SCIENCE OF SOIL

1

Why we need to know about soil?

2

Soil

A collection of natural bodies developed in the unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants and has properties due to the effects of climate and living matter acting upon parent material, as conditioned by topography, over a period of time.

3

GENESIS OF SOIL

Rocks are chief sources for the parent material over which soils are developed

Types of rocks-• Igneous• Sedimentary• MetamorphicGenesis includes –weathering of rocks &

formation of soil

4

Primary and Secondary Minerals

Primary Minerals: Minerals that have persisted with little change in composition since they were extruded in molten lava(eg. quartz, mica and feldspars).They are most prominent in sand and silt fractions.

Secondary Minerals: Minerals such as the silicate clays and iron oxides, have been formed by the breakdown and weathering of less resistant minerals as soil formation progressed.

5

Weathering of rocks

It is physical and chemical disintegration and decomposition of rocks. Weathering creates the parent material over which the soil formation takes place. Later weathering, soil formation and development proceeds simultaneously.

6

Physical weathering

• Temperature• Water• Wind• Plants & animals

7

Chemical weathering

• Solution• Hydration• Hydrolysis• Acidification• Oxidation• Reduction

8

Chemical weatheringAs soon as physical disintegration of rock and mineral begins, chemical decomposition starts.

Water and its solution – hydrolysis, hydration, dissolution

KAlSi3O8 + H2O ------> HAlSi3O8 + K+ + OH-

2 HAlSi3O8 + 11 H2O ---- Al2O3 + 6 H4SiO4

Al2O3 + 3H2O ----- Al2O3.3H2O

Acid solution weathering

• CaCO3 + H2CO3 -----> Ca2+ + 2 HCO3-

9

• Oxidation

3 MgFeSiO4 + 2 H2O H4Mg3Si2O9 + SiO2 + 3FeO

• 4 FeO + O2 + 2 H2O -- 4 FeOOH

It is particularly manifest in rocks containing iron

10

Soil formation

The mineral weathering combines with the associated physical and chemical phenomena constitute the process of soil formation.

It includes-1. The addition of organic & mineral materials2. The loss of these materials from the soil3. Translocation of materials from one point to

Another within the soil column4. Transformation of minerals & organic substances

within the soil

11

Two Approaches:

• Pedological• Edaphological

12

Edaphology (edaphos means soil or ground in greek) is the study of soil from the stand point of higher plants. Edaphologists consider the various properties of soils in relation to plant production. They are practical and have the production of food and fibre as their ultimate goal.

The origin of the soil ,its classification, and its description are examined in pedology (pedon-soil or earth in greek). Pedology is the study of the soil as a natural body and does not focus primarily on the soli’s immediate practical use. A pedologist studies, examines, and classifies soils as they occur in their natural environment.

13

14

Air

Water

Mineral

Organic45%

25%

25%

5%

Composition of soil

Soil Profile and its Layers(Horizons)

• Examination of a vertical section of a soil as seen in a roadside cut or in the walls of a pit dug in the field, reveals the presence of more or less distinct horizontal layers. Such a section is called a profile, and the individual layers are known as horizons

15

16

Topsoil and Subsoil

• When a soil is ploughed and cultivated, the natural state of the upper 12-18 centimeters(5-7 inches) is modified. This manipulated part of the soil is referred to as the surface soil or the topsoil.

• The subsoil is comprised of those soils layers underneath the top soil.

17

Mineral (inorganic) and organic soils

• Mineral soils: Mineral or inorganic in composition, low in organic matter ranges from 1 -6%.

• Organic soils: 50% organic matter by volume (at least 20% by weight).

18

Soil Texture and Soil Structure• Soil Texture: Proportions of different sized

particles present in soil.

• Soil Structure: The arrangement of the sand silt and clay particles within the soil.

19

Table: General properties of three major inorganic soil particles

20

Property Sand (0.05-2mm)

Silt (0.002-0.05mm) Clay(<0.002mm)

1. Means of observation Naked eye Microscopic Electron Microscope

2.Dominant minerals Primary Primary and Secondary

Secondary

3.Attraction of particles for each other

Low Medium High

4. Attraction of particles for water Low Medium High

5.Ability to hold chemical nutrients and supply them to plants

Very low Low High

6.Consistency properties when wet Loose , gritty Smooth Sticky, plastic

7.Consistency properties when dry Very loose, gritty

Powdery, some clods

Hard clods

Soil Air

Soil air differs from the atmospheric air in several respects-

First ,the composition of soil air is quite dynamic and varies greatly from place to place within a given soil.

Second, soil air generally has a higher moisture content than the atmosphere; the relative humidity of soil air approaches 100% when the soil moisture is optimum.

Third, carbon dioxide in soil air is often several times higher than the 0.03% commonly found in the atmosphere, Oxygen decreases accordingly and, in extreme cases 5-10%, or even less, as compared to about 20% for normal atmosphere.

21

Composition of soil air

22

Particulars Percentage by volume

Nitrogen Oxygen Carbon dioxide

Atmospheric air

79.00 20.95 0.03

Soil air 79.20 20.60 0.25

Sandy soil air 79.20 19.95 0.30

Loamy soil air 79.20 19.20 0.62

Clay soil air 79.20 19.69 0.66

Manured soil air

79.20 18.23 1.85

Soil Organic Matter

Soil organic matter comprises an accumulation of partially disintegrated and decomposed plant and animal residues and other organic compounds synthesized by the soil microbes as the decay occurs. Such material is continually being broken down and re-synthesized by soil microorganisms. Consequently, organic matter is a rather transitory soil constituent, lasting for a few hours to several hundred years.

Organic matter binds mineral particles into granules that are largely responsible for the loose. easily managed condition of productive soils and increases the number of water a soil can hold.

It is also major soil source of phosphorus and sulfur and the primary source of nitrogen (3 elements essential for plant growth)

23

• Organic matter, including plant and animal residues, is the main source of energy for soil organisms. Without it biochemical activity would come to a near standstill.

• In addition to the original plant and animal residues and to their partial breakdown products, soil organic matter includes complex compounds that are relatively resistant to decay. These complex materials, along with some that are synthesized by the soil microorganisms, are collectively known as humus. This material is usually black and brown in colour, is very fine(colloidal) in nature.

24

Soil Water Water is hold in the soil for varying degree of tenacity

depending on the amount of water present and the size of the pores.

Together with its soluble constituents, including nutrient elements(eg. Ca, P, N and K), soil water makes up the soil solution, which is the critical medium for supplying nutrients to growing plants.

The movement can be in any direction; downward in response to gravity, upward as water moves to the soil surface to replace that lost by evaporation, and in any direction toward plant roots as they absorb this important liquid. Although some of the soil moisture is removed by the growing plants, some remains in the tiny pores and in thin films around soil particles. The soil solids strongly attract the soil water and consequently compete for it with plant roots.

25

Soil Solution The soil solution contains small but significant

quantities of soluble inorganic and organic compounds, some of which contain elements that are essential for plant growth

Critical property of the soil solution is its acidity or alkalinity. Many chemical and biological reactions are dependent on the levels of hydrogen ions and hydroxide ions in the soil. These levels influence the solubility, and in turn the availability to plants, of several essential nutrient elements such as Fe, Mn, P, Zn and Mo.

26

• The concentration of hydrogen(H+) and hydroxide ions(OH-) in the soil solution is commonly ascertained by determining its pH. Technically the pH is the negative logarithm of the concentration of hydrogen ion in the soil solution. Thus each unit change in pH represents a tenfold change in the activity of the H+ and OH- ions.

Acidity Alkalinity

27

3 4

Very strong

Strong Moderate Slight Neutral

Slight Moderate Strong Very strong

3-4 4-5 5-6 6-7 7 7-8 8-9 9-10 10-11

Clay and Humus• The attraction of ions such as Ca2+, Mg2+, and K+ on the

surfaces of colloidal clay and humus is not as exciting as is the exchange of these ions for other ions in the soil solution. For example, an H+ ion released to the soil solution by a plant root exchange readily with a potassium ion(K+) adsorbed on the colloidal surface .The K+ ion is then available in the soil solution for uptake by the roots of crop plants. A simple example of such cation exchange illustrates this point.

K+ + H+(aq) H+ + K+(aq) (adsorbed) (in soil solution) (adsorbed) (in soil solution)

28

colloid colloid

29

Clay Micelle

-ve charge +ve charge

Ionic double layer

Al

Ca

Mg

K

H

Na

30

pH-dependent chargeCommon in humus, allophane, Fe & Al hydroxides

Negative Charges

Al – OH + OH- = Al – O- + H2O

-CO-OH + OH- = -CO-O- + H2ONo Charge -ve charge

These reactions are reversible. If the pH increases, more OH ions are available to force the reaction to the right

Positive charge

Under moderate to extreme acid soil conditions

Al – OH + H+ = Al–OH2+. In some cases,

Al-O- + H+ = ALOH + H+ = Al–OH2+

High pH Low pH

31

Essential nutrient element and their sources

Use in relatively large amounts

Use in relatively large amounts

Use in relatively small amounts

Mostly from air and water

From soil From soil

Carbon(C ) Nitrogen(N) Iron(Fe)

Hydrogen(H) Phosphorus(P) Manganese(Mn)

Oxygen(O) Calcium(Ca) Boron(B)

Magnesium(Mg) Molybdenum(Mo)

Sulfur(S) Copper(Cu)

Potassium (K) Zinc(Zn)

Chlorine(Cl)

Cobalt(Co)

Nickel (Ni)32

Soil Degradation Soil degradation is a concept in which the

value of the biophysical environment is affected by one or more combination of human-induced processes acting upon the land. It is viewed as any change or disturbance to the land perceived to be deleterious or undesirable. Natural hazards are excluded as a cause, however human activities can indirectly affect phenomena such as floods and bushfires.

It is estimated that up to 40% of the world's agricultural land is seriously degraded.

33

CausesThe major causes include: Land clearance, such as clear cutting and

deforestation Agricultural depletion of soil nutrients through

poor farming practices Overgrazing Inappropriate Irrigation and over-drafting Urban sprawl and commercial development Land pollution including industrial waste

. 34

• Vehicle off-roading

• Quarrying of stone, sand, ore and minerals Overcutting of vegetation

• Overgrazing

• shifting cultivation without adequate fallow periods, absence of soil conservation measures,

• Population pressure

35

Effects The major stresses on vulnerable land include: Accelerated soil erosion by wind and water Soil acidification and the formation of acid sulfate soil

resulting in barren soil Soil alkalinisation owing to irrigation with water

containing sodium bicarbonate leading to poor soil structure and reduced crop yields

Soil salinization in irrigated land requiring soil salinity control to reclaim the land

Waterlogging in irrigated land which calls for some form of subsurface land drainage to remediate the negative effects

Destruction of soil structure including loss of organic matter

Ultimately results into low vegetation cover, extensive soil erosion which leads towards desertification 36

• Every year 84 billion tonnes of productive top soil are lost world wide through degradation.

• Degradation has already affected 1900 m ha of land globally (De Man et. al. 2007).

• Additionally each year over 14 million acres of productive lands are oversalted because of improper water management.

37

Soil Erosion

Soil erosion is the process of detachment of soil particles from the parent body and transportation of the detached soil particles by wind or water.

Mechanism of Water Erosion:

a. Detachment

b. Transportation

Causes:

a. Natural

b. Anthropogenic

38

Forms of Water Erosion Sheet Erosion: uniform removal of top soil in thin layer from the

field, least conspicuous. Rill Erosion: channelization begins ,no longer uniform. Gully Erosion: unchecked rills result in increased channelization of

runoff. Ravines: manifestation of prolonged process of gully erosion.

Deepening & Widening of gullies used to form ravines. Landslides: occur in mountain slopes when the slope exceeds 20

per cent and width 6 m. Stream-bank Erosion: Seasonal streams or rivulets often change

their course from season to season due to blockage of their previous course by transported rocks, clods of soil & vegetation grown during lean periods.

39

40

41

Gully erosion Ravine erosion

42

Forms of wind erosion Suspension- Most spectacular method of transporting soil

particles is by suspension. Dust particles of fine sand ( less than 0.1 mm dia) are moved parallel to ground surface and upward. About 5-15 % of wind erosion afftected soil is transported by this process.

Saltation- Particles in the range 0.1-0.5 mm diameter are lifted by the wind, then fall back to the ground, so they move in a hopping or bouncing fashion. These particles cause abrasion of the soil surface and as they hit other particles they break into smaller particles, a process called attrition. Depending on conditions, this process may account for 50-70% of the total movement of soil.

Surface creep- Rolling and sliding of larger particles (more than 0.5 mm dia) along the surface. Surface creep account to 5-25% of total movement due to action of wind.

43

44

Soil Conservation

Definition Soil conservation is using and managing

land based on the capabilities of the land itself.

45

Soil Conservation Measures Agronomic Measures Contour Cultivation – By ploughing and sowing

across the slope, each ridge of plough furrow and each row of the crop act as an obstruction to runoff, providing more opportune time for water to enter into the soil and reduce soil loss.

Tillage – Tillage alters soil physical characters like porosity, bulk density, surface roughness and hardness of pans. Conventional tillage includes ploughing twice or thrice followed by some secondary operations like harrowing and planking that smoothen and pack the soil in seed-bed and/or control weeds.

Mulching – Mulches are any material such as straw, plant residues, leaves, loose soil or plastic film placed on the soil surface to reduce evaporation, erosion or to protect plant roots from extremely low or high temperature. 46

Mechanical Measures

Contour Bunding – Runoff from any given surface is along the line of greatest slope and the velocity of runoff increases with the vertical distance through which it is moved. The contour bund being on the same elevation, assures that the depth of water against the bund is uniform throughout its length. It ensures uniform distribution of water above the bunds and therefore, better cultivation possibilities than any other type of bund. As the bunds are at regular intervals, they intercept the runoff from attaining erosive velocity and causing erosion. The velocity of flowing water is slowed down and water thus held on the field for a longer time, soaks into the soils.

Broad Base Terrace - A terrace is a combination of ridge and channel built across the slope. These terraces have wide base and low height of ridge and usually formed with machinery. BBTs are constructed in soils with high clay content which develop deep cracks in summer (e.g. Black soil).

47

48

Bench Terracing - Bench terracing consists of transforming relatively steep land into a series of level strips or platforms across the slope of the land. It reduces the slope length and consequently erosion. The field is made into a series of benches by excavating the soil from upper part of the terrace and filling in the lower part. On steeply sloping and undulated land, farming practices is possible only with bench terracing. It is usually practiced on slopes ranging from 16 to 33%.

Trenching –Contour trenches are made in non-agricultural land for providing adequate moisture conditions in order to raise trees or grass species. The trenches are usually 60 cm × 48 cm in size. The spacing varies from 10 to 30 m.

Vegetative Barriers – these are closely spaced plantations-usually a few rows of grasses or shrubs --- grown along contours . They act as barrier to check the velocity of overland flow and entrapment of silt load behind them. Khus (Vettiveria zelanica) is the most suitable plant for this purpose.

49

Grassed Waterways – These are drainage channel either developed by shaping the existing drainage ways or constructed separately. Suitable perennial grasses that are not edible by cattle, deep rooted and spreading type are established subsequently for the stability of the waterway (e.g Panicum repens, Brachiara mutica, Cynodon dactylon, Paspalum notatum). The objectives are- 1. to provide drainage, 2. to convert gullies or unstable channels into stable channels by providing grass cover, and 3. for leading water at non-erosive velocity into a water body.

Gully Control – The basic approach to gully control involves reduction of peak flow rates through the gully and provision of stable channel for the flow that has to be handled. Temporary and permanent structures such as check dams, drop-spill ways are constructed.

Agrostological Measures Grasses prevent soil erosion by intercepting rainfall, by

binding the soil particles and by improving soil structure. A grass-legume association is ideal for soil conservation. E.g Pennisetum pupureum, Cenchrus ciliaris, Setaria sphacelata.

Forestry Measure Afforestation and re-forestation in wastelands

50

51

AA

52

Semi-circular & triangular contour bunds

53

54

55

56

57

Check dam

58

Waste Management

What are Wastes?Waste (also known as rubbish, trash, refuse, garbage, junk, litter,

and ort) is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea and sweat.

Basel Convention Definition of Wastes“substances or objects which are disposed of or are intended to be disposed of or are required to be disposed of by the provisions of the law”

Disposal means“any operation which may lead to resource recovery, recycling, reclamation, direct re-use or alternative uses (Annex IVB of the Basel convention)”

Basel Convention

• The Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and Their Disposal, usually known simply as Basel Convention, is an international treaty that was designed to reduce the movements of hazardous waste between nations, specially to prevent transfer of hazardous waste from developed to less developed countries (LDCs). It does not, however, address the movement of radioactive waste. The convention is also intended to minimize the amount and toxicity of wastes generated, to ensure their environmentally sound management as closely as possible to the source of generation, and to assist LDCs in environmentally sound management of the hazardous and other wastes they generate.

• The Convention was opened for signature on 22nd March 1989, and entered into force on 5 May 1992.

Classification of Wastes according to their Properties

Bio-degradable can be degraded (paper, wood, fruits

and others)

Non-biodegradablecannot be degraded (plastics, bottles,

old machines,cans, styrofoam containers and others)

Classification of Wastes according totheir Effects on Human Health and the

Environment• Hazardous wastes• Substances unsafe to use commercially,

industrially, agriculturally, or economically and have any of the following properties- ignitability, corrosivity, reactivity & toxicity.

• Non-hazardous • Substances safe to use commercially,

industrially, agriculturally, or economically and do not have any of those properties mentioned above. These substances usually create disposal problems.

Classification of wastes according to their origin and type

• Municipal Solid wastes: Solid wastes that include household garbage, rubbish, construction & demolition debris, sanitation residues, packaging materials, trade refuges etc. are managed by any municipality.

• Bio-medical wastes: Solid or liquid wastes including containers, intermediate or end products generated during diagnosis, treatment & research activities of medical sciences.

• Industrial wastes: Liquid and solid wastes that are generated by manufacturing & processing units of various industries like chemical, petroleum, coal, metal gas, sanitary & paper etc.

• Agricultural wastes: Wastes generated from farming activities. These substances are mostly biodegradable.

• Fishery wastes: Wastes generated due to fishery activities. These are extensively found in coastal & estuarine areas.

• Radioactive wastes: Waste containing radioactive materials. Usually these are byproducts of nuclear processes. Sometimes industries that are not directly involved in nuclear activities, may also produce some radioactive wastes, e.g. radio-isotopes, chemical sludge etc.

• E-wastes: Electronic wastes generated from any modern establishments. They may be described as discarded electrical or electronic devices. Some electronic scrap components, such as CRTs, may contain contaminants such as Pb, Cd, Be or brominated flame retardants.

Waste hierarchyWaste hierarchy refers to 3 Rs Reduce, Reuse, Recycle

Waste to Wealth

(A) Waste to compost (i.e. resource recovery)

Aerobic/Anaerobic composting

Vermicomposting – A major component of organic farming.

(B) Waste to energy Anaerobic digestion

(Biomethanation) Incineration Pyrolysis Gasification Pelletization (Refuse

derived fuel or RDF) Landfill gas recovery

66

Basic techniques of energy recovery

Energy can be recovered from the organic fraction of waste (bio-degradable as well as non bio-degradable) through two methods:

i. Thermo-chemical conversion: This process entails thermal decomposition of organic matter to produce either heat energy or fuel oil or gas.

67

Contd…ii. Biochemical conversion: This process is

based on enzymatic decomposition of organic matter by microbial action to produce methane gas or alcohol.

Some of the important energy recovery techniques are discussed under the following heads:

a) Anaerobic digestion (AD): Also known as bio-methanation Segregating the organic fractions of waste

68

Contd.. Feeding them into a closed container (biogas

digester) under anaerobic conditionOrganic wastes undergone bio-degradation

and produce methane rich biogas and effluent/ sludge

Biogas produced, 50-150 m3/ton depending upon waste composition

Fundamentally, anaerobic digestion process can be divided into three stages with 3 distinct physiological groups of micro-organisms.

69

Contd..Stage I: Fermentative bacteria (anaerobic &

facultative micro-organisms) e.g. Bacteroides succinogens, Clostridium sp.

Complex organic materials, carbohydrates, proteins and lipids hydrolyzed & fermented into fatty acids, alcohol, CO2, H2, NH3 and sulfides.

Stage II: Acetogenic bacteria consume these primary products and produce H2, CO2 & acetic acid (CH3COOH). e.g. Syntrophobactor wolinii, Syntrophomonas wolfei 70

Contd..

Stage III: Two types of methanogenic bacteriaFirst one (reduces) CO2 to CH4 (e.g.

Methanosprillium sp.)Second one (decarboxylates) CH3COO- to CH4 (e.g.

Methanosarcina sp.)

71

Contd..b) Incineration:Direct burning of wastes in the presence of

excess air (oxygen)Liberates heat energy, inert gas and ashAbout 65-80% of energy content of organic

matter can be recovered

72

Contd..

c) Pyrolysis:Also known as destructive distillation or

carbonizationThermal decomposition or organic matter at

high temperature (about 900oC) in an inert (O2 deficient) atmosphere or vacuum.

Produces a mixture of combustible and non-combustible gases

73

Contd..c) Gasification:Thermal decomposition of organic matter at

high temperature in presence of limited amount of oxygen

Produces mainly a mixture of combustible & non-combustible gases

Temperature > 1000oCThe gas can be cooled, cleaned and utilized to

generate electricity 74