Summary of Important Concepts

• Soil is a layer of weathered rock, minerals, and organic matter at the earth’s surface that supports plant life.

• The main factors that determine the characteristics of a soil are climate (mainly temperature and rainfall), topography, type of parent rock material, organic activity, and the amount of time that the soil has been forming.

• Soil forms from regolith - broken up rock material at the earth’s surface. Soil forms as regolith undergoes weathering - the physical and chemical breakdown of rock. - Physical weathering is breakdown of rock by physical forces. Example: rock wedged apart by freezing water or by plant roots.- Chemical weathering is breakdown of rock by chemical reactions. Examples: solution, oxidation, and hydrolysis reactions.

Summary of Important Concepts, continued

• A soil profile is a set of distinct layers, or soil horizons, that occur in a soil. These horizons have different features in different types of soil.

• The main types of soil on earth are:

- Loess (pronounced “luss”): fertile soils developed on wind-blown glacial silt deposited during the Ice Ages.

- Laterites: red, iron oxide-rich soils of wet, hot tropical areas, created by intense chemical weathering of parent rock material.

- Pedalfers: rich soils with brown color, high in aluminum and iron; typical of cooler, wet temperate climates world-wide.

- Pedocals: soils typical of warm, arid regions; high in calcium and commonly contain caliche (white deposits of calcium carbonate)

- Tundra soils: soils forming in polar climates of permafrost (permanently frozen ground).

Remember : These are NOT soil orders !!!

What is SOIL?

It has been said that soil represents “a few inches between humanity and starvation”. This phrase puts the importance of soil in perspective. Soil is a layer of weathered rock, minerals, and organic matter at the earth’s surface that supports plant life. Without soil, human life would not be possible.

This figure illustrates the composition of a typical soil. Soil is composed of mineral matter from weathered rock; water, gases, and organic matter (the remains of plant and animal material and bacteria).

The water table or phreatic surface is the upper limit of abundant groundwater. In the vadose zone, above the water table, the interstices between particles of earth are filled by air, or by air and water (with the exception of the capillary fringe). Below it, every available space is saturated with water. The definition of the water table is surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0).

Soil’s main important uses for humanity are summarized here.

Soil Profiles

Digging down into a soil, you would notice that the soil zone has a layered appearance. This layering is called a soil profile. Each layer of a soil profile is called a horizon.

Soil profiles vary between different types of soils, but one can often recognize the “O”, “A”, “B”, and “C” horizons in many soils.

Simplified Soil Profiles

Birchwood Castelleia Haven Nickerson

Examples of Soil Profiles

Creek versusfloodplain soil profiles

horizon description of detailed soil horizonsO consists mainly of organic matter from the vegetation, which accumulates under conditions of free aeration.A eluvial (outwash) horizon consisting mainly of mineral matter mixed with some humified (decomposed) organic matter.E strongly eluviated horizons having much less organic matter and/or iron and/or clay than the horizons underneath. Usually pale coloured and high in quartz.B illuvial (inwashed) horizon characterised by concentrations in clay, iron or organic matter. Some lime may accumulate, but if the accumulation is excessive, the horizon is named K.K horizon containing appreciable carbonate, usually mainly lime or calcium carbonate.G gleyed horizons which form under reducing (anoxic) conditions with impeded aeration, reflected in blueish, greenish or greyish colour.C weathered parent material lacking the properties of the solum and resembling more the fresh parent material.R regolith, the unconsolidated bedrock or parent material.

Types of Soils

Many schemes have been proposed for classifiying soils. For our purposes, we can recognize FIVE great soil types on earth. • Loess• Laterites• Pedalfers• Pedocals• Tundra soils

Loess (pronounced “luss”) covers about 10% of the earth’s land area, and is perhaps the most fertile soil on earth. Vast areas of farmland in the U.S. and in Asia are underlain by loess.

Loess is formed on deposits of wind-blown glacial silt that were laid down over vast areas of the continents during the Ice Ages. As glaciers moved across the high northern latitudes they ground up rock material to a powder. This was washed out from the glaciers by streams, then picked up by winds and blown over great distances.

Some would consider this heretical !!

This map shows the world-wide distribution of loess. Notice the vast areas of the U.S. and Asia that are covered by loess. These areas are some of the world’s most productive farmland.

Loess is tan to yellow in color, and porous and light. The small mineral grains weather readily, producing nutrients that get taken up by plants. The porous soil drains well, and is easily tilled to make fields.

This photo shows an excavated layer of loess in China -- home to the largest loess deposits in the world.

Loess is a soil defined by its origin (deposits of wind-blown glacial silt). The four other great soil types we will consider are defined not by their origin, but by the climate in which they form. The two climatic factors that are most important here are precipitation and temperature. As shown in the figure below:

• Laterites form in hot, wet climates.• Pedalfers form in cool, wet climates.• Pedocals form in hot, dry climates.• Tundra soils form in cold, dry climates.

Very generalized !

Soil Order Formative Terms PronunciationAlfisols Alf, meaningless syllable PedalferAndisols Modified from ando AndoAridisols Latin, aridies, dry AridEntisols Ent, meaningless RecentGelisols Latin gelare, to freeze JellHistosols Greek, histos, tissue HistologyInceptisols Latin, incepum, beginning InceptionMollisols Latin, mollis, soft MollifyOxisols French oxide OxideSpodosols Greek spodos, wood ash OddUltisols Latin ultimus, last UltimateVertisols Latin verto, turn Invert

Puerto Rico Crater Lake

Rocky Mountains Saint John

Also don’t forget parent material and

Time !!!

Transported versus Residual Overburden

Residual = soil ( developed from bedrock )

Transported ; may have a soil developed on it !

Laterite

Terra rossa clay replaces Salem limestone, Bloomington, via a set of moving alteration zones – a unique outcrop

Clay+FeOx replace calcite, terra rossa, Bloomington: early start of a diffuse FeOx patch

Merino et al, Am J Sci 93

Weathering’s crucial feedback, deduced by adjusting bottom reaction on volume (since it’s a replacement)

Gibbsite partly replaces Plagioclase, left, and Quartz, below

Soil ProfilesPit 1 - Peaty Gley

The peaty gley is found on a low lying flat land, the vegetation cover consists of wetland vegetation such as sedges, marsh type plants and rushes.

The texture of this soil is course and sandy with lumpy particles of wood etc. Within the structure there is capillary action from ground water taking place.

The peaty gley is a very dark colour this is because their is alot of organic matter, the surface layer is 40cm which is not mixed in through the A horizon. Also as it is very wet so no oxidation can take place.

The bioto found in the gley is limited because it is too cold and too wet. There are very few stones in this soil profile.

The parent material is the material deposited by melt water at the end of the ice-age. The ground water provides moisture and the water table is near the surface.

Soil Profile Pit 2 - Cultivated Gley

The land is used for crops with grass present. The land form is hollow and low-lying. The texture of the soil is fine and has clay particles. The structure of the soil profile is wet and heavy on the top layer. The next layer has humus which binds the fine soil together. The C Horizon has got wet sticky clay. The colour of this soil is grey with orange specs on the A Horizon and on the B Horizon it is an orange colour with bits of iron compounds. The organic matter is a thick layer but it is mixed in with the A Horizon and the biota consists of worms and there are also small stones in the soil. The soil is also poorly drained.

Soil ProfilePit 3 - Cultivated Podsol

The cultivated podsol is found in a gently sloping field. The vegetation cover consists of trees and crops. The texture of this profile is coarse and sandy. The structure is better than podols because of humus, there are small rounded particles and the structure is stable. The colour in the A horizon is dark, then there is an iron pan and the B horizon is dark aswell. The soil is acidic and the humus has been mixed through the top thirty centimetres. There are many worms because the weather is warmer here. This helps to make it less acidic than it could be. Within this soil profile there are small stones and it’s well drained.

Soil Profiles Pit 4 - Iron Podsol

The iron podsal is found in dry moorland areas. The typeof vegetation they like is dry heeth, and heather.The landform is undulating hilltop.The texture of the soil is sandy, fairly course for allhorizons.The structure of the horizons are poor, weak structure.There is no organic material mixed through the or B-horizon to blend the soil together.There are big rocks, which will be found 35cm from thesurface. The colour of the soil in the AO horizon is black.In the A horizon its light orange and the B horizon is anorange/ grey colour.The organic matter is 2-3 cm its black humus and is belowthe surface. Biota is the type of life found in the soil. Inthe A horizon there are no worms as its too acidic.There are a lot of stones in this soil podsal. The iron podsalis very well drained soil.The climate that the iron podsal most prefers is cool, humidconditions with precipitation evaporation so thattransiocation or leaching of material is active

Parent Material

The parent material is significant in the early development of the soil and it’s mineral content. It can vary from bedrock to a range of deposits including alluvium and sand.

The parent material in Podsols are weathered rock and are acidic.

The parent material in Gley soils are impermeable clay which results in waterlogging.

TimeTime is important in the development of soils before they fully mature. When they are young soils retain the features of the parent material.

As time increases the young soils gradually change their characteristics from their parent material to form their own type of soil.

ConclusionPeaty gley is useful for nothing due to it’s covering of heather and marsh. This covering ensures that no other crops would be able to grow there.

Cultivated gley is used for growing crops as it is low lying so that it is warmer as it is further down. Crops can grow in warmer conditions.

Cultivated podsol is used for farming because it has a better, smaller stable structure. It also has remnants of rich iron material.

Iron podsol is not used for anything as it is an acidic soil and is covered in dry heather, long grass and conifers. It is also very bumpy, hilly and stony.

Chapter 7Clay structure and properties

Soil Colloids• “Organic and inorganic matter with very small

particle size and a correspondingly large surface area per unit mass” (“Soil bank”)

• Four categories:– Crystalline silicate clays (phyllosilicates)– Noncrystalline silicate clays– Iron and aluminum oxide clays– Organic matter (humus)

“Clay” is . . .

• A particle size class (<0.002 mm)

• A mineral type with specific properties and characteristics (secondary mineral)

“Big” smaller really small

Sand silt clay

Relative Size Comparison of Soil Particles

Shape of silicon tetrahedron and aluminumaluminum octahedron

O OH

OO

O

OH

OH

OH

OH

Si

Al

O

OO

Octahedral sheet

Octahedral sheet

Tetrahedral sheet

Tetrahedral sheet

Tetrahedral sheet

Tetrahedral sheet

Ionic Radii of elements in silicate clays – Tetrahedral & Octahedral sheets

Types of clay minerals

• Based on numbers and combinations of structural units (tetrahedral and octahedral sheets)

• Number of cations in octahedral sheet• Size and location of layer charge

(due to isomorphic substitution)

• Absence or presence of interlayer cations

• Two general categories: 1:1, 2:1

Clay minerals

(one tetrahedral sheet for each

octahedral sheet)

nacrite, dickite, halloysite, etc.

(two tetrahedral sheets for each

octahedral sheet)

Montmorillonite,beidellite, saponite, etc.

Illite, muscovite, biotite, etc.

Tri- or di-vermiculite

Cookeite, chamosite ETC

Weird-o, not truly 2:1

1:1 clays 2:1 clays

Kaolinite

1:1 Silicate Clays• Layers composed of one tetrahedral

sheet bound to one octahedral sheet• Kaolinite: one of the most widespread

clay minerals in soils; most abundant in warm moist climates

• Stable at low pH, the most weathered of the silicate clays

• Synthesized under equal concentrations of Al3+ and Si4+

Kaolinite• A 1:1 clay

• Little or no isomorphous substitution

• “nutrient poor”

• No shrink-swell (stable because of H-bonding between adjacent layers)

• A product of acid weathering (low pH, common in soils of the SE USA

Sheets of silicon tetrahedra and aluminum octahedra linked by shared oxygen atoms.

Structure of KaoliniteNO ISOMORPHOUS SUBSTITUTION!!!

2:1 Silicate Clays• Two silica tetrahedral sheets linked to

one aluminum octahedral sheet

• Three key groups:– Smectites (e.g., montmorillonite)– Vermiculites– Micas (e.g., illite)

• And one weirdo (the chlorites)

Smectite (2:1, Montmorillonite)

• Layer charge originates from the substitution of Mg2+ for Al3+ in the octahedral sheet

• Unstable (weathers to something else) under low pH and high moisture

• Most swelling of all clays

• “Nutrient rich”

Structure of montmorillonite (a smectite): it is built of two sheets of silicon tetrahedra and one sheet of aluminum octahedra, linked by shared oxygen atoms.

Structure of basic Smectite (Montmorillonite)

Structure of basic Smectite (Montmorillonite)

Isomorphous substitution in the octahedral sheet

Causes cations to move into the interlayer space, where they can be replaced by other cations

= Mg (this slide only)

(2:1, Fine-grained Mica: Illite)• Al3+ substitution for Si4+ on the tetrahedral

sheet

• Strong surface charge

• “fairly nutrient poor”

• Non-swelling, only moderately plastic

• Stable under moderate to low pH, common in midwestern US

Structure of Illite

Structure of Illite

1. Isomorphous substitution is in the tetrahedral sheets

2. K+ comes into the interlayer space to satisfy the charge and “locks up” the structure

K+ K+

Chlorites (2:1:1)

• Hydroxy sheet in the interlayer space

• Restricted swelling

• “Nutrient poor”

• Common in sedimentary rocks and the soils derived from them

• Isomorphic substitution in both tetrahedral and octahedral sheets

Structure of Chlorite

Mg-Al hydroxy sheet

Mg-Al hydroxy sheet

1. Iron-rich

2. “locked” structure

3. Low nutrient supply capacity

= Al = Fe = Mg

Comparison of common silicate clays

Property Kaolinite Smectite Fine-grained mica

Swelling

Bonding

Net negative charge (CEC)

Charge location

General class 1:1 (TO) 2:1 (TOT) 2:1 (TOT)

LowLow to none

High

Hydrogen (strong)

Van der Waal’s (weak)

Potassium ions (strong)

Low: 2-5 cmolc/kg

High: 80-120 cmolc/kg

Mod: 15-40 cmolc/kg

Edges only – NO isomorphic substitution

Octahedral sheets

Tetrahedral sheets

Fertility

Where to find different clays

Laterite

New Caledonia Laterite

Reactions :

2Fe2+ + 3H2O = Fe2O3 + 6H+ + 2e-

3Fe2+ + 4H2O = Fe3O4 + 8H+ + 2e-

2Fe3+ + 3H2O = Fe2O3 + 6H+

Al3+ + 3H2O = Al(OH)3 + 3H+

Al(OH)4- + H+ = Al(OH)3 + H2O

Eh-pHdiagrams

Supergene Enrichment

Al

Eh

pH

1 0.1 ppm

Gibbsite forms

Fe

Eh-pHdiagrams

New Caledonia Laterite