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Lecture 5

ERS 234 Lecture 5: Forest Soils – Week Five – Lecture Five

8 Pages

Environment and Resource Studies
Course Code
Maren Oelbermann

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find more resources at Forest Soils – Week Five – Lecture Five Concept of Forest Soil 1. Forest soil is a medium for plant growth  Mechanical support  Offers moisture and nutrients  Forest soil differs from agricultural and other soils o Forest soil has a forest floor soil horizon (L, F, H) 2. Natural body which chemical, physical and biological properties governed by 5 soil forming factors  Climate  Topography  Biology  Geology  Time 3. Soil is a component of the forest ecosystem  Addition, transformation, translocation and loss of materials in naturally occurring biogeochemical cycles. Soil Morphology The Soil Profile - A two-dimensional section or lateral view of a soil excavation - Soil profile is divided into a number of sections referred to as ▯horizons▯ - Organic horizon further divided into L, F, H (Canadian System) o Bedrock is also referred to as R horizon - Elluviated: leached soil horizon - Illuvated: receives leached materials - Lower letters o Used as suffixes o Design specific properties of the master horizon  b = buried  e = elluviated  g = grey color  h = enriched with OM  k = presence of carbonates  s = horizon with salts Physical Properties of Forest Soil Soil Color: - Wide array of colors - Different soil colors occur: o Mineral Composition  Iron = red color o Organic matter content  Black or dark brown o Drainage find more resources at find more resources at  Green or blue = poor aeration - Color can be determined in the field using Musell Soil Color Chart Soil Texture: - Relative properties of sand, silt, and clay in soil - Sand, silt, and clay based on their particle size o Also referred to as soil separates o Sand = 2 to 0.05mm o Silt = 0.05 to 0.002mm o Silt = smaller than 0.002mm - This is determined by setting a column or using a textural triangle Soil Structure: - The arrangement of soil particles based on: o Particle size o Shape o Soil texture also influences structure (e.g. clay vs. sand) - What contributes to different soil structure? o Organic matter content & presence of microorganisms o Chemical reactions o Wetting and drying - A well-structured soil is able to retain and transmit water and provide nutrients compared to a poorly structured soil - Many different soil structures o Most common soil structures:  Granular, blocky, platy, prismatic, columnar  Granular: resembles cookie crumbs and is usually less than 0.5cm in diameter. Commonly found in surface horizons where roots have been growing  Blocky: Irregular blocks that are usually 1.5-5cm in diameter  Prismatic: Vertical columns of soil that might be a number of cm long. Usually found in lower horizons.  Columnar: Vertical columns of soil that have a salt ▯cap▯ at the top. Found in soils of arid climates.  Platy: thin, flat plates of soil that lie horizontally. Usually found in compacted soil.  Single Grained: Soil is broken into individual particles that do not sick together. Always accompanies a loose consistence. Commonly found in sandy soils.  Figure 5.5 in textbook Soil Organic Matter (SOM) - Several important functions in forest soil o Provides structure by binding/gluing soil particles o Increases soil porosity and aeration o Moderates soil temperature fluxuations find more resources at find more resources at o Increases moisture holding capacity o Important source of nutrients and carbon  Soil carbon sequestration - Where does SOM come from? o Autumnal litterfall  Annual input of leaves is the largest source of organic matter in forests  Fine root turnover also contributes  Leaves decompose – release of CO2, water, and energy  By product of decomposition is humus  Dark material  Complex chemical structure: no further decomposition by microbes.  SOM in undisturbed forest is in equilibrium (C sink)  Forest disturbances (harvest) causes SOM loss (C source)  Can manage forests for harvest that minimize SOM loss o Retention of slash o Site preparation techniques o Harvest techniques Soil Water - Soil moisture influences the distribution and growth of forest vegetation o Too much water (flooding) kills trees o Water scarcity can be controlled by eliminating competing vegetation to maximize water availability to trees in a newly planted forest plantation. - Soil moisture also transports nutrients to tree roots - Soil moisture influences other factors o Soil temperature o Soil aeration o Microbial activity o Runoff and erosion o Accumulation of toxic substances - Soil retains water through adhesive forces o Strong adhesion of water molecules to soil prevents plants from taking up the water  This occurs when water availability is low o If water supply is sufficient, water molecules have a weak adhesion and allows for plant uptake. - In soil, water moves from an area of saturated flow to an area of unsaturated flow - Saturated Flow: o Occurs in areas of soil with high water availability (below water table) o Occurs in old root channels or burrowing animal channels find more resources at find more resources at o Usually associated with macro-pores - Unsaturated Flow: o Occurs when water availability is low (above water table) o Usually associated with micro-pores Soil Organisms - Play an important role in tree growth and forest soil - Decompose OM and release nutrients taken up by trees - Influence soil profile development o Translocate nutrients from upper to lower horizons o Are key players in development of the O horizon - Form symbiotic relationship with tree roots to enhance nutrient uptake o Mycorrhizae o Nitrogen fixing bacteria Role of Roots in Soil 1. Add organic matter to the soil 2. Stimulate microorganisms via root exudates 3. Produce organic acids that solubilize compounds otherwise insoluble in water 4. Hold and exchange nutrients within the soil 5. Produce toxic compounds to inhibit the establishment and growth of other plants. 6. Protect against erosion Soil Chemical Properties - Soil pH o pH influences microbial populations o Influences nutrient availability (N, P, Ca, Mg) o Influences rate of nitrification o Forest soil is usually more acidic (coniferous forest)  Soil developed from calcareous bedrock is not acidic o Soil acidity can be ameliorated by adding lime o Soil pH usually decreases with N fertilizers are added. - Cation Exchange o Complex chemical reaction between soil particle (humus or clay have negative charge) and plant roots and nutrients in soil solution/water o CEC is the capacity of
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