Soil

Soil – An Introduction• Along with water, light and air, it is the

basic necessity of the living world• It is the upper layer of the earth’s crust• It occurs everywhere on the earth except

over rocky land and the land under snow• Soil mainly contains Oxygen (47.3%) and

Silicon (27.7%)• Aluminium, iron, calcium, sodium,

potassium and magnesium are also present

Soil – An Introduction• Soil is a mixture of mineral and

organic constituents that are in solid, gaseous and aqueous states.

• Most soils have a density between 1 and 2 g/cm³

• Soil is also known as earth: it is the substance from which our planet takes its name.

Types of Soil• According to the kinds of

particles and their proportions in the soil, three kinds of soils are possible:– Sandy soil– Clay soil– Loam soil

Soil types by Sand, Silt and Clay composition

Types of Soil• The proportion of sand and gravel is

higher• The soil is loose and porous• Water holding capacity is low• More air occurs in these pores• The amount of nutrients is negligible• These reasons render sandy soil as

unfertile

Sandy Soil

Types of Soil• The proportion of clay and silt

particles is higher• Clay soil has higher porosity • This type of soil is poorly aerated• The spaces among the small

colloidal particles are very narrow, hence it hold a large amount of water

Clay Soil

Types of Soil• There is a proper mixture of the various-

sized particles• Sand, silt, and clay are present in

relatively even concentration (about 40-40-20% concentration respectively)

• Loam soil is considered ideal for gardening and agricultural uses.

• Loams are gritty, moist, and retain water easily.

• Loam soil feels mellow and is easy to work over a wide range of moisture conditions.

Loam Soil

Types of Soil• Loam soils generally contain more

nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils.

• Loam is ideal for growing crops because it retains nutrients well and retains water while still allowing the water to flow freely.

• This soil is found in a majority of successful farms in regions around the world known for their fertile land.

Loam Soil

Soil Formation

Soil Formation• Soil formation is the combined effect of

physical, chemical, and biological processes on soil parent material.

• Soil genesis involves processes that develop layers or horizons in the soil profile.

• These processes involve additions, losses, transformations and translocations of material that compose the soil.

• The weathering of bedrock produces the parent material that soils form from.

Soil Formation• Weather is the first stage in the

transforming of parent material into soil material.

• In soils forming from bedrock, a thick layer of weathered material is formed.

• This is the result of weathering processes that include: – Hydrolysis, hydration & solution of

minerals by water– physical processes that include freezing

and thawing or wetting and drying.

Weather

Soil Formation• The mineralogical and chemical

composition of the primary bedrock material, plus physical features, including grain size and degree of consolidation, plus the rate and type of weathering, transforms it into different soil materials.

Weather

Soil Formation• Soil formation greatly depends on the

climate, and soils from different climate zones show distinctive characteristics.

• Temperature and moisture affect weathering and leaching.

• Wind moves sand and other particles, especially in arid regions where there is little plant cover.

• Temperature and precipitation rates affect biological activity, rates of chemical reactions, and types of vegetation cover.

Climate

Soil Formation• The type and amount of precipitation

influence soil formation by affecting the movement of ions and particles through the soil, aiding in the development of different soil profiles.

• Seasonal and daily temperature fluctuations affect the effectiveness of water in weathering parent rock material and affect soil dynamics, freezing and thawing is an affective mechanism to break up rocks and other consolidated materials.

Climate

Soil Formation• Plants, animals, fungi, bacteria and

humans affect soil formation. • Animals and micro-organisms mix soils and

form burrows and pores allowing moisture and gases to seep into deeper layers.

• In the same way, plant roots open channels in the soils, especially plants with deep taproots which can penetrate many meters through the different soil layers bringing up nutrients from deeper in the soil.

Biological Factors

Soil Formation• Plants with fibrous roots that spread out

near the soil surface, have roots that are easily decomposed, adding organic matter.

• Micro-organisms, including fungi and bacteria affect chemical exchanges between roots and soil and act as a reserve of nutrients.

• Humans can impact soil formation by removing vegetation cover, which promotes erosion, or by mixing different soil layers which restarts the soil formation process

Biological Factors

Soil Formation• Vegetation impacts soils in numerous

ways. • It can prevent erosion from rain or

surface runoff. • It shades soils, keeping them cooler and

slowing evaporation of soil moisture. • Or it can cause soils to dry out by

transpiration. • Plants can form new chemicals that

break down or build up soil particles.

Biological Factors

Soil Formation• Dead plants and dropped leaves

and stems of plants fall to the surface of the soil and decompose.

• There, organisms feed on them and mix the organic material with the upper soil layers; these organic compounds become part of the soil formation process, ultimately shaping the type of soil formed

Biological Factors

Soil Formation• Time is a factor in the interactions of all

the above factors as they develop soil. • Over time, soils evolve features

dependent on the other forming factors, and soil formation is a time-responsive process dependent on how the other factors interplay with each other.

• Soil-forming factors continue to affect soils during their existence

Time

Soil Formation• Materials are deposited on top and

materials are blown or washed away from the surface.

• With additions, removals, and alterations, soils are always subject to new conditions.

• Whether these are slow or rapid changes, depends on climate, landscape position, and biological activity.

Time

Soil Profile

Soil Profile• In a vertical section of soil dug out

at any place on earth, we find many horizontal layers from the upper surface to the deeper regions

• Such a multi-layered structuring of the soil is called Soil Profile

• Each observed layer is called a horizon

Soil Horizon• A soil horizon is a specific layer in the

soil which measures parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath.

• Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time periods.

• Soils vary in the degree to which horizons are expressed.

Soil Horizon• Each soil type has at least one, usually

three or four different horizons• Horizons are defined in most cases by

obvious physical features, colour and texture being chief among them.

• These may be described both in absolute terms (particle size distribution for texture) and in terms relative to the surrounding material, i.e., ‘coarser’ or ‘sandier’ than the horizons above and below

Soil Horizons

Soil Horizon• There are 4 main horizons – A, B, C and D –

from the outside towards inside respectively• Layer A has 5 sub-layers A00, A0, A1, A2 and A3

• A00 sub-layer is well developed in forest areas

and is rich in fallen leaves, flowers, petals and excretory matter. It is called the litter layer and is not a part of the soil itself.

• A0 sub-layer is called the duff layer and comprises partially decomposed debris. It is not observed in grassland soils

Soil Horizon• A1 sub-layer is rich in organic matter and

called the humus layer• A2 sub-layer is constantly leached by

rainwater It is a sub-layer of mineral soil with most organic matter accumulation and soil life.

• This layer is depleted of iron, clay, aluminum, organic compounds, and other soluble constituents. This is called the surface soil

• A3 is a transition layer between layers A and B

Soil Horizon• B layer is called the Established layer

and is a store-house of material being leached from the upper regions

• C layer contains a store of degraded inorganic material

• Water is stored here and also hard material like calcium carbonate and calcium sulphate are observed

• D is the innermost layer made of sand, rock or clay

Soil Constituents

Soil Constituents• Any soil consists of the following:

– Parental material or inorganic constituents

– Organic matter– Soil solution– Soil gases– Soil organisms

Soil Constituents• Composed of mineral substances• Formed from the parental rock lying in

the soil• Only quartz (among parental rocks)

remains in its original form• Other rocks are degraded into stones,

sand, silt and clay• 90% of soil is composed of parental

material and their characteristics influences the nature of plant life

Parental material

Soil Constituents• These are dead bodies of organisms

and animal excreta• Makes soil suitable for plant

inhabitation• Improves water-holding capacity and

aeration of the soil• Starch, glucose, proteins and other

organic substances are supplemented through them

• Provide a source of nutrient substances

Organic Matter

Soil Constituents• The dilute solution of water

occurring around the soil particles and in the pore spaces in the soil

• Contains dissolved solids, liquids and gases

• Plants absorb their essential nutrients from this soil solution

Soil Solution

Soil Constituents• Helps plants in respiration by

making available oxygen and arranging for the removal of carbon dioxide

• Amount of oxygen decreases with depth

• The presence of water and air in the soil is inversely proportional

Soil Gases

Soil Constituents• Numerous organisms can be found

living in soil.• Among plants, we can find bacteria,

fungi, algae and the underground organs of plants

• Among animals, we can find protozoa and other animals like ants, snakes, mites, earthworms as well as burrowing animals like rats and moles

Soil Organisms

Soil Constituents• Organisms bring about useful or harmful

changes to the soil they live in• Decomposer animals like ants and worms

degrade the dead bodies of organisms• Several fungi and bacteria produce

growth-stimulating chemicals• Some produce toxic substances through

their metabolic activities• Earthworms bring soil from deeper

regions to the surface, improving soil constitution and fertility

Soil Organisms

Soil – A Resource• Soil is used in agriculture, where

it serves as the primary nutrient base for the plants.

• The types of soil used in agriculture vary with respect to the species of plants that are cultivated.

Soil – A Resource• Soil material is a critical component in

the mining and construction industries. • Soil serves as a foundation for most

construction projects. • Massive volumes of soil can be involved

in surface mining, road building, and dam construction.

• Earth sheltering is the architectural practice of using soil for external thermal mass against building walls.

Soil – A Resource• Soil resources are critical to the

environment, as well as to food and fiber production.

• Soil provides minerals and water to plants.

• Soil absorbs rainwater and releases it later thus preventing floods and drought.

• Soil is the habitat for many organisms: the major part of known and unknown biodiversity is in the soil

Soil – A Resource• Waste management often has a

soil component. • Septic drain fields treat septic

tank effluent using aerobic soil processes.

• Landfills use soil for daily cover.• Organic soils, especially peat,

serve as a significant fuel resource.

Soil – A Resource• Both animals and humans in

many cultures, occasionally consume soil.

• It has been shown that some monkeys consume soil, together with their preferred food (tree foliage and fruits) in order to alleviate tannin toxicity.

Soil – A Resource• Soil cleans the water as it

percolates. • Soils filter and purify water and

effect its chemistry. • Rain water and pooled water from

ponds, lakes and rivers percolate through the soil horizons and the upper rock strata, and thus become groundwater.

Soil – A Resource• Pests (viruses) and pollutants such as

oils, heavy metals and excess nutrients are filtered out by the soil and soil organisms metabolize them or immobilize them in their biomass and necromass, thereby incorporating them into stable humus.

• The physical integrity of soil is also a prerequisite for avoiding landslides in rugged landscapes

Soil Pollution

Soil Pollution - Causes• The main causes of soil pollution are:• Increasing population, urbanization and

industrialization• Dead bodies of animals and waste disposal

of various materials utilized by human beings

• The use of chemical fertilizers and various insecticides and pesticides in agriculture

• Excessive irrigation increases soil salinity and a decreasing nitrogen fixing capacity of bacteria

Soil Pollution - Causes• Littering in the form of plastics and

polythene carry-bags is another source of soil pollution

• Random deforestation is another cause of soil pollution, as it leads to erosion and top soil being washed away

• Urban land fills is another source of soil pollution

Soil Pollution - Control• Proper disposal and collection of garbage• Use of bio-degradable material like jute,

paper instead of plastic and polythene• Planting more trees• Proper agricultural practice of

decreasing use of pesticides and insecticides and more use of organic farming and biological control methods

• Recycling of waste• Modern irrigation techniques to

minimize wastage of water

Soil Erosion – An Introduction• Erosion is the removal of solids

(sediment, soil, rock and other particles) in the natural environment.

• It usually occurs due to transport by wind, water, or ice; by down-slope creep of soil and other material under the force of gravity; or by living organisms, such as burrowing animals, in the case of bio-erosion

Soil Erosion – An Introduction• Erosion is a noticeable intrinsic natural

process but in many places it is increased by human land use.

• Poor land use practices include deforestation, overgrazing, unmanaged construction activity and road-building.

• Land that is used for the production of agricultural crops generally experiences a significant greater rate of erosion than that of land under natural vegetation.

Soil Erosion – An Introduction• This is particularly true if tillage is

used, which reduces vegetation cover on the surface of the soil and disturbs both soil structure and plant roots that would otherwise hold the soil in place.

• However, improved land use practices can limit erosion, using techniques such as terrace-building, conservation tillage practices, and tree planting.

Soil Erosion – An Introduction• A certain amount of erosion is

natural and, in fact, healthy for the ecosystem.

• For example, gravels continuously move downstream in watercourses.

• Excessive erosion, however, does cause problems, such as receiving water sedimentation, ecosystem damage and outright loss of soil.

Examples of Erosion

Soil Erosion - Causes• The rate of erosion depends on many

factors.• Climatic factors include the amount and

intensity of precipitation, the average temperature, as well as the typical temperature range, and seasonality, the wind speed, storm frequency.

• Geologic factors include the sediment or rock type, its porosity and permeability, the slope (gradient) of the land, and whether the rocks are tilted, faulted, folded, or weathered.

Soil Erosion - Causes• Biological factors include ground

cover from vegetation or lack thereof, the type of organisms inhabiting the area, and the land use.

• Activities like logging, building of roads and embankments, and heavy grazing of animals increase the chances of soil erosion

Erosion Processes• Mass wasting is the down-slope

movement of rock and sediments, mainly due to the force of gravity.

• Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results.

• Any down-slope movement of rocks is often referred to in general terms as a landslide.

Gravity Erosion

Erosion Processes• Different types of erosion caused by water

are:• Splash erosion is the detachment and

airborne movement of small soil particles caused by the impact of raindrops on soil.

• Sheet erosion is the detachment of soil particles by raindrop impact and their removal down slope by water flowing overland as a sheet.

• Rill erosion refers to the development of small, concentrated flow paths, which function as both sediment source and sediment delivery systems for erosion on hill-slopes.

Water Erosion

Erosion Processes• Gully erosion results where water flows

along a linear depression eroding a trench or gully.

• Valley or stream erosion occurs with continued water flow along a linear feature.

• At extremely high flows, kolks, or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called Rock-cut basins.

Water Erosion

Erosion Processes• Shoreline erosion, which occurs

on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role

Shoreline Erosion

Erosion Processes• Ice erosion is caused by

movement of ice, typically as glaciers

• Sometimes, cold weather causes water trapped in tiny rock cracks to freeze and expand, breaking the rock into several pieces. This can lead to gravity erosion on steep slopes

Ice Erosion

Erosion Processes• Wind erosion is the result of material

movement by the wind. • There are two main effects. • First, wind causes small particles to

be lifted and therefore moved to another region. This is called deflation.

• Second, these suspended particles may impact on solid objects causing erosion by abrasion

Wind Erosion

Erosion Processes• Wind erosion generally occurs in

areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation.

• An example is the formation of sand dunes, on a beach or in a desert

Wind Erosion

Erosion - Control• Covering of open land through

cultivation• Crops like groundnut, pulses, lucerne

provide a protective covering to the soil• Creation of horizontal contours at right

angles to the gradient of the slope increase water absorption and slows down the process of erosion

• Planting of trees in ravines, and that of sand-binding vegetation in deserts can help stop erosion