NRES 201 1nd Edition Exam 2 Study Guide Lectures 14 26 Soil Organic Matter What 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 microbes o Supplies N P and S to plants and microbes o Increases nutrient holding capacity Binds micronutrients Contributes buffer capacity Physical o Improves soil structure aggregation and macroporosity critical for temperate soils o Enhances water infiltration and aeration o Increases water holding capacity important for sandy soils o Reduces erosion and compaction o Promotes root growth o Decreases evaporation o Reduces crusting Important for clayey soils Biological o Essential for all heterotrophs including most microbes macro and micro fauna Ecological o 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 use o 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 rapid o N is liberated mineralized o Plant N availability increases Rate of decomposition o Faster with a low C N ratio o Slower with a high C N ratio How 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 matter Tillage 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 rotation Soil Water How 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 atom o Two electropositive H atoms Hydrogen 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 viscosity What 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 cohesion What 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 tension o greater adhesive attraction to the tube surface Rise Occurs in all moist soils Limited by o Tortuous nature of soil pores o Entrapment of air Height of rise clays loams sands Rate of rise sands loams clays How does water flow in relation to potential Water runs downhill from a place of higher potential energy to a place of lower potential energy What 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 hardpan Field 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 water o Tillage or excavation o Aeration of roots and microbes Wilting Coefficient Also known as the permanent wilting point or percentage Soil water content that supplies no water to plants o Herbaceous plants wilt and cannot recover Soil is dusty dry Hygroscopic Coefficient Lower water content than the wilting coefficient Soil water content when evaporation ceases Hygroscopic water exists in very thin films o Unavailable to plants o Moves only in the vapor phase o Behaves like moisture in books Oven 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 air How is gravimetric water content calculated Wet weight of soil Oven dry weight of soil Oven dry weight of soil Expressed as A percentage or a proportion What 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 soils o But affected by surface aggregation Decreases with time due to o Breakdown of soil structure upon wetting o Clogging of soil pores by displaced solids o Swelling of clay particles o The filling of surface soil pores when drainage is limited by low subsoil permeability What distinguishes saturated from