Soil Properties and its development

[kheteshwar borawat]

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Soil Systems Soil Properties Soils can be enormously complex systems of organic and inorganic components. Here, we'll concentrate on a few of the most significant properties, texture, structure, color, and chemistry Figure SS.3 Loamy agricultural soil (Source NRCS Used with permission) Soil Texture Soil texture refers to the relative proportion of sand, silt and clay size particles in a sample of soil. Clay size particles are the smallest being less than .002 mm in size. Silt is a medium size particle falling between .002 and .05 mm in size. The largest particle is sand with diameters between .05 for fine sand to 2.0 mm for very coarse sand. Soils that are dominated by clay are called fine textured soils while those dominated by larger particles are referred to as coarse textured soils. Soil scientists group soil textures into soil texture classes. A soil texture triangle is used to classify the texture class (see below). Click image to enlarge Figure SS.  4 Soil Texture Triangle The sides of the soil texture triangle are scaled for the percentages of sand, silt, and clay. Clay percentages on the left side of the triangle are read from left to right across the triangle (dashed lines). Silt runs from the top to the bottom along the right side and is read from the upper right to lower left (light, dotted lines). The percentage of sand increases from right to left along the base of the triangle. Sand is read from the lower right towards the upper left portion of the triangle (bold, solid lines). The boundaries of the soil texture classes are highlighted in blue. The intersection of the three sizes on the triangle give the texture class. For instance, if you have a soil with 20% clay, 60% silt, and 20% sand it falls in the "silt loam" class.

Self-Assessment What is the texture class for a soil having 45% clay, 45% silt, and 10% sand? Choose one of the answers below by clicking a button

Soil texture effects many other properties like structure, chemistry, and most notably, soil porosity, and permeability. Soil porosity refers to the amount of pore, or open space between soil particles. Pores are created by the contacts made between irregular shaped soil particles. Fine textured soil has more pore space than coarse textured because you can pack more small particles into a unit volume than larger ones. More particles in a unit volume creates more contacts between the irregular shaped surfaces and hence more pore space. As a result, fine textured clay soils hold more water than coarse textured sandy soils. Permeability is the degree of connectivity between soil pores. A highly permeable soil is one in which water runs through it quite readily. Coarse textured soils tend to have large, well-connected pore spaces and hence high permeability.  Soil Structure Soil structure is the way soil particles aggregate together into what are called peds. Peds come in a variety of shapes depending on the texture, composition, and environment. Figure SS. 5 Common soil structure forms Granular, or crumb structures, look like cookie crumbs. They tend to form an open structure that allows water and air to penetrate the soil. Platy structure looks like stacks of dinner plates overlaying one another. Platy structure tends to impede the downward movement of water and plant roots through the soil. Therefore, open structures tend to be better agricultural soils. Bulk density of a soil is the mass per unit volume including the pore space. Bulk density increases with clay content and is considered a measure of the compactness of the soil. The greater the bulk density, the more compact the soil. Compact soils have low permeability, inhibiting the movement of water. The use of heavy agricultural equipment can cause compaction of soil, especially in wet clay soil. Soil compaction results in reduced infiltration and increase runoff and erosion. Soil Chemistry As plant material dies and decays it adds organic matter in the form of humus to the soil. Humus improves soil moisture retention while affecting soil chemistry. Cations such as calcium, magnesium, sodium, and potassium are attracted and held to humus. These cations are rather weakly held to the humus and can be replaced by metallic ions like iron and aluminum, releasing them into the soil for plants to use. Soils with the ability to absorb and retain exchangeable cations have a high cation-exchange capacity. Soils with a high cation-exchange capacity are more fertile than those with a low exchange capacity. Hydrogen ion concentration in the soil is measured in terms of the pH scale.  Soil pH ranges from 3 to 10. Pure water has a pH of 7 which is considered neutral, pH values greater than seven are considered basic or alkaline, below seven acidic. Most good agricultural soils have a pH between 5 and 7. Though acidic soils pose a problem for agriculture due to their lack of nutrients, alkaline soils can pose a problem as well. Alkaline soils may contain appreciable amounts of sodium that exceed the tolerances of plants, contribute to high bulk density and poor soil structure. Alkaline soils are common in semiarid regions. Figure SS. 6 Soil pH Previous | Continue

Contents |Glossary | Atlas |  Index  |  Blog | Podcast |  Updates | Top of page WebActive: Active Learning on the Web About TPE | Who's Using TPE |  Earth Online Please contact the author for inquiries, permissions, corrections or other feedback. For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography. 2006. Date visited.  http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html © 2003-2009 Michael Ritter (tpea...@mac.com) Last revised 12/24/08

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Soil Systems Soil Profiles Soil formation begins first with the break down of rock into regolith. Continued weathering and soil horizon development process leads to the development of a soil profile, the vertical display of soil horizons. Watch the typical progression of a soil profile then read the description below of a generic, fully developed soil. Figure SS. 7 A Typical Soil Profile (after Oberlander & Muller, 1987) O Horizon At the top of the profile is the O horizon. The O horizon is primarily composed of organic matter. Fresh litter is found at the surface, while at depth all signs of vegetation structure has been destroyed by decomposition. The decomposed organic matter, or humus, enriches the soil with nutrients (nitrogen, potassium, etc.), aids soil structure (acts to bind particles), and enhances soil moisture retention. A Horizon Beneath the O horizon is the A horizon. The A horizon marks the beginning of the true mineral soil. In this horizon organic material mixes with inorganic products of weathering. The A horizon typically is dark colored horizon due to the presence organic matter. Eluviation, the removal of inorganic and organic substances from a horizon by leaching occurs in the A horizon. Eluviation is driven by the downward movement of soil water. E Horizon The E horizon generally is a light-colored horizon with eluviation being the dominant process. Leaching, or the removal of clay particles, organic matter, and/or oxides of iron and aluminum is active in this horizon.  Under coniferous forests, the E horizon often has a high concentration of quartz giving the horizon an ashy-gray appearance. B Horizon Beneath the E horizon lies the B horizon. The B horizon is a zone of illuviation where downward moving, especially fine material, is accumulated.  The accumulation of fine material leads to the creation of a dense layer in the soil. In some soils the B horizon is enriched with calcium carbonate in the form of nodules or as a layer. This occurs when the carbonate precipitates out of downward moving soil water or from capillary action. The diagram below illustrates the effect of climate on eluviation and illuviation. Eluviation is significant in humid climates where ample precipitation exists and a surplus in the water balance occurs. Illuvial layers are found low in the soil profile. Illuvial zones are found closer to the surface in semiarid and arid climates where precipitation is scarce. Capillary action brings cations like calcium and sodium dissolved in soil water upwards where they precipitate from the water. Figure SS.8 Eluviation and illuviation under humid, semiarid and arid conditions. (after Marsh, 1987) C Horizon The C horizon represents the soil parent material, either created in situ or transported into its present location. Beneath the C horizon lies bedrock. Figure SS.9 Glacial till exposed in a moraine; a typical parent material for soils in the central United States. (Image Source: Agriculture Agri-Food Canada. Used with permission) The preceding paragraphs describe a generic soil profile, yet not all soils have each one of the horizons, nor are they all the same with respect to thickness composition and structure. Newly formed "immature" soils may only have an O-A-C sequence while older more "mature" soils display the full profile of horizons as described above. The particular compositional, structural and chemical composition of the soil depends on the various factors that influence soil formation. Previous | Continue

 

Contents |Glossary | Atlas |  Index  |  Blog | Podcast |  Updates | Top of page WebActive: Active Learning on the Web About TPE | Who's Using TPE |  Earth Online Please contact the author for inquiries, permissions, corrections or other feedback. For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography. 2006. Date visited.  http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html © 2003-2009 Michael Ritter (tpea...@mac.com) Last revised 12/24/08

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Soil Systems Factors Affecting Soil Development Soil research has shown that soil profiles are influenced by five separate, yet interacting, factors: parent material, climate, topography, organisms, and time. Soil scientists call these the factors of soil formation. These factors give soil profiles their distinctive character. Parent Material Soil parent material is the material that soil develops from, and may be rock that has decomposed in place, or material that has been deposited by wind, water, or ice. The character and chemical composition of the parent material plays an important role in determining soil properties, especially during the early stages of development. Figure SS.9 Stabilized dunes are a form of Eolian (wind deposited) parent material (Source: Agriculture Agri-Food Canada) Soils developed on parent material that is coarse grained and composed of minerals resistant to weathering are likely to exhibit coarse grain texture. Fine grain soil develop where the parent material is composed of unstable minerals that readily weather. Parent material composition has a direct impact on soil chemistry and fertility. Parent materials rich in soluble ions-calcium, magnesium, potassium, and sodium, are easily dissolved in water and made available to plants. Limestone and basaltic lava both have a high content of soluble bases and produce fertile soil in humid climates. If parent materials are low in soluble ions, water moving through the soil removes the bases and substitutes them with hydrogen ions making the soil acidic and unsuitable for agriculture. Soils developed over sandstone are low in soluble bases and coarse in texture which facilitates leaching. Parent material influence on soil properties tends to decrease with time as it is altered and climate becomes more important. Climate Soils tend to show a strong geographical correlation with climate, especially at the global scale. Energy and precipitation strongly influence physical and chemical reactions on parent material. Climate also determines vegetation cover which in turn influences soil development. Precipitation also affects horizon development factors like the translocation of dissolved ions through the soil. As time passes, climate tends to be a prime influence on soil properties while the influence of parent material is less. Climate, vegetation, and weathering Climate affects both vegetative production and the activity of organisms. Hot, dry desert regions have sparse vegetation and hence limited organic material available for the soil. The lack of precipitation inhibits chemical weathering leading to coarse textured soil in arid regions. Bacterial activity is limited by the cold temperatures in the tundra causing organic matter to build up. In the warm and wet tropics, bacterial activity proceeds at a rapid rate, thoroughly decomposing leaf litter. Under the lush tropical forest vegetation, available nutrients are rapidly taken back up by the trees. The high annual precipitation also flushes some organic material from the soil. These factors combine to create soils lacking much organic matter in their upper horizons. Climate, interacting with vegetation, also affects soil chemistry. Pine forests tend to dominate cool, humid climates. Decomposing pine needles in the presence of water creates a weak acid that strips soluble bases from the soil leaving it in an acidic state. Additionally, pine trees have low nutrient demands so few soil nutrients are taken back up by the trees to be later recycled by decaying needle litter. Broadleaf deciduous trees like oak and maple have higher nutrient demand and thus continually recycle soil nutrients keeping soils high in soluble bases.  Topography  Topography has a significant impact on soil formation as it determines runoff of water, and its orientation affects microclimate which in turn affects vegetation. For soil to form, the parent material needs to lie relatively undisturbed so soil horizon processes can proceed. Water moving across the surface strips parent material away impeding soil development. Water erosion is more effective on steeper, unvegetated slopes. Effect on soil erosion Slope angle and length affects runoff generated when rain falls to the surface. Examine the diagram below showing the relationship between hill slope position, runoff, and erosion. Figure SS.10 Hill slope position, runoff & erosion The amount of water on a particular hill slope segment is dependent on what falls from precipitation and what runs into it from an upslope hill slope segment. The hill slope in Figure SS.10 has been divided into several segments and the amount of precipitation falling on each segment is the same. As water runs down slope, the water that has accumulated in segment A runs off adding to what falls into segment B by precipitation. The water in B runs into C, and C into D, and so on. The amount of water increases in the down slope direction as water is contributed of water from upslope segments. The velocity of the water increases as well as it moves towards the base of the slope. As a result, the amount and velocity of water, and hence rate of erosion increases as you near the base of the slope. Rather than infiltrating into the soil to promote weathering and soil development, water runs off. Erosion causes stripping of the soil thus preventing parent material to stay in place to develop into a soil. So we should expect to find weakly developed soil at the mid- and near the bottom of the slope. Effect on deposition and soil texture Water velocity not only determines the rate of erosion but the deposition of soil material in suspension too.  Figure SS.11 shows the relationship between location and texture. Sites A, B, and C, are located progressively further from the base of a slope. A soil texture triangle is used to illustrate the variation in soil textures at the three sites.  Figure SS.11 Location, Deposition and Soil Texture (after Marsh, 1984) As water empties from a mountain stream, its velocity starts to decrease. The largest size particles, like sand, are the first to drop out of suspension (Site A). Fine, clay size particles can be carried further away from the base of the slope before they are deposited. As a result, coarse textured soils tend to be found near the base of the mountain and fine textured soils are located further away (Site C). Microclimatic effects Hill slope orientation affects the microclimate of a place. As the slope of the surface increases, so does the local sun angle, up to a point. As the local sun angle increases, the intensity of heating increases, causing warmer surface temperatures and, likely, increased evaporation. Orientation of the hill slope is certainly important too. Those slopes which face into the sun receive more insolation than those facing away. Thus inclined surfaces facing into the sun tend to be warmer and drier, than flatter surfaces facing way from the sun. The microclimate also impact vegetation type. Previous | Continue

 

Contents |Glossary | Atlas |  Index  |  Blog | Podcast |  Updates | Top of page WebActive: Active Learning on the Web About TPE | Who's Using TPE |  Earth Online Please contact the author for inquiries, permissions, corrections or other feedback. For Citation: Ritter, Michael E. The Physical Environment: an Introduction to Physical Geography. 2006. Date visited.  http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html © 2003-2009 Michael Ritter (tpea...@mac.com) Last revised 12/24/08

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