Soils Study Guide: Functions, Components, and Properties - Prof. Yuji Arai, Study notes of Environmental Science

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Soils Study Guide Chapter 1 6 major functions of soil -Medium for plant growth - Systems for water supply and purification - recycling system for nutrients and organic waste - Habitat for soil organisms -engineering medium - modifier of the atmosphere Plant medium functions: -physical support for anchoring the root system - Ventiliation of O2 and CO2 through pore space in soils - Water holding capacity via deep roots extension - Insulation: protect from heat damage Soils act as a modifier of the atmosphere - When soils are dry they can be eroded by wind - The evaporation of soil moisture = major source of water vapor in the atmosphere: affects air temp, and composition and weather patterns Horizons: O, A, E, B, C O- litter or freshly added debris, may be thin or nonexistent, depends on vegetation A- rich mineral particles and organic matter, and darker in color than the rest B- accumulation of weathered materials 4 major components in loam surface soils - Mineral(45%), Air (20-30%), Water (20-30%), Organic (5%) Mineral Groups: Primary: Chemically not altered since its deposition and crystallization from molten lava (Quartz, SiO2) Secondary: one resulting from weathering of primary materials (clays) (Kaolinite) 3 major size classes: Sand: 2-0.05 mm Silt: 0.05- 0.002 mm Clay: < 0.002 mm Soil Organic Matter: organic carbonaceous: rich in carbon; substances including living organisms (biomass) and decomposed remains of organisms and plant matters (acts a “natural glue” to stabilize soil particles) Macronutrients: Carbon (C), Hydrogen (H), Oxygen(O), Phosphorous (P), Potassium (K), Nitrogen (N), Calcium (Ca), Magnesium (Mg), Sulfur (S) Micronutrients: Boron (B), Iron (Fe), Cobalt (Co), Chlorine (Cl), Manganese ( Mn) Molybdenum (Mo), Nickel (Ni), Copper (Cu), Zinc (Zn) Soil-air exchange at the soil-atmosphere interface -About 25 % air content in soils is an optimum condition for plants -Ventiliation of air through the network of soil pore (CO2 and O2 exchange) : CO2 in the air- 0.035 % : CO2 in soils- 0.1-10 % (higher) Nutrients are present in soil solutions, and they are constantly replenished away 3 principal mechanisms for plant nutrient uptake: (Root interception, Mass flow, and Diffusion) Once nutrients are available in soil solutions, plant roots can uptake nutrient via mass flow and diffusion at the rhizophere Soil Quality: a measure of the ability of a soil to carry out particular ecological functions Degradation: Anthropogenic inputs and mismanagement destroy the soil quality CH 4--------- 3 Components of soil color: provides properties and conditions Hue: redness or yellowness Chroma: Intensity or brightness Value: lightness or darkness Causes of soil color: -Organic matter content( high OM exhibits dark color - Water content: moist soil- dark in color - Presence of oxidized iron and manganese oxides: decrease in water content--- more soil air (oxygen)—makes soil color more reddish Gley: low chroma indicates the waterlogged history, presence of reduced iron (Fe) Soil texture is “size of distribution of particles” Surface area is the unique relationship between particle size and surface area -the smaller the particle size, the greater the surface area 5 properties influenced by surface area: 1. water retention: the greater the SA, the greater the retention 2. Nutrient and other chemical retention: the greater the SA, the greater the retention of nutrients and contaminents 3. The greater the SA, the greater the rate of release of nutrients from weatherable materials 3 major forces affecting the energy level of soil water: Matric force: responsible for absorption and capillarity Osmotic force: reduce the energy state of water in the soil solution Gravity: pulls the water downward Gravitational Potential: (ψg)= g*hg)= g*h Pressure Potentials: Hydrostatic: ψg)= g*hs Matric Potential: ψg)= g*hm Osmotic Potential: ψg)= g*ho It is attributable to the presence of both inorganic and inorganic solutes (Na+ and ethanol) in soil solutions The greater the solute concentration, the lower the osmotic potential -Water moves in response to differences in osmotic potential only if semipermeable membranes exist Understand the relationship between the soil moisture content and the energy levels in kPa Understand the soil water versus energy curves ( as a function of aggregation and texture) Hysteresis- the unique relationship between the soil water potential and the soil water content when soils are drying or re- wetting Expressing soil moisture content -Gravimetric Water Content: Wt of wet soil – wt of dry soil/ wt of dry soil -Volumetric water content: Volume of water cm3/ volume of dry soil, cm3 -Volumetric water content * total depth of soil Maximum Retention Capacity: When all soil pores are filled with water (saturated) MRC and soil depth are used to predict how much water can be stored in a soil. (used to help avoid flooding downstream) Field Capacity: With no addition of water, soil pores will begin to drain, Water in largest pores drain first -Air replaces water in macro-pores, water movement occurs but it is now due to the capillary force. Wilting Coefficient: process to reach wilting point -Soil is not completely dry at this stage, water is still held in small pores- held tightly by matric potential Hygroscopic coefficient: The water content gets reduced via evapotranspiration and evaporation - Water is tightly held by soil particles (mostly absorbed on colloidal surfaces 3 types of water movement Saturated flow: pores are filled with water Unsaturated flow: large pores in soil are completely filled with air, smaller pores hold and transmit water. Vapor movement: vapor pressure differences develop in relatively dry soils Water always flows from a zone of higher potential to one of lower water potential Darcy’s Law: Ksat = QL/ delta H At (cm/s) Factors that Influence Hydraulic Condictivity -Alteration of the size and configuration of soil pores - Preferential flow- water moves much faster than we predict Unsaturated Flow: Most soil water movement occurs when soils are unsaturated, movement occurs in a more complicated environment in which characterizes saturated water flow When the moisture content is low in soils hydraulic conductivity occurs at higher rates in clayey soils When there is higher moisture content in soil, hydraulic conductivity occurs at higher rates in sandy soils. Infiltration: the process by which water enters the soil pore spaces and becomes soil water -As pores are filled with water the infiltration rate levels off -The rate is not constant over time. It decreases with increasing time Percolation: the movement of water, within the soil, downward through the soil profiles. The rate of percolation is related to the hydraulic conductivity Stratified soil profile: -Water is applied to medium textured soils - The downward movement of water stops when a coarse texture layered is encountered -After 400 min the water content of the overlying layer becomes sufficiently high and the downward movement of water into the sand takes place.