soil formation

Soil Formation

Soil Physical Properties

• Slope is defined as the angle of the soil surface from horizontal.

• It is expressed as the % of rise over run.


• Slope effects the productive potential in numerous ways: Rain runoff, soil erosion, the use of farm machinery, and contour farming.

Soil Physical Properties• Texture refers to the proportions of

sand silt and clay in the soil.

• Course-textured soils are and sandy and do not hold water well, while fine-textured soils contain clay and tend to hold more surface moisture.


• Flood hazard refers to the likelihood that the soil will flood.

• This may occur in flood plains near rivers and greatly reduce plant production.


• Erosion as a soil property, refers to the degree that the soil has already been damaged.

• May range from none to severe.


• A field used for crop production that has little or no erosion can continue to be used for crops.

• But a severely eroded field may need to be turned into pasture where it is always covered.

Soil Texture• Soil Texture

• What is Soil Texture?– It is the proportion of three sizes of soil

particles.• Which are:

– Sand (Large)

– Silt (Medium)

– Clay (Small)

Effects of Particle Size?

Soil particle size affects two important soil features:They are internal surface area

The numbers and size of the pore spaces

Soil Texture

What is the internal surface area ?It is the area of soil that the total surface are of the particles in the soil.

So the smaller the soil particles the greater the internal surface area will be.

Soil Texture

Pore size and number: this depends on the particle size.

So there are more pores that are found between the larger particles.

Soil Texture

What is a Soil Separates?

They are categories that scientist have divided up into three groups

The three groups are:

SandSilt Clay

Soil Texture

What is Sand?It is the largest and is further divided into four subcategories.They are:

Very coarse sandCoarse sandMedium sandFine Sand

Soil Texture

Sand it is the larges soil separate and is composed mainly of weathered grains of quartz. Sand is also gritty to the touch.Sand grains will not stick to each other.

Soil Texture

SandWhat will sand do to the soil?

It will improve the soil by improving the water infiltration and aeration

Soil Texture

Silt is the medium sized soil separated.Silt particles are silky or powdery to the touchSilt grains will not stick to one another just like sand grains.Silt is the best soil as it has the ability to hold large amounts of water in a form plants can use.

Soil Texture

Soil Texture• Clay

– is the smallest size soil separate.– It is composed of tiny crystals– Clay is formed by chemical reactions between

weathered minerals to form tiny particle of new minerals.

– Clay will hold more plant nutrients than any other separate.

– Clay grains will stick to one another.

Pass out picture of triangle and lets see if we can classify some soils.

Soil Texture

Soil Texture Triangle

1.40% sand 22% clay and 38% silt

What is the soil classified as?

Soil Texture

2.90% sand 10 % clay and 25 % silt

What is it?

Soil Texture

Lets go and texture some of the samples that we have.

See if you can make a ribbon and with which one?

Which one feels grittiest?

Which one feel powdery?

Soil Texture

Soil StructureWhat are the three grades of structured soils?

A. Weak structure: peds are hard to distinguish. Only a few can be separated from the soil.

B. Moderate structure: peds are visible and can be handled without breaking up.

C. Strong structure: most of the soil is formed into peds.

Soil StructureThe six principle soil structural classes:

A. Granular

B. Prismatic

C. Massive

D. Blocky

E. Platy

F. Single grain

Soil Structure• Soil structure: Soil particles cling together

by various reasons such as moisture, earthworms, and the movements of plant roots. Groups of clinging particles are called peds or soil aggregates.

Soil Drainage• Permeability

– Soil permeability is the property of the soil pore system that allows fluid to flow. It is generally the pore sizes and their connectivity that determines whether a soil has high or low permeability. Water will flow easily through soil with large pores with good connectivity between them. Small pores with the same degree of connectivity would have lower permeability, because water would flow through the soil more slowly. It is possible to have zero permeability (no flow) in a high porosity soil if the pores are isolated (not connected). It is also to have zero permeability if the pores are very small, such as in clay.

Soil Drainage• Porosity

–Porosity of a soil is the volume of all the open spaces (pores) between the solid grains of soil. For growing things in soil, the porosity is important as it defines the volume of water that can be held in a given volume of the soil.

Characterizing Soils• The Soil Profile

• Most soils have three distinct layers called horizons.

• The horizons are called A Horizon (topsoil), B horizon (subsoil), and C horizon (parent material).

Characterizing Soils

• The top soil is the most productive because that is where all the nutrients are.

The Soil Profile

• What is a horizon?

–They are the layers of the different types of soil and the different depths that the layers will be located at.

The Soil Profile• What is a soil profile?

–It is the vertical section through the soil and extending into the unwreathered parent materials and exposing all the horizons.

Master Horizons• What is the Master Horizons?

– The are A, B and C horizons are known as the master horizons. They are a part of a system for naming soil horizons in which each layer is identified by a code O, A, E, B, C and R

The O Horizon-

Is the organic layer made of wholly or partially decayed plant material and animal debris . You can normally find this layer in a forest with fallen leaves, branches and other debris.

Horizons

Horizons• The A Horizon-

– It is usually called the topsoil by most farmers. This is where the organic matter accumulates over time. This layer is very prone to leaching and losing iron and other minerals. The A horizon provides the best environment for the growth of plant roots, microorganisms and other life.

Horizons• The B Horizon-

– This is also called the subsoil is often called the zone of accumulation where chemicals leached out of the A horizon

The C Horizon- lacks the properties of the A and B horizon. It is the soil layer that is less touched by soil forming processes and is usually the parent material soil

Horizons

Parent Material• Soil parent materials are those

materials underlying the soil and from which the soil was formed.

• There are five major categories of parent material: minerals and rocks, glacial deposits, loess deposits, alluvial and marine deposits and organic deposits.

Parent Material• Minerals are solid, inorganic,

chemically uniform substance occurring naturally in the earth.

• Some common minerals for soil formation are feldspar, micas, silica, iron oxides, and calcium carbonates.

Parent Material

• Rocks are different from minerals because they are not uniform.

• There are three types of rocks, igneous, sedimentary, and metamorphic.

Parent Material

• Igneous rocks are those formed by the cooling of molten rock.

• Sedimentary rocks are those formed by the solidification of sediment.

Parent Material

• Metamorphic rocks are simply igneous or sedimentary rocks which have been reformed because of great heat or pressure.

Parent Material• During the ice age, glaciers moved

across areas of the northern hemisphere.

• They ground, pushed, piled, gouged, and eventually deposited great amounts of rocks, parent material, and already formed soil material.

Parent Material• Loess deposits are generally

thought of as windblown silt.

• Alluvial and marine deposits are water borne sediments.

• Alluvial deposits are left by moving fresh water.

• Marine deposits are formed on ancient ocean floors.

Parent Material

• Organic deposits are partially decayed plants that live plants are able to root and grow in.

• These are found in swamps and marshes.

Weathering

• When minerals are exposed to weather, they begin to break down into smaller pieces.

• This is mostly done by heating and cooling of the minerals and rock.

Weathering• Some minerals are water soluble

which means they dissolve when exposed to water.

• Some rocks may contain some minerals that are water soluble and only that part of the rock will dissolve. Ex: some caves.

Weathering

• When a tree or other types of plants begin growing in the cracks of rocks, this may speed up the break down of the rock because of the pressure the roots may exert.

Weathering

• Ice can also speed up the weathering process on rocks.

• If a rock has a crack that can fill up with water, when the water freezes, it can literally crumble the rock into small pieces.

Weathering

• Rocks can also be broken down by mechanical grinding such as wind blowing sand at high speeds or glaciers causing rocks to grind each other.

Weathering

• New soil is continually being made, but it takes a long time to create new soil and if it isn’t managed properly, soil can be eroded away quicker than it can be made.

Soil

• a layer of natural materials on the earth’s surface containing both organic and inorganic materials and capable of supporting plant life.

Soil

• The material covers the earth’s surface in a thin layer.

• It may be covered by water, or it may be exposed to the atmosphere.

Soil

• Soil contains four main components: inorganic material, organic matter, water, and air.

Soil

• Ideal soil should contain about 50% solid material and 50% pore space.

• About half of the pore space should contain water and half of the space should contain air.

Soil

• Inorganic material consists of rock slowly broken down into small particles.

• The organic material is made up of dead plants and animals varying in stages of decay.

Soil

• The percentages of the four main soil components varies depending on the kind of vegetation, amount of mechanical compaction, and the amount of soil water present.

Soil• Soil is formed very slowly.

• It results from natural forces acting on the mineral and rock portions of the earth’s surface.

• The rock is slowly broken down to small particles resulting in soil.

Organic Matter

• In most soils, the proportion of organic matter is relatively small (2-5%).

• Its importance in formation and production is much higher than the small % would suggest.

Organic Matter

• Soil organic matter decaying plant and animals.

• As they die, they are attacked by microorganisms: fungi, bacteria, and others.

Organic Matter• There are two types of organic matter.

• Original tissue is that portion of the organic matter that can still be recognized.

• Twigs and leaves covering a forest floor are good examples.

Organic Matter

• Humus is organic matter that is decomposed to the point where it is unrecognizable.

• The brown color you sometimes see in soil is a good example.

Organic Matter• Purposes of organic matter:

affects the soil structure by serving as a cementing agent, returns plant nutrients to soil (P, S, N), helps store soil moisture, makes soil more tillable for farming, provides food (energy) for soil microorganisms, which makes the soil capable of plant production

Additional Resources• http://soils.usda.gov/education/• Water Movement in Soil Video (Click Below)

soil formation

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