What is Soil? Depends on who you ask.

SOIL

What is Soil? Depends on who you ask. To a geologist: residual material formed in situ from weathering of a parent material either bedrock or sediment. not a sediment Profile from alteration of parent material residual vs. transported soils To a soil scientist: organic-rich material that supports plant growth. The life zone: intersection of hydrosphere, atmosphere, lithosphere, and biosphere. soil scientists largely trained/work in agricultural related areas. very specialized. consider economic significance of agricultural: grains, fruits, vegetables, feed stock, ethanol, etc. important considerations: fertility, texture, drainage

What Is Soil? Mixture of organic and inorganic material May range from 100% inorganic (sand) to nearly 100% organic (peat) Inorganic part is minerals Organic part is decayed plant and animal material and is sometimes called humas

Soil Definition Solid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life. Engineering definition: Anything that can be removed without blasting Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates.

SOIL Traditional definition: Material which nourishes and supports growing plants; includes rocks, water, snow, air. Component definition: Mixture of mineral matter, organic matter, water, and air.

SOIL As a portion of the landscape: Collection of natural bodies occupying portions of the earth s surface that support plants and that have properties due to the integrated effect of climate and living matter, acting upon parent material, as conditioned by relief, over periods of time.

HOW DOES SOIL FORM

Factors of formation Climate Organisms (Vegetation/Biology) Relief (Topography) Parent Material Time

Soil formation Key processes in forming soil: weathering and the accumulation and transformation of organic matter They are influenced by the following factors: Climate: soils form faster in warm, wet climates Organisms: plants and decomposers add organic matter Topography: hills and valleys affect exposure to sun, wind, and water Parent material: influences properties of resulting soil Time: soil can take decades to millennia to form

Soil formation Parent material = the base geologic material of soil Lava, volcanic ash, rock, dunes Bedrock = solid rock comprising the Earth s crust Weathering = processes that form soil Physical (mechanical) = wind and rain; no chemical changes in the parent material Chemical = parent material is chemically changed Biological = organisms produce soil through physical or chemical means Humus = spongy, fertile material formed by partial decomposition of organic matter

Soil formation Soil is formed as rocks are weathered and covered with organic material. It takes about 500 years for 1 inch of topsoil to form.

Soil formation

Soil structure Soil aggregation is the cementing of several soil particles into a secondary unit or aggregate. Soil particles are grouped together during the aggregation process to form structural units (or peds). These units vary in size, shape, and distinctness (also known as strength or grade). The structure of the soil affects pore space size and distribution and, therefore, rates of air and water movement. Well-developed structure allows favorable movement of air and water, and root development.

IMPORTANT SOIL PROPERTIES Color Texture Structure Consistence Shrink-swell Potential Bulk Density Porosity Permeability Infiltration Drainage Depth Available Water Holding Capacity Reaction Cation Exchange Capacity Landscape Position

Soil Major Components Mineral matter Organic matter Air Water

Soil Color Indicator of different soil types Indicator of certain physical and chemical characteristics Due to humus content and chemical nature of the iron compounds present in the soil

Soil parent material and weathering The mineral material of a soil is the product of the weathering of underlying rock in place, or the weathering of transported sediments or rock fragments. The material from which a soil has formed is called its parent material. The rate and extent of weathering depends on: the chemical composition of the minerals that comprise the rock or sediment the type, strength, and durability of the material that holds the mineral grains together the extent of rock flaws or fractures. the rate of leaching through the material the extent and type of vegetation at the surface

Layers of Soil Topsoil - where plants get their nutrients Subsoil - not as much organic material Parent Material - provides the minerals in soil Bedrock - mostly solid rock

Layers of Soil Horizon = each layer of soil Soil can have up to six horizons Soil profile = the cross-section of soil as a whole Leaching = dissolved particles move down through horizons Some materials in drinking water are hazardous Topsoil = inorganic and organic material most nutritive for plants

Layers of Soil Principal (master) soil horizons found in managed agricultural fields are: A horizon or mineral surface soil (if the soil has been plowed, this is called the Ap horizon). B horizon or subsoil. C horizon or partially weathered parent material. rock (R) or unconsolidated parent materials similar to that from which the soil developed. Unmanaged forest soils also commonly contain: O (organic) horizon on the surface E (eluviated) horizon: a light-colored leached zone just below the A horizon. A horizon B horizon C horizon

Surface soil horizons: Ap or A + E Ap or A+ E horizons: Contains more organic matter than the other soil layers. Often coarser than the subsoil layer. A or Ap horizon tends to be more fertile and have a greater concentration of plant roots than any other soil horizon. In unplowed soils, the eluviated (E) horizon below the A horizon is often light-colored, coarser-textured, and more acidic than either the A horizon or the horizons below it, because of leaching with time. Ap

Characteristics of the Soil Horizons

Subsurface soil horizons: B B horizon: Typically finer in texture, denser, and firmer than the surface soil. Organic matter content tends to be much lower than surface layer. Subsoil colors are often stronger and brighter: shades of red, brown, and yellow predominating due to the accumulation of iron on clays and other particles. Bt horizon: Subsoil layers with high clay accumulation relative to the A horizon. Bt

Subsurface soil horizons: C C horizon: Partially decomposed and weathered parent material that retains some characteristics of the parent material. More like the parent material from which it has weathered than the subsoil above it. C

Physical Properties of Soil Soil texture Soil structure Soil color Bulk density Three Fractions of Mineral Matter Sand Silt Clay

Soil particles Sand: Particles range in size from very fine (0.05 mm) to very coarse (2.0 mm) in average diameter. Most particles can be seen without a magnifying glass. Feel coarse and gritty when rubbed between the thumb and fingers, except for mica flakes.

Soil particles Silt: Particles range in size from 0.05 mm to 0.002 mm. Cannot usually be seen by the unaided eye When moistened, silt feels smooth but is not slick or sticky. When dry, it is smooth and floury

Soil particles Clay: Particles are finer than 0.002 mm. Can be seen only with the aid of an electron microscope. Feels extremely smooth or powdery when dry, and becomes plastic and sticky when wet.

Using the Soil Texture Triangle

Soil organic matter: Soil organic matter Plant and animal residues in various stages of decay. Sources: dead roots, root exudates, litter and leaf drop, and the bodies of soil animals such as insects and worms. Primary energy and nutrient source for insects, bacteria, fungi, and other soil organisms. After decomposition, nutrients released from the residues available for use by growing plants. Soil humus: Fully decomposed and stable organic matter. Most reactive and important component of soil organic matter. Form of soil organic material that is typically reported as organic matter on soil testing reports.

Factors affecting organic matter content Type of vegetation: Soils under grass generally have a relatively high percentage of organic matter in their surface. Soils that develop under trees often have a low organic matter percentage in the surface mineral soil, but do contain a surface litter layer (O horizon). Organic matter levels are typically higher in a topsoil supporting hay, pasture, or forest than in a topsoil used for cultivated crops.

Drainage: Factors affecting organic matter content Soil organic matter is usually higher in poorly-drained soils because of limited oxidation, which slows down the overall biological decomposition process.

Factors affecting organic matter content Tillage: Soils that are tilled frequently are often low in organic matter. Plowing and otherwise tilling the soil increases the amount of air in the soil, which increases the rate of organic matter decomposition.

Soil texture: Factors affecting organic matter content Soil organic matter is generally higher in fine-textured soils because soil humus forms stable complexes with clay particles. Coarse-textured soils have faster gas exchange, thus more CO 2 loss.

Soil drainage indicators Soil drainage is usually indicated by soil color patterns (such as mottles) and color variations with depth. Clear, bright red and yellow subsoil colors indicate well-drained conditions where iron and other compounds are present in their oxidized forms, as in the two soil profiles to the right.

Soil drainage indicators Poorly drained soils tend to accumulate large amounts of organic matter in their surface horizons because of limited oxidation, and may have very thick and dark A horizons.

Soil survey The soils of most counties have been mapped by the USDA-NRCS Cooperative Soil Survey Program, and these maps are available in soil survey reports. Each soil survey report contains information about soil morphology, soil genesis, soil conservation, and soil productivity.

Soil survey A soil survey report reveals the kinds of soils that exist in the county (or other area) covered by the report at a level of detail that is usually sufficient for agricultural interpretations. The soils are described in terms of their location on the landscape, their profile characteristics, their relationships to one another, their suitability for various uses, and their needs for particular types of management. Soil survey reports are available from county and state USDA-NRCS Cooperative Extension offices and on-line (for certain counties).

Soil porosity: Macropores and micropores Macropores allow the ready movement of air, roots, and percolating water. Movement of air and water through a coarsetextured sandy soil is often rapid despite its low total porosity because of the dominance of macropores. Micropores in moist soils are typically filled with water, and this does not permit much air movement into or out of the soil. Movement of air and water through a fine textured clay soil may be slow (see picture at right) despite high total porosity because of the dominance of micropores.

Altering Soil Structure Unlike texture, structure can be altered by tillage or traffic. Tilling soils that are too wet, or compacting soils with heavy equipment can break down the natural structural units.

Soil color Useful tool for providing information about other soil properties Organic matter content Soil minerals Seasonal high water tables Red color - hematite (iron)

Soil color Useful tool for providing information about other soil properties Organic matter content Soil minerals Seasonal high water tables

Soil color Redoximorphic Features Soil colors formed by the repeated chemical oxidation and reduction of iron and manganese compounds resulting from saturation. Useful for predicting the presence and depth of seasonal high water tables in the soil.

Texture Soils containing large amounts of sand exhibit little plasticity and cannot retain large amounts of water or nutrients. They have large voids between the particles and can readily transport water and air.

Texture The properties of silt are intermediate between sand and clay. Silty soils can retain large amounts of water but tend to have moderately slow to slow permeabilities. Soils high in silt can present problems for engineers since they will shift under stress and slide and flow when wet.

Texture Clays exert a great influence on soil chemical and physical properties. Clay particles are chemically active and the proportion of clay-sized particles greatly influences soil physical properties, including aggregation, porosity, water movement and storage, aeration and workability of the soil.

Structure The arrangement of primary soil particles into compound particles or aggregates. The type and grade of structure plays an important role in the movement of water within soils.

Structure Granular - Rounded aggregates usually less than 1/4 inch in diameter. These rounded complexes lie loosely on the surface and are readily shaken apart. The aggregates are called granules and the pattern is called granular. This is the most common type of structure found in topsoil.

Structure Single grain Each individual soil particle is separate and there is essentially no structure. This is only found in very sandy soils and is the type of structure commonly seen in sand dunes at the beach.

Structure Blocky - The original aggregates have been reduced to blocks, irregularly faced, and basically equal in height, width, and depth. Blocky structure is the most common type of structure seen in the subsoil (B horizon) in North Carolina.

Structure Prismatic Characterized by vertical oriented aggregates or pillars with flat tops. These elongated columns vary in length with different soils. Prismatic structure is commonly seen in soils with high clay content and in horizons dominated by high shrink-swell clays.

Structure Columnar Characterized by vertical oriented aggregates or pillars with rounded tops. These elongated columns with flat tops vary in length with different soils. Most commonly seen in soils that have a high sodium content in a dry climate.

Structure Platy The aggregates are arranged in thin horizontal plates or sheets. This structure is commonly found in soil layers that have been compacted. Platy structure inhibits the downward movement of water.

Consistence The degree and kind of cohesion and adhesion that soil exhibits, and/or the resistance of soil to deformation or rupture under applied stress. Field evaluations of consistence usually include rupture resistance, stickiness, and plasticity.

Consistence Rupture resistance is a measure of the soil s ability to withstand applied stress. For this test, moist soil is normally used. A naturally occurring soil aggregate is placed between the thumb and index finger. Pressure is slowly applied to estimate the amount of force that is required to rupture a soil aggregate.

Consistence Stickiness is the capacity of a soil to adhere to other objects. Stickiness is estimated at the moisture content that displays the greatest adherence when pressed between the thumb and forefinger. This normally occurs when the soil is quite wet.

Consistence Plasticity is the degree to which a reworked soil can be permanently deformed without rupturing. Plasticity is evaluated by forming a roll (wire) of soil that is 4 cm long.

Shrink-swell Potential Shrink-swell potential is a measurement of the amount of volume change that can occur when a soil wets and dries. Most of this volume change is due to the clay fraction of the soil. Clays swell when wet and shrink when dry. Soils high in 2:1 clays such as montmorillonite tend to have high shrink-swell potentials. Soils high in 1:1 clays such as kaolinite tend to have low shrink-swell potentials.

Shrink-swell Potential A soil with a high shrink-swell potential can cause severe problems when used for urban development unless the problem is recognized and proper engineering precautions taken. Shrinking and swelling can buckle roads, crack building foundations and walls, and even damage plant roots.

Bulk Density Compaction also decreases infiltration, thereby increasing runoff and the hazard of water erosion.

Bulk Density Soil compaction restricts rooting depth, which reduces the uptake of water and nutrients by plants.

Permeability Permeability refers to the movement of air and water within the soil. Permeability rate is the rate at which a saturated soil transmits water, usually expressed in inches per hour. Texture, structure, bulk density, and the type and connectivity of macropores influence permeability. Hydraulic conductivity is a measurement of the amount of water that can move downward through a unit area of unsaturated soil in a unit of time. Saturated hydraulic conductivity (k sat ) is a measurement of the amount of water that can move downward through a unit area of saturated soil in a unit of time.

Infiltration Infiltration is the downward entry of water into the immediate surface of the soil and is influenced by texture, structure, bulk density, and the type and connectivity of macropores. Soils with a high infiltration rate are resistant to erosion because there is little runoff.

Drainage Drainage refers to the frequency and duration of periods of saturation or partial saturation. Internal soil drainage is important because of its effect on land use and management decisions.

Drainage An apparent water table is a type of saturation in which all horizons between the upper boundary of saturation and a depth of 6 feet are saturated. Apparent water tables are frequently encountered in low-lying areas where the slope of the land is insufficient to provide good drainage such as flood plains, depressions, and nearly level areas.

Depth The useable depth of the soil is an important consideration when evaluating a soil for a particular land use. Groundwater can be easily impacted in soils with a shallow depth to rock. Root restrictive layers are any permanent zones in the soil that restrict the growth of plant roots. Bedrock is the most common root restrictive layer.

Reaction Reaction is a measure of acidity or alkalinity of a soil. Acidity or alkalinity is determined by the amount of hydrogen and hydroxyl ions in the soil. When hydrogen ions outnumber hydroxyl ions the soil is acidic. In the reverse condition the soil is basic. A ph scale is used to measure the level of acidity or alkalinity.

Cation-Exchange Capacity Cation-exchange capacity (CEC) is a measure of the ability of a soil to hold and exchange cations. It is one of the most important chemical properties in soil and is usually closely related to soil fertility. A few of the plant nutrient cations that are part of CEC include calcium, magnesium, potassium, and ammonium

Soil erosion Fertility decline Two major types of degradation: These are linked as erosion generally removes the most fertile soil, and the processes which lead to highly erodible soils are caused largely by soil fertility decline.

Soil consists of mineral and organic matter, air, and water Dead and living microorganisms Decaying material Bacteria, algae Habitat for earthworms, insects, mammals, reptiles, and amphibians Soil as a system Since soil is composed of interacting living and nonliving matter, it is considered an ecosystem

If all five factors are the same in two geographic regions, the soil will be the same in both. Some basic examples of different soil types include: Temperate deciduous soil Coniferous forest soil Grassland soil Tropical rain forest soil Desert soil

Soil is an excellent place to study interactions in the Earth System, including contributions from the Atmosphere Dew (moisture from the air begins chemical alteration of parent rock Rain erodes loose soil, preventing further alteration temperature controls rate and extent of chemical processes Hydrosphere Water seeps into the ground, dissolving and redistributing elements evaporation dries soil, changing its physical characteristics Biosphere plants add and remove chemicals plant roots anchor soil in place, enabling chemical reactions to be completed animals mix soil; transport seeds, etc. Geosphere solid rock and unconsolidated sediment are the parent material for soil geologic processes (surface and internal) expose and bury rock, etc.

Hydrologic Cycle and the Soil Soil Properties related to the hydrologic cycle. Soil moisture Color Structure Texture Bulk Density Temperature ph Horizon Depths

Soil erodes by several methods Erosion occurs through wind and four types of water erosion Rill erosion moves the most topsoil, followed by sheet and splash erosion Water erosion occurs most easily on steep slopes Land is made more vulnerable to erosion through: Overcultivating fields through poor planning or excessive tilling Overgrazing rangelands Clearing forests on steep slopes or with large clear-cuts

Erosion degrades ecosystems and agriculture Erosion = removal of material from one place to another By wind or water Deposition = arrival of eroded material at a new location Flowing water deposits nutrient-rich sediment in river valleys and deltas Floodplains are excellent for farming Flood control measures decrease long-term farming productivity Erosion occurs faster than soil is formed It also removes valuable topsoil

Erosion removes soil Water erosion removes soil from farmlands Erosion has declined due to soil conservation measures

Soil and Its Uses

Protecting soil: terracing and intercropping Terracing = level platforms cut into steep hillsides This staircase contains water Intercropping = planting different crops in alternating bands Increases ground cover Replenishes soil Decreases pests and disease

Soil erosion is a global problem Humans are the primary cause of erosion It is occurring at unnaturally high rates In Africa, erosion could reduce crop yields by half over the next 40 years Conservation farming decreases erosion When added to population growth, some describe agriculture s future as a crisis situation

Mosaic of closely packed pebbles, boulders Weak humusmineral mixture Dry, brown to reddish-brown, with variable accumulations of clay, calcium carbonate, and soluble salts Alkaline, dark, and rich in humus Clay, calcium compounds Desert Soil (hot, dry climate) Grassland Soil (semiarid climate)

Physical weathering breaks rocks into small mineral particles.

Decomposing organic material from plants and animals mixes with accumulated soil minerals.

Chemical weathering dissolves and changes minerals at the Earth s surface.

Soil Profiles of the Principal Terrestrial Soil Types