Soil (Nat Sci)

Soil

Ø  The loose material that coves the land surfaces of Earth and supports the growth of the plants

Ø  In general: soil is an unconsolidated, or loose combination of organic and inorganic materials

Ø  Introduction:

·        Soils vary widely from place to place. Many factors determine the chemical composition and physical structure of soil at any given location.

·        In some cases, human activity such as farming or building has caused disruption.

·        Takes time to develop (thousands or even millions of years)

·        Soil is a nonrenewable resource

Ø  Soil Science/ Pedology

·        The study of soil types and their properties

·        Plays a key role in agriculture, helping farmers to select and support the crops on their land and maintain fertile, healthy ground for planting

·        Soil engineers carry out detailed analysis of the soil prior to the building of roads, houses, industrial and retail complexes and other structures

Ø  Composition of rocks:

·        Soils comprise a mixture of organic and inorganic components: minerals, air, water, and plant and animal material.

·        Mineral and organic particles generally compose roughly 50% of soil’s volume

·        The other 50% consists of pores:

-          Open areas of various shapes and sizes

-          Networks of pores hold water within the soil and also provide a means of water transport

-          Oxygen and other gases move through pore spaces in soil

-          Pores also serve as passageways for small animals and provide room for the growth of plant roots

·        Components:

a.      Inorganic material

v  the products of rocks and minerals that have been gradually broken down by weather, chemical action and other natural processes

b.      Organic Material

v  composed of debris from plants and from the decomposition of the many tiny life forms that inhabit the soil

c.       Water

v  Soil scientists also characterize soils according to how effectively they retain and transport water

v  Once water enters the soil from rain or irrigation, gravity comes into play, causing water to trickle downward

v  Water is also taken up in great quantities by the roots of plants

v  Plants use anywhere from 200 to 1,000 kg (440 to 2,200 lbs) of water in the formation of 1kg (2.2 lbs) of dry matter

Ø  Soil Formation

·        Also known as “Pedogenesis” (from the Greek words pedon for “ground” and genesis = “birth” or “origin”)

·        An ongoing process that proceeds through the combined effects of five soil-forming factors: parent materials, climate, living organisms, topography and time.

·        Each combination of the five factors produces a unique type of soil that can be identified by its characteristic layers called horizons

·        Factors:

a.      Parent material

v  the first step in Pedogenesis is the formation of parent material from which the soil itself forms

v  roughly 99% of the world’s soils derive from mineral-based parent materials that are the result of weathering, the physical disintegration and chemical decomposition of exposed bedrock

b.      Climate

v  Water, ice, wind, heat and cold cause physical weathering by loosening of and breaking of rocks

v  Climate also influences the developing soil by determining the types of plant growth that occur

c.       Living organisms

v  As parent material accumulates, living organisms gradually gain a foothold in it.

v  The arrival of living organisms marks the beginning of the formation of true soil

d.      Topography

v  The degree of slope on which a soil forms helps to determine how much rainfall will run off the surface and how much will be retained by the soil

e.      Time

v  The amount of time a soil requires to develop varies widely according to the action of the other soil-forming factors.

v  Soil may take a hundreds of thousands of years to form

v  In some areas, the soils may be more than a million years old

f.        Horizons

v  Reflects the different properties and different degrees of weathering

Ø  Soil Characteristics

·        Scientists can learn a lot about a soil’s composition and origin by examining various features of the soil:

a.      Color

v  Color alone does not affect a soil but it is often a reliable indicator of other soil properties

§  A dark color – usually indicates the presence of organic matter

§  A red to yellow range – common soil hues, getting their color from iron oxide minerals coating soil particles

§  Gray, blue or green – saturated by water, the minerals that give them the red and yellow colors have been leached away

b.      Texture

v  Relative percentage of each particle size in a soil

v  Texture differences can affect many other physical and chemical properties and are therefore important in measures such as soil productivity:

§  Soil with predominantly large particles tend to drain quickly and have lower fertility

§  Very fine-textured soils may be poorly drained, tend to become waterlogged and therefore not suited for agriculture

§  Soils with medium texture and relatively even proportion of all particle sizes are most versatile

v  Ideal mixture for productive soil

§  10% to 20% clay

§  Sand and silt in roughly equal amounts

§  Good quantity organic materials

c.       Aggregation

v  Occurs as a result of complex chemical forces acting on small soil components or when organisms and organic matter in soil act as glue binding particles together

v  Soil aggregates (also known as soil peds: individual soil particles tend to bound together into larger unit)

v  3 main soil types:

§  Platelike – aggregates are flat and mostly horizontal

§  Prismlike – greater in vertical and horizontal dimension

§  Blocklike – roughly equal in horizontal and vertical dimensions and either angular or rounded

d.      Porosity

v  The part of the soil that is not solid is made up of pores of various sizes and shapes.

v  The size, number and arrangement of these pores

v  Affects water movement and gas exchange

v  Well-aggregated soils have numerous pores, which are important for organisms that live in the soil and require water and oxygen to survive

v  The transport of nutrients and contaminants will also be affected by soil structure and porosity

e.      Ion Content

v  a chemical characteristic

v  the of certain soil particles, particularly the clays, hold groupings of atoms known as ions

v  cations become attached to the soil particles in a process known as “cation exchange”

v  the chemical reactions in cation exchange make it possible for calcium and other elements to be exchanged into water-soluble forms that plants can use for food

v  therefore, a soil’s cation exchange capacity is an important measure of fertility

f.        pH (positively charged hydrogen atom)

v  another important chemical measure is soil pH, which refers to the soil’s acidity/ alkalinity

v  this property hinges on the concentration of hydrogen ions in solution

v  a greater concentration of hydrogen results in a lower pH, meaning greater acidity

v  scientists consider pure water with a pH of 7, neutral

Ø  Soil Classification

·        There is no worldwide, unified classification scheme for soil

·        Since the birth of the modern discipline of soil science roughly 100 years ago, scientists in different countries have used many systems to organize the various types of soils into groups

Ø  Soil Use

·        For most of human history, soil has not been treated as the valuable and essentially nonrenewable resource that it is

a.      Erosion

v  Removal of rock and soil material by natural processes

b.      Soil Management

v  Prevention for destruction through wise use

Ø  Soil Profile

·        refers to the layers of the soil

·        another term for soil horizon

a.      Organic Matter (O horizon)

v  Surroundings or the things that we can see above the soil

b.      Topsoil (A Horizon)

v  The uppermost layer of the soil

v  Rich in organic matter called humus

v  Holds most of the roots of the plants

c.       Subsoil (B Horizon)

v  Rich in minerals but contains less humus than the topsoil above it

d.      Rock Fragments (C Horizon)

v  Separates the subsoil and the bedrock

e.      Bedrock (D Horizon)

v  The solid rock of the earth. It may be exposed at the surface in an outcrop or be covered by soil, sand or gravely

Ø  Soil Degradation

·        It is when soil deteriorates because of human activity and loses its quality and productivity

·        It happens when soil loses its nutrients or its organic matter

·        Also happens when the soil structure breaks down or if soil becomes toxic because from pollution

·        Cause: erosion and depletion by nutrient-demanding crops

Ø  Mass Wasting/ Mass Movement

·        The down slope movement of rock and regolith near the Earth’s surface mainly due to the force of gravity

·        Important part of erosion process as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations

·        Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results

a.      Types of Mass Movement Processes

The down-slope movement of material, whether it is bedrock, regolith, or a mixture of these, is commonly referred to as a landslide. All of these processes generally grade into one another, so classification of such processes is somewhat difficult. We will use a classification that divides mass movement processes into two broad categories:

v  Slope Failures

§  a sudden failure of the slope resulting in transport of debris downhill by sliding, rolling, falling, or slumping

o   Slumps (also called Rotational Slides)

-          types of slides wherein downward rotation of rock or regolith occurs along a concave-upward curved surface (rotational slides)

-           The upper surface of each slump block remains relatively undisturbed, as do the individual blocks. Slumps leave actuate scars or depressions on the hill slope

-           Slumps can be isolated or may occur in large complexes covering thousands of square meters

-           They often form as a result of human activities, and thus are common along roads where slopes have been over steepening during construction. They are also common along river banks and sea coasts, where erosion has under-cut the slopes.  Heavy rains and earthquakes can also trigger slumps.

o   Falls/ Rock Falls

-          occur when a piece of rock on a steep slope becomes dislodged and falls down the slope

-          Debris falls are similar, except they involve a mixture of soil, regolith, vegetation, and rocks. A rock fall may be a single rock or a mass of rocks and the falling rocks can dislodge other rocks as they collide with the cliff

-          this process involves the free fall of material, falls commonly occur where there are steep cliffs

-          At the base of most cliffs is an accumulation of fallen material termed talus

o   Slides (also called Translational Slides)

-           Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane, foliation surface,  or joint surface (joints are regularly spaced fractures in rock that result from expansion during cooling or uplift of the rock mass)

-          Piles of talus are common at the base of a rock slide or debris slide

-          Slides differ from slumps in that there is no rotation of the sliding rock mass along a curved surface

v  Sediment Flows

§  Occur when sufficient force is applied to rocks and regolith that they begin to flow down slope. A sediment flow is a mixture of rock, and/or regolith with some water or air. They can be broken into two types depending on the amount of water present

o   Slurry Flows

-          Are sediment flows that contain between about 20 and 40% water. As the water content increases above about 40% slurry flows grade into streams.  Slurry flows are considered water-saturated flows

o   Granular Flows

-           are sediment flows that contain between 0 and 20% water. Note that granular flows are possible with little or no water. Fluid-like behavior is given these flows by mixing with air.  Granular flows are not saturated with water

o   Solifluction

-           flowage at rates measured on the order of centimeters per year of regolith containing water. Solifluction produces distinctive lobes on hill slopes. These occur in areas where the soil remains saturated with water for long periods of time

o   Debris Flows

-          These occur at higher velocities than solifluction, with velocities 100 meters/hr and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow downhill forming lobes with an irregular surface consisting of ridges and furrows

o   Mudflows

-           these are a highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like and wet concrete

-          They move at velocities greater than 1 km/hr and tend to travel along valley floors

-           These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be `picked up by streams

-          Thus after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment

-          Because of their high velocity and long distance of travel they are potentially very dangerous

-          Note that the media often refers to mudflows (and sometimes debris flows) as mudslides. This is inaccurate because mud flows rather than slides down a slope. Thus, in this course the word "mudslide" is an illegal word - one that you should never use

o   Creep

-          - the very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles

o   Earth flow

-          are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day. They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff

o   Grain flow

-          Usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope

o   Debris Avalanches

-          These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions

-          Snow Avalanches are similar to debris avalanches, but involve only snow, and are much more common than debris avalanches. Snow avalanches usually cause hundreds of deaths worldwide each year

b.      Mass Movements in Cold Climates

v  Rock Glaciers

§  a lobe of ice-cemented rock debris (mostly rocks with ice between the blocks) that slowly moves downhill

v  Frost Heaving

§  This process is large contributor to creep in cold climates. When water saturated soils freeze, they expand, pushing rocks and boulders on the surface upward perpendicular to the slope. When the soil thaws, the boulders move down vertically resulting in a net down slope movement

c.       3 types – of mass movements are common, based on degree of disintegration of the material during movements

v  Submarine Slumps

§  Coherent blocks break and slip

v  Submarine Debris Flows

§  Moving material breaks apart

v  Turbidity Currents

§  Sediment moves as a turbulent cloud, called a turbidity current