Introduction to Soil Science

LECTURES FOR UNDERGRADUATE STUDENTS SOIL SCIENCE

Dr. Osama Shaltami

Department of Earth Sciences Faculty of Science, Benghazi University, Libya

Introduction

* Soil is a three-phase system containing

solids,

liquids,

and

gasses that strongly interact with

each

other.

It

contains

four

components; air, minerals, organic matter and water.

* Soil is called the "Skin of the Earth" and interfaces with its lithosphere,

hydrosphere,

bio-

sphere and atmosphere. The term

pedolith, used commonly to refer to the soil.

* Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of

soils; and these properties in relation to the use and management of soils. * There are two main branches of soil science namely; pedology and

edaphology. * Pedology is the study of soils in their natural environment. It deals with pedogenesis, soil morphology, and soil classification,

while edaphology studies the way soils influence plants, fungi and other living things.

* Paleopedology: It is the discipline that studies soils of past geological eras, from quite recent (Quaternary) to the earliest periods of the Earth's history. Paleopedology can be seen either as

a branch of soil science (pedology) or of paleontology

* Pedosphere: It is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists

at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The sum total of all the organisms, soils, water and air is termed as the pedosphere.

* Agronomy: It is the science and technology of producing and using plants for food, fuel, fiber, and land reclamation.

* Agrology: It is the branch of soil science dealing with the production of crops. Agrology is synonymous with agricultural science.

* Regolith: It is a layer of loose, heterogeneous superficial material covering solid rock. It includes dust, soil, broken rock and other related materials.

* Humus: It is the dark brown to black complex decomposition product of organic matter turnover in soils. It is colloidal, much more highly charged than clay on a weight basis, and is typically

what we report as organic matter content in soil testing programs.

Regolith

Humus

Paleosols

* Paleosols are soils formed long periods ago that have no

relationship

chemical

and day

their

physical

characteristics present

in

to

the

climate

or

vegetation. Such soils form

on

extremely

old

continental cratons and as small scattered localities in outliers of ancient rock.

United

States

Department

of

Agriculture

(USDA)

Soil

Taxonomy: * USDA provides an elaborate classification of soil types according

to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series. Soil Orders

1) Alfisols - Rich in Fe and Al - Common in humid areas, semi-tropics and Mediterranean climates.

2) Aridisols - Commonly in deserts (dry soil)

3) Gelisols - Commonly at high latitudes and elevations

4) Entisols - Least soil profile development - No B horizons - Most common order by surface area

5) Inzeptisols - Similar to Entisol, but beginning of a B horizon is evident - No diagnostic subsurface horizons

- Na > 15%

6) Oxisols - Most soil profile development - Must have oxic horizon within 150 cm of soil surface

- Dominated by clays (Al and Fe oxides) - Commonly in old landscapes in tropics 7) Ultisols

- Known as red clay soils -Common in subtropical regions 8) Histosols

- No diagnostic subsurface horizons - Commonly in wetlands (swamps, marshes, etc.);

9) Andisols - Form from volcanic eject - High in phosphorus, organic matter, glass and amorphous colloidal materials such as allophane 10) Mollisols - Dark soils - Common in grasslands 11) Spodosols - High in Fe and Al oxides, and humus accumulation - Common in coniferous or boreal forests;

12) Vertisols - High in shrinking and swelling clays (> 30%) - Deep cracks (called gilgai) form when soil dries.

Example: Order: Alfisols Suborder: Xeralfs

Great Group: Durixeralfs Subgroup: Abruptic Durixeralfs Family: Fine, Mixed, Active, thermic Abruptic Durixeralfs Series: San Joaquin (soil)

Alfisol

Aridisol

Gelisol

Entisol

Inzeptisol

Oxisol

Ultisol

Histosol

Andisol

Mollisol

Spodosol

Vertisol

Soil Temperature Regimes (STR)

Soil Moisture Regimes (SMR) 1) Aquic - Soil is saturated with water and virtually free of gaseous oxygen

for sufficient periods of time. - Common in wetlands. 2) Udic

- Soil moisture is sufficiently high year-round in most years to meet plant requirement. - Common in humid regions.

3) Aridic - Soil is dry for at least half of the growing season and moist for less than 90 consecutive days.

- Common in arid regions (e.g. desert). 4) Ustic - Soil moisture is intermediate between Udic and Aridic regimes;

generally, plant-available moisture during the growing season, but severe periods of drought may occur. - Common in semiarid regions.

5) Xeric - Soil moisture regime is found in Mediterranean type climates, with cool, moist winters and warm, dry summers.

- Like the Ustic Regime, it is characterized as having long periods of drought in the summer

Soil Structure And Texture

Soil structure: It is the arrangement of sand, silt and clay particles to form larger aggregates. Organic matter is the glue that holds the aggregates together Large

pores

(spaces)

between

aggregates are filled with air in a

moist soil. Small pores are filled with water in a moist soil. Even smaller pores

1/10 inch

inside the aggregates (not shown) are also filled with water.

Soil texture: It is the mineral part of soil consists of sand, silt, and clay particles.

Soil separates: They are specific ranges of particle sizes.

Where: USDA = United States Department of Agriculture WRB = World Reference Base

Soil classification 1) USDA classification * This classification is based on grain size.

2) OSHA classification The U.S. Occupational Safety and Health Administration (OSHA) requires the classification of soils to protect workers from injury

when working in excavations and trenches. This classification is based primarily on unconfined compressive strength: a)

Stable

Rock

(natural solid mineral matter that can be

excavated with vertical sides and remain intact while exposed).

b) Type A (cohesive plastic soils with unconfined compressive strength greater than 1.5 ton per square foot) c) Type B (cohesive soils with unconfined compressive strength between 0.5 and 1.5 ton per square foot).

d) Type C (cohesive soils with unconfined compressive strength less than 0.5 ton per square foot).

3)

Engineering

classification

or

Unified

Soil

Classification

System (USCS) * Engineers, typically

geotechnical engineers, classify

soils

according to their soil engineering properties and behaviors as they relate to use for foundation support or building material. * There are three major groups

a) Coarse grained soils (e.g. sands and gravels); b) Fine grained soils (e.g. silts and clays) c) Highly organic soils (peat).

Letters:

* The USCS distinguishes sands from gravels by grain size and further classifying some as well graded and the rest as poorly graded. Silts and clays are distinguished by the soils' Atterberg

limits, and separates high plasticity from low plasticity soils as well. Organic soils are distinguished from inorganic soils by changes in their Atterberg limits.

Note: * The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit.

Soil Horizon

* Soil profiles reveal soil horizons *A soil horizon is a layer generally parallel to the soil crust, whose

physical characteristics differ from the layers above and beneath. Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly color and texture.

*Six master soil horizons are commonly recognized and are designated using the capital letters O, A, E, B, C, and R. The following horizons are listed by their position from top to bottom

within the soil profile. Not all of these layers are present in every location.

Master Horizons

P (between O and A)

D (between C and R) L (under R)

O = Organic matter P = Peat A = Surface soil E = Eluviated zone B = Subsoil C = Parent rock or substratum R = Bedrock L = Limnic horizon

* O horizon -predominantly organic matter (litter and humus)) -it is generally absent in grassland regions.

-it usually occurs in forested areas and is commonly referred to as the forest floor.

* P horizon (these horizons are also heavily organic (peat), but are distinct from O horizons in that they form under waterlogged

conditions).

* A horizon -organic carbon accumulation, some removal of clay) -Soil

organisms

such

as

earthworms,

potworms

(enchytraeids), arthropods, nematodes, fungi, and many

species of bacteria and archaea are concentrated here, often in close association with plant roots.

* E horizon -zone of maximum removal (loss of organic carbon, Fe, Mn, Al and clay)

-it is commonly found in soils developed under forests, but are rare in soils developed under grasslands.

* B horizon - forms below O, A, and E horizons -zone of maximum accumulation (clay, Fe, Al, CaCO3 and salts)

- most developed part of subsoil (structure, texture and color) - < 50% rock structure or thin bedding from water deposition * C horizon

- little or no pedogenic alteration - unconsolidated parent material or soft bedrock -< 50% soil structure

* D horizon (they are not universally distinguished, but it may be recognized by the contrast in pedologic organization between it and the overlying horizons).

* R horizon (hard, continuous bedrock) * L horizon (in these horizons, mineral or organic material were deposited in water by precipitation or through the actions of aquatic organisms).

Soils develop horizons due to the combined process of: (1) organic matter deposition and

decomposition (2) illuviation of clays, oxides and other mobile compounds downward with the wetting front.

In moist environments free salts (Cl

and

SO4)

are

leached

completely out of the profile, but

they accumulate in desert soils.

Pedogenesis

*Pedogenesis (soil formation or soil genesis) is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy

order within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry.

* In general, formation of soil takes place by four methods: a) Physical weathering b) Chemical weathering c) Biological weathering

d) Human activities (such as excavation, blasting and waste disposal).

Clorpt or Corpt It is a mnemonic for Hans Jenny's famous state equation for soil

formation: s = ƒ (cl, o, r, p, t) where

S = soil cl (sometimes c) = climate o = organic activity r = relief p = parent material

t = time

Factors of soil formation 1) Climate

2) Organisms 3) Topography 4) Parental material 5) Time

1) Climate * Climate regulates soil formation. Soils are more developed in

areas with higher rainfall and more warmth. The rate of chemical weathering increases by 23 times when the temperature increases by 10 degrees Celsius. Climate also affects which organisms are present, affecting the soil chemically and physically (movement of

roots).

• (Fig. 2.15)

Effective precipitation Effective precipitation depends on:

a) Seasonal distribution b) Temperature and evaporation

c) Topography d) Permeability

a) Seasonal distribution of precipitation

Location A 600 mm/yr

50mm

Every month

Location B 600 mm/yr

100mm

Six rainy months only

b) Temperature and evaporation

Location A hot

High evapotranspiration

Location B cool

Low evapotranspiration

600 mm

600 mm

Lower effective ppt

Higher effective ppt

c) Topography

level

slope

concave or bottom of slope (receiving)

d) Permeability

Impermeable

Permeable

2) Organisms (Plants and Animals) * The organisms living in and on the soil form distinct soil types. Classification of Living plants and animals on and in soil: 1) Macroanimals (insects, mammals, gastropods, earthworms) 2) Microanimals (nematodes, protozoa)

3) Macroplants (the green plants) 4) Microplants (fungi, bacteria, actinomycetes, algae)

* Effects of organisms on soil formation depend on: a) Type of vegetation influences soil type

b) Base pumping c) Sources of organic matter d) Nutrient recycling e) Vegetation prevents erosion

a) Type of vegetation influences soil type

b) Base pumping * Deciduous trees are more effective base pumpers than conifers.

-deciduous litter is easy to break down -cations (bases) are released so surface soils are not acidic

-needles are hard to break down -basic cations leach away: soil is acidic

3) Relief (Topography) * Topography has a strong influence on soil development. Soils on

the side of hills tend to be shallow, due to erosional losses. Soils on the tops of hills tend to be deep, but lighter in color, due to downward leaching losses. Soils in the valleys tend to be deeper, darker, and contain more horizons. This is due to increased

material deposition from hillside erosion, material accumulation from downward leaching from the tops of hills, and the collection of greater quantities of water in the low lying areas.

Topography's effect on soil formation

4) Parent material * The rock from which soil is formed is called parent material. The

main types are: aeolian sediments, glacial till, glacial outwash, alluvium, lacustrine sediments and residual parent material (coral or bedrock).

5) Time * Amount of time soil has been exposed to weathering and soil

forming processes influences soil properties. * Rates of soil formation:

Slow--------------------------------------Fast ~ 1 cm/1000 yr ~ 30cm/50yr

Soil forming processes (Pedogenic) * There are four key processes by which soil is formed, they are:

1) Additions: Materials added to the soil, such as decomposing vegetation and organisms (organic matter), or new mineral materials deposited by wind or water.

2) Losses: Through the movement of wind or water, or uptake by plants, soil particles (sand, silt, clay, and organic matter) or chemical compounds can be eroded, leached, or harvested from the

soil, altering the chemical and physical makeup of the soil.

3)

Transformations: The chemical weathering of sand and

formation of clay minerals, transformation of coarse OM into decay resistant organic compounds (humus).

4)

Translocations: Movement of soil constituents (organic or

mineral) within the profile and/or between horizons. Over time, this process is one of the more visibly noticeable as alterations in

color, texture, and structure become apparent. *Key soil forming processes especially important to macroscale patterns

of

soil

formation

are

laterization,

calcification, salinization, and gleization.

podzolization,

Soil Chemistry

Soil components: There are four parts of soil

About ½ of the soil volume is solid particles

Mineral Matter 45%

Organic Matter 5%

Soil Air 25% Soil Water 25%

About ½ of the soil volume is pore space

Mineral Constituents * The majority of soil solids are primary mineral fragments like quartz and feldspars along with synthesized secondary minerals

like clays and iron oxides. * Particles > 2 mm are largely unreactive and are called coarse fragments.

Supplying plant nutrients: Nutrients that plants obtain from the soil Macronutrients:

Micronutrients:

(needed in large amounts)

(needed in small amounts)

• Nitrogen (N)

• Chlorine (Cl)

• Phosphorus (P)

• Cobalt (Co)

• Potassium (K)

• Copper (Cu)

• Calcium (Ca)

• Iron (Fe)

• Magnesium (Mg)

• Manganese (Mn)

• Sulfur (S)

• Molybdenum (Mo) • Nickel (Ni)

• Zinc (Zn)

Where do plant nutrients come from?

* Decaying plant litter

* Breakdown of soil minerals

* Addition by humans – Commercial fertilizer – Manure – Lime – Other

Recycling plant nutrients

Breakdown of soil minerals

Acid

Water Zn

Ca Ni

K

Mg Cu

Nutrient additions by humans * Commercial fertilizers – Nutrients are in a form that is available to plants – Dissolve quickly and nutrients go into soil water * Lime – Dissolves slowly as it neutralizes soil acidity – Releases calcium and magnesium

* Organic nutrient sources – Manure, compost, sewage sludge – Decay and nutrient release is similar to crop litter

The soil solution * Soil water is a complex solution that contains

– Many types of nutrients –Other trace elements –Complex organic molecules * Nutrients in the soil solution

can be readily taken up by

N

plant roots * If nutrients remained in

solution

they

could

all

quickly lost from the soil.

be

Zn

Ni

P

Ca

K

Mg

Cu

Adsorption *Adsorption refers to the ability of an object to attract and hold particles on

+

-

its surface. Solid particles in soil have the ability to adsorb: 1) Water 2) Nutrients and other chemicals

* The most important adsorbers in soil are clays and organic matter.

Surface area of clay

¼ cup

¼ cup of clay has more surface area than a football field

The large surface area of clay allows it to

- Adsorb a lot of water - Retain nutrients - Stick to other soil particles

Properties of soil clays * Clay particles are stacked in layers like sheets of paper.

Each

clay

sheet

is

slightly

separated from those on either side.

*

Each

sheet

has

negative

charges on it. * Negative charges have to be

balanced by positive charges called cations.

Cation retention on soil clays Calcium, +2 Magnesium, +2 Potassium, +1 Ammonium, +1 Sodium, +1 Copper, +2 Aluminum, +3 Hydrogen, +1

Cation retention on organic matter

Hydrogen Nutrients

Increasing pH increases cation exchange capacity of organic matter

Low pH, 4 to 5 (acidic soil)

Neutral pH, 7 (“sweet” soil)

Cation Exchange Capacity * Cation exchange capacity (CEC) is the total amount of cations that a

soil can retain. The higher the soil CEC the greater ability it has to store plant nutrients. * Soil CEC increases as

-The amount of clay increases -The amount of organic matter increases -The soil pH increases

Negatively charged nutrients (anions) * Some very important plant nutrients are anions.

Nitrate

Phosphate

Sulfate

Chloride

* Soils are able to retain some of these nutrient anions.

* Retention of nutrient anions varies from one anion to another

Soil Mechanics

* Soil mechanics is a branch of soil physics and engineering mechanics that describes the behavior of soils.

* Soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology.

* This article describes the genesis and composition of soil, the distinction between pore water pressure and intergranular effective stress, capillary action of fluids in the soil pore spaces, soil classification, seepage and permeability, time dependent

change of volume due to squeezing water out of tiny pore spaces, shear strength and stiffness of soils.

Sieve analysis * The size distribution of gravel and sand particles are typically measured using sieve analysis. The formal procedure is described in ASTM D691304

(2009).

Wentworth Scale Remove Cobbles and Boulders from Analysis (>75mm) “Gravel” 75-2 mm “Sand” 2-0.075 mm

“Silt and Clay” 50% would, for example, be classified as CH.

Indices related to soil strength 1) Liquidity index (LI) * The effects of the water content on the strength of saturated

remolded soils can be quantified by the use of the liquidity index, LI: LI = (w - PL)/(LL – PL)

* When the LI is 1, remolded soil is at the liquid limit. When the soil is at the plastic limit, the LI is 0.

Plasticity chart

2) Relative density (Dr) * The density of sands is often characterized by the relative density: Dr = [(emax - e)/( emax - emin)]100 * Where: emax is the maximum void ratio corresponding to a very loose state, emin is the minimum void ratio corresponding to a very

dense state and e is the in situ void ratio. * Thus if Dr = 100% the sand or gravel is very dense, and if Dr = 0% the soil is extremely loose and unstable.

3) Plasticity index (PI) * The plasticity index is a measure of the plasticity of a soil. The plasticity index is the size of the range of water contents where

the soil exhibits plastic properties. The PI is the difference between the liquid limit and the plastic limit: PI = LL – PL

* Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay.

PI and their meanings

Categories Nonplastic Slightly plastic Medium plastic Highly plastic

PI 0 to 3 3 to 15 15 to 30 >30

4) Consistency index (CI) * The consistency index indicates the consistency (firmness) of a soil. It is calculated as:

CI = (LL ‐ w)/(LL ‐ PL) * Soil at the liquid limit will have a consistency index of 0, while

soil at the plastic limit will have a consistency index of 1.

5) Activity (A) * The activity of a soil is the PI divided by the percent of claysized particles present. Normally the activity of clay is between

0.75 and 1.25, and in this range clay is called normal. It is assumed that the plasticity index is approximately equal to the clay fraction (A = 1). When A is less than 0.75, it is considered inactive. When it is greater than 1.25, it is considered active.

Friction, interlocking and dilation * Soil is an assemblage of particles that have little to no cementation while rock may consist of an assembly of particles

that are strongly cemented together by chemical bonds. * The shear strength of soil is primarily due to interparticle friction and therefore, the shear resistance on a plane is

approximately proportional to the effective normal stress on that plane. * The angle of internal friction is thus closely related to the

maximum stable slope angle, often called the angle of repose.

Angle of repose

Thanks