Phyllosilicate compositions 2:1 - Illite - K y (Si 8-y Al y )(Al 4 )O 20 (OH) 4 y < 2 2T O K + in the interlayer 2:1 - Smectite - X 0.8 (Si 7.7 Al 0.3 )(Al 2.6 Fe +3 0.9Mg 0.5 )O 20 (OH) 4 2T O Individual particles are 1 layer thick; hydrated cations are associated with both surfaces when water is present.
Phyllosilicate compositions 2:1:1 - Chlorite (Mg,Fe +2,Al) 6 (OH) 12.(Si,Al) 8 (Mg,Fe +2,Al) 6 O 20 (OH) 4 O 2T O 1:1 - Kaolinite - Si 4 Al 4 O 10 (OH) 8 Hydrogen bonds between the octahedral surface of one sheet and the tetrahedral surface of the next.
Frost Heave The fundamentals of how: chemical factors, mineral surfaces, soil texture and freezing temperatures combine to produce frost heaving
Frost heave An elevation of the soil surface associated with the growth of pure ice (ice lenses) within the soil. Requirements for frost heave Freezing temperatures Water Freezing point depression A frost-susceptible soil
Frost Heave in a Silty Clay Soil
Freezing point depression of water Factors: Osmotic pressure: dissolved salts and ions. Physical pressure: Capillary reasons: Confinement in a small space: the smaller the radius of curvature, the greater the internal pressure, and the lower the freezing temperature
Schematic diagram of an ice lens, on/in soil, that lacks a frozen fringe
Schematic diagram of an ice lens with a frozen fringe
Schematic diagram of a portion of the frozen fringe from previous figure The relative thicknesses of the unfrozen water films, their cation and anion concentrations and the relative solution concentrations at points A, B, C, and D on the ice/water interface, as governed by the temperature at each location in the frozen fringe.
Frost susceptibility (FS) of a soil Is Determined by: Ability of pores to support a freezing point depression (FPD) Ability to transmit water to the freezing front (hydraulic conductivity (HC) Ability to transmit water to the ice lens bases above the soil particles (UFC) Sand Clay Silt FPD low high moderate HC high low moderate UFC insignificant high moderate FS low low to moderate high
Questions on frost heave?
Soils as ecosystems Soils, as the skin of the Earth, are located at the geosphere/atmosphere boundary and are part of the biosphere. Their location makes them vulnerable, yet easy to ignore Study of the soil ecology is greatly hampered by the inability to study soils without greatly disturbing them. NEVERTHELESS There are more organisms per unit area under the soil surface than above, and in many cases the total biomass is greater below the soil surface than above.
Diversity of soil organisms Fauna: range from single-celled Protozoa, through insects and worms to large vertibrates. Flora: range from bacteria and fungi to the roots of herbaceous plants and trees. It s a hugely diverse jungle underground!
Roles of soil organisms 1) To build up soil organic matter: accomplished by green plants (from algae to trees); 2) To destroy organic matter: accomplished by bacteria, fungi, other micro-organisms and soil animals; 3) To redistribute organic matter: accomplished by soil animals (mainly).
Soil fauna Microfauna Mesofauna Macrofauna Herbivores Protozoa springtails insects mites vertebrates nematodes slugs & snails Detritivores Protozoa springtails woodlice earthworms millipedes insects Predators nematodes centipedes insects vertebrates spiders
Bacteria: - single-celled Soil flora - 1 to 4,000 million per gram of soil - tremendous variety - most active above ph 5.5 - It is believed that we have identified only a small proportion of the types present those that we can culture in the laboratory. Bulk soil DNA studies indicate that there must be many, many more. - some are pathogens Two major types: Autotrophic: obtain their energy by oxidizing mineral substances, particularly iron and sulfur. Heterotrophic: obtain their energy solely from organic sources. The majority are free-living forms involved in the breakdown of organic compounds. Some are symbiotic.
Rhizobium and other nitrogen fixers Rhizobium bacteria are symbiotic and form nodules on the roots of leguminous plants (clovers, alfalfa, beans, peas, soybeans, etc.). They are capable of taking atmospheric nitrogen and converting it to plant-available forms (amides and amino acids). They use what they need and pass the rest on to their host plant in return for the carbohydrates (their energy source) supplied by the host plant. They are said to fix nitrogen. There are several other N-fixing bacteria that form similar relationships with other types of plants. There are also free-living N-fixing bacteria. These N-fixing bacteria are the major agents that make atmospheric N available for use by living organisms. Once N is in organic forms, these forms are repeatedly recycled.
Fungi Fungi: - multi-cellular - 5,000 to 1 million per gram of soil - greater total mass than the bacteria - heterotrophic - very important in the early stages of organic decomposition - predominate at low ph, and active over wide ph range - some act as mycorrhizae - some are pathogens Mycorrhizae: These fungi either form a sheath around plant rootlets or invade plant root cells. Both forms send hyphae out into the soil. These hyphae facilitate the obtaining of nutrients by the plant. They are symbiotic.
Actinomycetes Actinomycetes: Intermediate in complexity between bacteria and fungi. They are able to break down some highly resistant organic compounds. The best known is Streptomycetes, the source of the antibiotic streptomycin. The discovery of streptomycin earned Sterling Waksman (whom we soil scientists claim as a soil microbiologist), the Nobel Prize in Medicine. Since this discovery, soil microorganisms have been a major source of antibiotics Some are pathogens potato scab is an example.
Green plants Algae: - single celled - photosynthetic must live near the soil surface - some fix nitrogen (in rice paddies) Lichens: - symbiotic, photosynthetic combinations of algae and fungi. Higher plants: - supply the greatest amount of the organic matter in soils - protect the soil surface from erosion by wind and water - cycle plant nutrients and minimize nutrient loss by leaching - the nature of the vegetation influences the pedological development of the soil.
Soils as Natural Bodies How do soils get to be as they are? In North America, this approach to studying soils is called Pedology. Soil forming factors, horizon development and regional differences
Soil forming factors The soil that develops at a site is a function of the interaction of five soil forming factors: Parent material Climate Organisms Relief (Topography) Time The relative importance of these factors changes with location.
Soil profiles and soil horizons As soils develop under the influence of the soil forming factors, differentiation of a series of layers occurs. These differentiated layers are called soil horizons. The soil horizons are designated by the letters O, A, E, B, and C modified by other letter subscripts to provide more specific information
Soil horizons dominant features O - a zone of organic matter accumulated on the soil surface A - the mineral horizon enriched in organic matter E - a mineral horizon characterized by intense leaching B - a mineral horizon characterized by accumulations of materials washed down from overlying horizons C - the relatively unchanged parent material of the soil
Parent material Originally, investigations of soil development and soil distributions were most commonly conducted by chemists and geologists and were conducted at local or state scales, hence, it is probably not surprising that parent material was initially considered to be the most important of the soil forming factors in determining the characteristics that a soil would exhibit. Influences Mineralogical composition (primary and secondary) Soil texture and the properties that texture influences most notably the finer the texture, the lower the permeability, and the finer the texture, the less advanced is the state of soil development because of slow water movement, higher buffer capacity and more work needing to be done
Climate At the continental and global scales, it becomes obvious that climate is the dominant factor determining soil distribution. Climate acts through the agencies of temperature, moisture, their seasonalities and climate s influence on vegetation.
Climatic zones Climatic zones can be established on the basis of the effects of temperature [3 provinces ] and precipitation [5 provinces ] Dry, cold Wet, cold Perpetual ice and snow (no vegetation) Tundra (mosses, lichens, stunted trees) Taiga (coniferous forest) Arid Semi-arid Subhumid Humid Wet <25 cm 25-50 cm 50-100 cm 100-200 cm >200 cm Desert shrubs and grasses Short grasses Long grasses Forests Rain forests Dry, hot Wet, hot
Temperature Main effects: 1) Effects on amount and type of plant growth 2) Direct effect on the soil temperature - rates of chemical reaction (doubles with a 10ºC rise) - amount of biological activity (optimum is about 35ºC) 3) Effect on evapotranspiration: - 30-70% of annual rainfall in humid temperate region - 70-100% in semi-arid region
Moisture Main effects 1) Must be present for chemical weathering to occur. 2) The ratio between precipitation and potential evapotranspiration ip/e) is important: - the more by which pptn > evapotranspiration, the greater the amount of leaching of soluble materials. The subsoil is normally wet. - if potential evapotranspiration > pptn, there is little leaching, but there is redistribution of soluble materials. The subsoil is normally dry.
Seasonality Rainfall that occurs during season of major plant growth tends to cause relatively little leaching of nutrients because plant nutrient uptake is occurring and evapo-transpiration losses are at a maximum. These combine to limit the downward movement of both water and nutrients. Surface erosion is also limited by plant cover and by plant uptake of water. Rainfall that occurs during the non-growing season for plants,in regions where the soil does not freeze, has a greater proportion penetrate to the water table, and nutrient loss is enhanced by both the increased downward flow and the lesser plant uptake. Surface erosion is also more likely to occur. In regions where the soils freeze, very little water moves downward during the freezing period, but surface erosion can be severe on non-vegetated areas during the spring melt season.
P/E regional relationships Humid regions: soluble alkali (Na, K) compounds are lacking and Ca and Mg salts are in lower concentration than in the parent material. Semi-arid regions: Ca and Mg salts are present and have commonly accumulated somewhere within the top meter to concentrations greater than in the parent material. Arid regions: Ca, Mg, K and Na salts may all be present in amounts greater than in the parent material.
Organisms The roles of organisms in soil development are to: Produce organic matter and contribute it to the soil, commonly onto the surface (tree leaves, needles and above ground parts of herbaceous plants) and in the near surface (from root death, mainly of herbaceous plants). Break down organic matter to humus A role of microorganisms and soil animals Redistribute the organic matter within the soil to produce A horizons A role of soil animals, particularly worms and insects
Effects of Relief Drainage Sloping areas are better drained than valley bottoms, so aeration and plant communities differ with location on the slope Large flat areas of fine textured soils tend to have poor drainage Erosion Erosion of slopes tends to prevent a mature soil from forming by constantly truncating the upper surface Erosion onto the base of the slope has the same effect by constantly bringing new material onto the surface Slope aspect The sunny side is warmer and drier: enhances soil development in both warm humid and cold regions and slows it down in dry regions The windward side is drier, which slows development in dry regions
Time The effect of time is a hard time to quantify, but: It requires time to develop a mature soil profile (one which has the sequence of horizons that normally develops in the specific soilforming conditions that are present) Longer periods of time may deepen the horizons and strengthen the expression of their characteristics Soils developed in sand may reach a mature state in less than 100 years, but the depth of occurrence and strength of expression of horizons may continue to change over the long term In fact, the soil is an ever changing entity. Most of the material that now comprises sedimentary rocks was once soil material, the mineralogy of the sedimentary rock is influenced by production of secondary minerals during its time as soil material, and ultimately the sedimentary rock may be uplifted, form mountains, be weathered and eroded and contribute to the next interval of soil formation in the rock-soil-sediment-rock-etc cycle.
Soil development Soil development refers to the effects of the various processes that contribute to the differentiation of soil horizons and the development of soil profiles In the most general sense, the processes of soil formation can be categorized as a series of: Additions Losses Transformations Translocations
(After Simonson) General Model of Soil Profile Development Additions precipitation, with its dissolved materials particles moved by wind and water organic matter and materials cycled by plants Translocations organic matter clays and oxides other elements Transformations organic matter to humus primary to secondary minerals oxidation/reduction processes Losses water and dissolved material erosion (from the surface)
Specific processes of soil development 1 Additions 2 Losses 3 Translocations 4 Transformations Eluviation (3) movement of material out of Illuviation (3) movement of material into Leaching (2) - eluviation out of the solum (A + B horizons) Enrichment (1) addition of material to the soil Erosion (2) loss of particles from the surface Cumulization (1) addition of mineral particles to the surface Salinization (3) accumulation of soluble salts Desalinization (3) removal of soluble salts Alkalinizarion (3) accumulation of Na ions on CEC sites Dealkalinization (3) leaching of accumulated Na and Na salts Calcification (3) accumulation of calcium carbonate Decalcification (3) removal of calcium carbonate Lessivage (3) mechanical migration of small mineral particles from A and E to B horizons Pedoturbation (3) biological /physical mixing that homogenizes the solum After Buol et al.
Specific processes of soil development 1 Additions 2 Losses 3 Translocations 4 Transformations Podzolization (3,4) chemical migration of Fe, Al, &/or OM, leaving Silica behind Desilication (4,2) migration of Si, resulting in concentration of Fe /Al oxides/hydroxides Decomposition (4) breakdown of minerals and OM Synthesis (4) formation of secondary minerals and new organic molecules Melanization (1,3) darkening of soil by OM addition Leucinization (4,2) lightening of soil by OM transformation or removal Littering (1) accumulation of organic litter Humification (4) transformation of OM to humus Paludization (1,4) accumulation of deep organic deposits (muck and peat) Ripening (4) chemical, biological and physical of organic soil after drainage Mineralization (4) release of oxide solids by OM decomposition Braunification, Rubification, Ferrugination (3,4) release of iron by weathering to yield brownish, reddish-brown and red colours Gleization (3,4) reduction of ferric iron to ferrous iron (gray) under anaerobic conditions Loosening (4) increase in pore space by any means Hardening (4) decrease in pore space by collapse, compaction of infilling After Buol et al
Development Scenario Humid cool temperate, coniferous forest, sand, well-drained, 10,000 yr Translocations Very little mixing of OM into mineral surface layer Soluble OM compounds and Fe & Al oxides translocated from O & E to the B horizon (eluviation) to the B horizon where they accumulate podzolization). The B horizon is red to black Additions: leaf litter on surface; rain + dissolved; recycled nutrients (few bases) O E Bsh C Transformations OM to humus Primary to secondary minerals Mainly Fe & Al oxides A bit of 2:1 clay formed Spodosol developed (Podzol Canada) Losses: water + dissolved Serious loss of bases
Spodosol - Quebec
Development Scenario Humid cool temperate, mixed forest, loam, well-drained, 10,000 yr Translocations OM mixed into A Some secondary clays and oxides translocated from A & E (eluviation) to the B horizon where they accumulate (illuviation) - lessivage ( cutans developed. Additions: leaf litter; rain + dissolved; recycled nutrients O Ah E? Bt C Transformations OM to humus OM to humus Primary to secondary minerals 2:1 and iron oxides Some primary to secondary Alfisol developed (Luvisol Canada) Losses: water + dissolved Carbonates (ph increases with depth)
Alfisol (Michigan)
Development Scenario Humid warm temperate, mixed forest, loam, well-drained, 100,000 yr Translocations OM mixed into A Some secondary clays and oxides translocate from A & E (eluviation) to the B horizon (illuviation) Cutans developed in B(lessivage) Additions: leaf litter; rain + dissolved; recycled nutrients O (thin) Ah E Bt C Transformations OM to humus OM to humus Primary to secondary minerals 2:1, 1:1 and iron oxides Primary to secondary minerals in both the B and upper C horizons Ultisol developed (Not in Canada) Losses: water + dissolved Carbonates (ph decreases with depth)
Ultisol N. Carolina
Development Scenario Humid tropical, tropical forest, loam, well-drained, 1,000,000 yr Translocations OM mixed into A Residual accumulation of Fe & Al oxides as the layer silicate clays are broken down and the soluble products (including Si) are leached Oxisol developed (Not in Canada) Additions: Regular fall of leaf litter; Abundant rain, with some dissolved material; Efficient plant recycling of nutrients A (thin Bo may be very thick C Losses: Water + dissolved bases and silica Extreme leaching Transformations OM to humus quickly Mineral weathering is extreme; very resistant primary minerals remain; 2:1 clays completely weathered; 1:1 clays remain; Fe and Al oxides and hydroxides dominate (desilication) Colour depends on whether the parent rock was Fe-rich or Alrich
Oxisol - Brazil High Fe near Sao Paolo High Al near Manaus
Development Scenario Semi-arid to sub-humid,(60 cm) grassland, loam, well-drained, 20,000 yr Translocations Additions: rain + dissolved; Particles brought by wind and water erosion Organic matter (shoots and roots) Transformations OM mixed into A, A is fairly deep; thins as rainfall decreases Clays translocated from A to B ((illuviation) - lessivage (cutans). Calcium carbonates accumulates at about the annual depth of percolation of rainfall (lower part of B to upper part of C) Ah Bt prismatic C OM to humus Primary to secondary minerals (2:1 clays) Some primary to secondary minerals Mollisol developed (Chernozem Canada) Losses: Water & dissolved - periodically
Mollisol S. Dakota Well-drained, with a calcic horizon With Na dominating CEC, with a natric horizon
Development Scenario Humid and sub-humid temperate, mixed forest, loam, poorly-drained, 10,000 yr Translocations OM mixed into A Some clay may be transported from A to B Aquic suborders developed (Gleysol Canada) Additions: OM rain & run-on, with dissolved material and eroded particles Ah Btg or Bg Cg Losses: water + dissolved carbonates Transformations OM to humus, but oxidation of OM retarded high OM content, often black Little mineral weathering Reduction process to produce gray colours and rust coloured mottles. Mottles are in the zone where the water table fluctuates.
Aquic soil Aquic Mollisol - Aquic Alfisol - Ontario