall by adhesion and they pull others up by cohesionWhat affects the height and rate of capillary rise in soils?Height --Determined by the upward - acting force of attracting to the tube wall, which effectively reduces the downward - acting force of gravity for water in the tube, relative to free water outside the tube. -Increases with o decrease in tube radius because the wall is in contact with a greater proportion of water in the tube. o lower liquid density o greater surface tensiono greater adhesive attraction to the tube surface Rise - -Occurs in all moist soils-Limited by o Tortuous nature of soil poreso Entrapment of air-Height of rise: clays > loams > sands-Rate of rise: sands > loams > claysHow does water flow in relation to potential?Water runs downhill - from a place of higher potential energy to a place of lower potential energyWhat are the four potentials that determine the energetics of soil water? Which one is most important?Matric Potential (most important)-Water is attracted to soil solids. This attraction (adhesion) lowers the "free" energy of the water.Osmotic Potential-Due to its polarity, water is attracted to ions and other solutes. These attractions lower the "free"energy of water.Gravitational Potential-The amount of work required to move a given mass of water against gravity. -Positive value when above the reference elevation. Usually chosen within or at the lower boundary of the soil profile. -Important for removing excess water from the upper profile.Hydrostatic Potential-Positive pressure of water in the saturated zone below the water table. How is soil water classified with respect to saturation, field capacity, the wilting coefficient, the hygroscopic coefficient, and oven dryness?‐Saturation-Also known as maximum retentive capacity -Also soil pores filled with water - macro and micropores -Free water on the soil surface-Water will drain by gravity unless drainage is restricted by a hardpanField Capacity -Soil water content after gravitational water has drained from macro-pores-Occurs 2-3 days after rainfall-Macro-pores still hold water = the larger ones supply plants-Ideal for - o Supplying plant - available watero Tillage or excavationo Aeration of roots and microbesWilting Coefficient-Also known as the permanent wilting point or percentage -Soil water content that supplies no water to plantso Herbaceous plants wilt and cannot recover -Soil is dusty dryHygroscopic Coefficient-Lower water content than the wilting coefficient -Soil water content when evaporation ceases-Hygroscopic water exists in very thin filmso Unavailable to plantso Moves only in the vapor phaseo Behaves like moisture in booksOven - dryness-Soil water content after drying at 110 C for 48 hours-Only chemically bonded water is present-Dried samples readily gain water from the airHow is gravimetric water content calculated?(Wet weight of soil - Oven dry weight of soil)/Oven dry weight of soilExpressed as - -A percentage or a proportionWhat two factors cause infiltration, and how do infiltration rates compare for fine‐ versus coarse‐ textured soils?Infiltration-Caused in part by gravity and also by the affinity of water for soil solids-Most rapid for course - textured soils due to extensive macroporosity -Slowest for fine - textured soilso But affected by surface aggregation-Decreases with time due to:o