The Complete Soil

By Moira Whitehouse PhD

The continents have a layer of solid rock, the crust, covering the hot stuff in the mantle.

The continental crust, then, is mostly covered with thick layers of soil.

Here we see a slab of Earth taken out of the crust with the soil on top showing where we live.

Bedrock (crust of the Earth)

USDA

Soil, from the bedrock to the top, is our subject.

Below the top, there are coherent layers of soil—similar in characteristics such as composition, texture, and color.

Thankfully, soil covers most of the Earth’s solid crust (bedrock); however, in some places it is thin or nonexistent.

Plant growth, that allows us to live, occurs on the top layer of soil.

bedrock D

layers of soil

http://www.nrcs.usda.gov

Below that layer are several other layers, some that provide minerals and ores for our use.

Why do we care?

We should understand that soil is a valuable natural resource that is essential to life on Earth.

Since top soil can be nurtured and protected, or squandered, our well being as a specie may well depend on which we choose.

Listing the good things provided by soil might be useful for our students: soil is where we grow our food; it helps clean our water; it provides the material for constructing many of our buildings and roads; it absorbs rainwater and helps prevent floods.

Soil is made of.....?

Speaking mainly of top soil and the upper soil horizons

Four things:

• weathered pieces of rock made up of minerals (All rock is a mixture of minerals.)

• organic material (remains of dead plants and animals)

• air (containing oxygen)

• water

The other half is made up of vast numbers of interconnecting holes between the rock pieces and organic matter.Depending on location, the time of year or rainfall, these holes, called pores, are filled with either air or water.

The weathered rock pieces makes up about half of the total mass of most soil. Less than 10% of the mass would be organic matter and living things.

Soil

Tiny pieces of weathered rock

WaterRemains of dead plantsand animals

Air with oxygen

In pores, spaces between the pieces of weathered rock

airwater

Those ingredients that make up soil are necessary for the plants and micro-organisms that live there.

• The weathered pieces of rock and the organic material provide many of the nutrients such as iron, nitrogen, potassium that plants need to grow and to carry out their life processes.

• The weathered pieces of rock anchor the plants roots so a plant doesn’t blow or wash away.

• Plant roots and soil microorganisms get the oxygen and water they need to live from the spaces (pores) in the soil.

Liquid—the water and the minerals dissolved in the water that plants need.

Gas—oxygen is needed for respiration by the roots, the bacteria and other microorganisms.

Though we usually think of soil as the solid combination of weathered rock (minerals) and organic material we can hold in our hands, the liquid and the gas components are crucial to the life processes that are supported by soil.

In this presentation we will focus mainly on the solid parts of soil—weathered rock and the humus—the organic material (dead plants and animals). Soil:

Weathered rock(tiny pieces of rock)

Humus—organic matter (remains of dead plants and animals)

First we will explore the weathered rock part of the soil.

What is weathered rock?

To answer, we will review “weathering”.

Weathering is the breaking down of rocks into smaller pieces.

Weathering can happen as a result of physical processes

or as the result of chemical processes.

Knowing that soil is made up of tiny pieces of weathered rock, it is obvious that the rocks in these pictures have a long way to go before becoming soil.

Weathering will take many years to change this sized rocks into soil sized pieces.

We will find, however, that some rock weathers (breaks down) into soil size fragments faster than other rock.

Some of the reasons for rock to weather at different rates include:

Particle size--The smaller the particles making up the rock, the faster it will weather.

Porosity--the more holes in the rock the faster it weathers.

Exposed surface area--The more surface area that is exposed, the more it will weather.

The minerals that make up the rock-- Softer minerals weather faster than harder ones.

Knowing that weathering of rock takes time, and that some rocks weather at different rates than others, we will now consider what causes weathering.

The two major types of weathering are physical and chemical.

1. Physical weathering causes the parent rock to break into smaller fragments without changing the chemical makeup of the rock.

2. Chemical weathering changes the basic chemical makeup of the parent rock through chemical reactions.

Physical weathering

•Breaking bigger rocks into smaller fragments without changing the chemical makeup of the rock can be done by nonliving things and by living things.

We will look first at non living things that break up rock.

Non living things that causeweathering

1. Water running over the rock

4. The abrasion of rock by the blowing wind carrying sand

3. The temperature of rocks changing from hot to cold

2. Water freezing in cracks in the rocks

5. Salt wedging

Nonliving things that break rocks into pieces.

Running water

http://www.ngu.no/en-gb/hm/

Rapidly moving water particularly high up in the mountains or a canyon can wear away (breaking off fragments of rock).

Seen here, a small, fast running stream of water is dramatically cutting through this huge granite monolith.

http://www.flickr.com kia4067

Rocks carried by fast moving water hit other rocks breaking them into pieces. Moving sand and silt acts like sandpaper on the larger rocks in the river bed rubbing off pieces of rock. Theses smaller

pieces become part of the suspended rock and sediment carried by the fast moving river.

http://www.flickr.com Wildcat Dunny

Gradually larger rocks are broken into smaller and smaller rocks eventually becoming sand and silt.

http://www.flickr.com Randy OHC

Here you can see fragments of rock created by water that was moving quickly.

http://www.flickr.comDawn

Rocks that have been tumbled for a long time in rivers and streams become smooth and rounded.

Freezing water

This process occurs most often in cold wet climates usually in high mountain ranges. It does need to periodically warm up enough for liquid water to run into the cracks.

2. Water freezing in cracks in the rocks

http://www.flickr.com/reurinkjan

http://www.ct.gov/

Here we can see cracks in large mountain rock.

Copyright 2004 by Andrew Alden, geology.about.com, reproduced under educational fair use

Rain fills these cracks with water.

When the water freezes, it expands within the crack pushing the rock apart.

Image courtesy of the geology department umd

As the ice melts, the water penetrates more deeply into the fracture.

Then the water freezes again, further cracking the rock.

This repeated cycle of freezing and thawing forces the crack open more and more, eventually shattering the large rock into smaller pieces.

Changing temperature

3. Changing temperature also causes physical weathering. As rocks heat up, they expand. As they cool, they contract. This process is repeated over and over again in nature. Eventually this process causes them to break apart.

The expansion and contraction of rock due to changes in temperature occurs most often in desert environments where the temperature fluctuates dramatically from day to night. The sky above a desert is usually clear and dry.

The surrounding air is likewise heated and cooled quickly by the ground.

As a result, during the day, the ground heats up and at night cools down rapidly.

This repeated expansion and contraction of weakens the rock causing it to break.

http://www.flickr.comHoggheff aka Hank Ashby aka Mr. Freshtags'

Here we see large rock in a desert environment that has probably been exposed to the freeze-melt cycle.

Abrasion

4. Abrasion by windblown sand also weathers rocks particularly in deserts. It is similar to rubbing sandpaper over a piece of wood or sandblasting concrete.

http://www.flickr.com beige alert

http://www.flickr.com nukeit1

Frequent sandstorms in the desert weather exposed rocks.

http://www.flickr.com lumierefl

The scouring of the rock by the wind carrying sand wears off fragments of rock.

So much for those non living things that cause physical weathering, now we will consider the living things that also cause physical weathering.

Living things that causeweathering

Living things also break rocks into smaller pieces:

1. Plant roots—particularly tree roots

2. Lichen growing on rocks

3. Burrowing animals

Plant roots

The roots of plants, particularly tree roots, are amazingly strong. When they start growing as tiny root hairs they can fit into the smallest of cracks.

As these tree roots continue to grow in diameter, they put stress on the rock and cause the cracks to expand breaking the rock apart.

http://www.flickr.comChazz Layne

Here the roots of the tree are growing in the cracks in the rocks making the fractures larger.

http://www.flickr.com iandavid

http://media.photobucket.com/ city bumpkins

Lichen growth

2. Lichen growing on rocks.

Lichens are unusual creatures, they are not single organisms the way most other living things are. Rather, lichens are a combination of two organisms—a fungus and an algae-- which live together in a symbiotic relationship.

A little memory prodder for this, “Freddy fungus met Alice algae and took a lichen to her.”

Lichens appear in the form of small patchy crusty outgrowths in colors of green, brown, orange and other colors. They often grow on rocks.

http://www.flickr.com brian http://www.flickr.comSeaDavid

Lichens expand and contract as they are moistened and then dry out. In this process they loosen microscopic rock fragments creating tiny cracks in the rock.

Their rock-breaking power is greatly enhanced when seeds fall into cracks and then germinate. This is particularly true in the case of woody shrubs and conifers with powerful expanding root systems.

Burrowing animals

3. Burrowing animals

When animals burrow in rocks or between the rocks, they introduce seeds which germinate, or water that freezes, both of which encourage rock breakup.

So far, we have looked at the agents of physical weathering—the process of breaking up parent rock into smaller pieces without changing the chemical makeup of the rock.

Now we will investigate chemical weathering.

Chemical weathering

Causes of chemical weathering:

Chemical weathering breaks up rock by changing the basic chemical makeup of the parent rock through chemical reactions.

2. Oxidation (rusting of iron in rock) 1. Acidic water

3. Hydrolysis

Chemical weathering creates holes or soft spots in rock, so the rock breaks apart more easily.

Carbonic acid is very common in nature. It is produced when carbon dioxide is released from decaying material or when carbon dioxide in the air combines with water.

When this weak carbonic acid trickles into cracks in limestone, it can dissolve large amounts of calcium carbonate (calcite) in the form of calcium bicarbonate.

1. Acidic water:

The mildly acidic rain water seeps into cracks in the ground.

http://www.esi.utexas.edu

The carbonic acid in the water dissolves the calcium carbonate in limestone rock leaving behind holes in the rock.

Carbonic acid also works on limestone found on the surface. We don’t see the classic holes here, but there is evidence that softer parts of the rock have been dissolved away allowing the rock to break apart more easily.

http://www.geolab.unc.edu/

2. Oxidation, another type of chemical weathering, is the process in which oxygen chemically combines with another substance. Many rocks contain iron. When water and air are present, this iron often oxides (rusts), similar to an iron bar rusting.

The rust formed by oxidation makes rocks softer. As a result, the rock deteriorates.

When you see red rocks you know oxidation (rusting) is happening and that those rocks are weathering.

pics4learning

This process is common is arid regions.

3. Hydrolysis occurs when minerals in rocks react with water to form other weaker compounds.These weaker compounds are then more easily worn away by physical weathering.

Feldspar changes to clay.

parent rockWeathering breaks the rock into smaller and smaller pieces

soil

Freezing and thawing of water

Wind with sand

Running water

Heat and cold

Ice

Physical Weathering Chemical Weathering

Acids

Watersand

silt

clay

To review, both physical and chemical weathering are important in creating soil out of rock.

In the study of soil, the rock that is weathered to produce soil is called the “parent rock”. The parent rock may be igneous, sedimentary or metamorphic.

Soil:

Weathered rock Humus

(remains of deadplants & animals)

igneous

sedimentarymetamorphic

It may be cooled above the surface, usually on the ocean floor (as in basalt).

http://rst.gsfc.nasa.gov/

Igneous rock: is rock that is formed from solidified magma.

Or the magma may be extruded underground where it gradually cooled and solidified (as in granite)

http://rst.gsfc.nasa.gov/

Sedimentary rock: is rock

that is formed from sediment deposited by water or wind or minerals deposited when water evaporated.Seventy-five to eightypercent of the rock exposed on the Earth’s surface is sedimentary rock; therefore most of the parent rock is sedimentary.

Metamorphic rock: is rock that has been changed chemically by heat and pressure deep under the surface of the Earth.

Igneous and metamorphic rock tend to be buried underground and therefore, generally not subject to weathering.

Gniess

In science, it is often found that things occur in cycles. The rock cycle is one of the more interesting ones, but it takes a very long time to complete. This picture of the

rock cycle shows that each type of rock can be weathered and changed into sediments including soil (small pieces of weathered rock).

Looking again at the solid part of soil we will consider the three types of soil:

Soil:

Weathered rock Humus(remains of deadplants & animals)

Sand ClaySiltlargest smallest particles

1. Sand—largest particles (created by physical weathering.

2. Silt—medium sized particles (created by physical weathering.

3. Clay—smallest particles (created by chemical weathering.

These three types of soil, sand, silt and clay are identified largely based of the size of their weathered rock pieces.

The weathered rock in soil probably started out as a huge boulder.

In the process of being broken down, the size of the particles of rock become smaller and smaller—boulders to large rocks, to smaller pieces of rock to pebbles to sand, silt and clay.

The following particle size guidelines are used to differentiate the various rocks as well as the three types of soil.

Size of Particles of Rock (Diameter)• 2 m boulders • coarse fragments such as pebbles > 2 mm• sand < 2 mm to 0.05 mm• silt < 0.05 mm to 0.002 mm• clay < 0.002 m

Wikipedia Commons

Still looking at the solid part of soil, we now examine the humus:

Soil:

Humus(remains of deadplants & animals)

Weathered rock

Sand ClaySiltlargest smallest particles

Humus is formed when dead plants and animals decay.

What is humus?

How is it formed?

The organic part of soil.

Special organisms in the soil, called decomposers, cause dead plants and animals to decay or rot changing their bodies into the humus part of soil.

What causes these dead things to change into soil?

When plants and animals die, they become food for these decomposers--bacteria, fungi, arthropods, nematodes and earthworms.

Decomposers recycle dead plants and animals into chemical nutrients like carbon and nitrogen that are released back into the soil, air and water.

Bacteria are the smallest living organisms, and the most numerous of the decomposers; they make up 90% of the billions of microorganisms typically found in a gram of soil. A teaspoon of fertile soil generally contains between 100 million and 1 billion bacteria.

They carry out the majority of decomposing that occurs in the soil.

USDA

Bacteria are important in the carbon cycle. During respiration, bacteria release carbon dioxide into the atmosphere.

Bacteria are an essential parts of the nitrogen cycle.

Wikipedia commons

http://soils.usda.gov

Magnified bacteria found in the soil.

Bacteria dot the surface of strands of fungal hyphae.http://soils.usda.gov

Actinomycetes are a large group of bacteria that grow as hyphae like fungi. They are especially important in degrading hard-to-decompose compounds, such as chitin and cellulose.

http://soils.usda.gov/

Fungi are not plants; they can't make their own food.

Fungi is the name for simple organisms including mushrooms, molds and yeasts.Next to bacteria, fungi are the most efficient decomposers.

They absorb their nutrients from the organisms they are decomposing. In the process they release enzymes that decompose dead plants and animalsMany fungi can break down tough organic materials such as the cellulose in leaves and the cellulose and lignin in wood.Many plants depend on fungi to help extract nutrients from the soil.

http://www.flickr.com Benimoto

http://www.flickr.com Futurilla

http://www.flickr.com scoobygirl

Mushrooms growing on logs

http://www.flickr.com mill56

http://www.flickr.com photogirl7

Mushrooms growing on a forest floor

Mushrooms growing in dead grass

Fungus beginning to decompose leaf veins in grass clippings.

http://soils.usda.gov/

Other important decomposers found in the soil are numerous invertebrates—animals without backbones.

Thus, the saying “FungiBacteriaInvertebrates” as the three main decomposers.

Included under the invertebrate grouping are worms called nematodes, mites, pillbugs and millipedes.

Nematodes, a group of invertebrate decomposers living in the soil are tiny non-segmented worms typically 1/500 of an inch in diameter and 1/20 of an inch in length.

Nematodes are important in releasing nutrients into the soil in forms plants can use.

Nematodes are play an crucial role in the return of nitrogen to the soil (nitrogen cycle).

One square yard of woodland or agricultural soil can contain up to several million nematodes.

http://soils.usda.gov/

Nematodes magnified in soil.

http://www.flickr.com zimpdenfish

Pill bug

Other important invertebrate decomposers

Decomposing mites

Photo credit: Joseph O'Brien, USDA Forest Service,

Millipede

Organisms such pill bugs, millipedes and mites are important to the soil because they stir up and churn the soil, mixing in air which is needed by other organisms in the soil habitat.They shred organic matter into small pieces, assisting other soil organisms in the decomposition process.Finally, along with other decomposers, they change nutrients into forms plants can use.

The lowly earthworm is also an important decomposer.

Earthworms eat dead plants and animals, thereby, absorbing the nutrients that they need to survive. Earthworms excrete wastes in the form of casts which are rich in nutrients such as nitrogen, phosphorous and potash that plants need.In addition to breaking down organic materials and adding nutrients to the soil, earthworms also help loosen the soil, thereby, creating space for the oxygen that plant roots and microorganisms need to live.

Decomposer food web

All these decomposers along with the dead plants and animals form a soil food web.

And how are soil food webs different than food webs on the surface of the Earth?

In a food web above ground, the decomposers are at the end of the energy transfer in the food web.

In a food web below ground the decomposers are at the beginning of the web.

While living plants are the largest source of energy above ground, the most abundant energy resource in the soil is dead plant and animal matter.

http://www.blm.gov/

In turn, these primary consumers are the food for many other secondary consumers that live in the soils such as mites, protozoa and other nematodes.

The primary consumers in soil are often microbes such as bacteria, fungi and nematodes (tiny microscopic worms).

They consume the remains of dead plant and animal.

http://soils.usda.gov/

The natural cycle of plants absorbing minerals from the soil and and these minerals being returned to the soil through decomposition is repeated over and over in nature.

http://www.flickr.com/ angus clyne

Decomposition creates fertile soil containingthe nutrients plants need in a form that they can use to carry out their life processes.

There are 13 main mineral nutrients that when dissolved in water and absorbed through a plant's roots provide what is needed by plants to grow healthy forests, prairies, or even front lawns.

These are: nitrogen, phosphorous, potassium, calcium, magnesium, sulfur, boron, copper, iron, copper, manganese, molybdenum and zinc.

USDA

Where there is lots of vegetation to decay and enrich the soil, such as in deciduous forests and grasslands, the soil is rich in humus and very fertile.

Wikipedia Commons

Where there is little or no vegetation to provide the organic debris, such as at the seashore or in the desert, the soil has little or no humus and is not very fertile.

Desert in Saudi Arabia The Chihuahuan Desert along the Rio Grande

Wikipedia Commons http://www.flickr.com/ Cory Leopold

What are the physical properties of these kinds of soil, sand, silt, clay and humus?

Now that we have explored how the two solid parts of soil are formed (weathered rock and humus), let’s move on to another topic.

Remember physical properties are the things we can observe about a substance using our five senses.

For soil, the two main physical properties are: •color

•texture

Soil Color

The most obvious property when looking at soil is its color.

Geologist officially recognize over 170 different soil colors.

The most common color of soils are shades of black, brown, red, gray, and white.

The darker color often indicates an increase in decomposed organic matter known as humus.

Generally speaking, the darker a soil is, the more nutrients it contains.

Gray soils often indicate poor drainage, while red soils may indicate a lack of nutrients.

black fertile soil infertile red soilPhotos courtesy of USDA

• 3) ability to sustain plant growth

Texture is based on the size of the particles making up the soil and is the single most important physical property of the soil.

• 1) how much water will flow through it

• 2) water holding capacity

• Soil texture is the identifying characteristic of sand, silt and clay, the three types of soil formed from weathered rock.

The size of theparticles making up a soil, therefore, greatly affects:

Sand

• if the sand grains look white it is usually weathered quartz; if they look brown it contains many minerals Images from Wikipedia Commons

Particles under a microscope

• particles are visible without microscope• < 2 mm to > 0.05 mm

• rounded or angular in shape

• holds little water and prone to drought

Sand• feels gritty

• does not stick together in a mass

unless it is very wet.

• fewer nutrients for plants than silt or clay

• pores between sand particles promote free drainage and entry of air

Sandy soil

Also, water-soluble nutrients leach out rapidly before the plants can use them.

Sandy soil has large particles with large air spaces between them. Therefore, it drains very quickly and dries out faster than the other soils.

Humus added to sandy soil acts like a sponge, absorbing and holding moisture and any nutrients dissolved in it.

• erosion by glaciers often responsible for formation of silt

Silt

• particles < 0.05 mm to > 0.002 mm

• particles not visible without a microscope

• wet silt does not stick together and cannot mold it into different shapes.

• has more plant nutrients than sand.

• does not feel gritty.• floury powdery feel –

smooth like silly putty when wet.

• bigger particles than sand -- retains more water for plants and have slower drainage than sand.• easily washed away by flowing water –

highly erosive.

Clay

• when stirred in water small particles of clay do not settle but remain suspended.

Clay particles under a microscope

http://photojournal.jpl.na sa.gov

• < 0.002 mm • particles are flat plates or tiny flakes

• clay swells when water is added and shrinks when the water evaporates. This results in bothersome shrinking and swelling that can adversely affect buildings, roads and walls.

• wet clay is very sticky and slippery and can be molded readily into any shape like a rod or a long ribbon.

• pores (spaces between particles), like the particles are very small

somovement of water and air through clay is

slow giving clay a tremendous ability to hold water.

Clay soil

Since moisture does not drain from this soil well, if clay is too wet plant roots cannot respire so they rot.

The tiny particles that make up clay tend to stick together causing water to fill up the air spaces.

Adding humus to clay soils discourages the small particles from sticking so tightly together, resulting in larger spaces that drain water more easily and hold more air.

Determining Soil Texture - Feel Method

• Wet a sample of the soil and place in your hand

• Try to roll it into a ribbon.• If it makes a ribbon, it contains mostly clay.

• The longer the the more clay in the soil.

• If it does not roll into a ribbon, it is sand or silt

• If it feels grainy or gritty it is mostly sand

• If it feels smooth, silky or flowery, it is mostly silt

Humus is the soil formed when dead plants and animals decay.

It is a dark brown or black (color).

It feels crumbly and loose when dry and spongy when wet (texture).

http://www.edupic.net/sci_gr.htmFree clip are for educational use

Properties of humus

When dead plants and animals decay leaving behind the humus, it accumulates just under the top layer of soil (shown as Horizon “A”)

Humus inherits the minerals that were contained in the decayed plants and animals.Humus therefore contains the nutrients (minerals such as, nitrates, phosphates, potassium, copper, zinc dissolved in water) that plant roots can absorb.

Comparing soil... ability to hold waterClay soils hold more water than sand, silt or humus.

Humus holds more water than sand or silt.

Sand and silt hold the least water, often too little for many kinds of plants to survive.

For growing plants, clay holds too much water for most kinds to survive.

But water does drain through humus quickly.

Clay has more nutrients (minerals) than sandy soils mainly because it hangs on to its mineral abundant water for longer.

Comparing soil...nutrients (minerals) providedHumus soils have an abundance of nutrients for plants.

Adding humus to infertile soil increases its fertility.

Since clay soils hold a lot of water, fewer nutrients are leached out of the soil.However, as clay soil ages and plants remove its nutrients, it becomes infertile.

When nutrients leach out of the soil, they are not available for plants to use.

Comparing soil...nutrients (minerals) providedSandy soils have fewer minerals because when water drains through sandy soils, it often dissolves the minerals in the rock and carries them along with it. This condition

is called leaching.

Loam is a mixture of clay, silt, sand and humus and is the best soil for growing plants.

Because loam is a mixture of four kinds of soil, it holds the proper amount of water and provides all the nutrients plants need.

Loam is formed in nature when the dead plants and animals are left to rot and mix in clay, sand or silt.

Comparing soil...nutrients (minerals) provided

We previously mentioned that humus is found in the “A” soil horizon.

What do we mean by soil horizons?

bedrockR

layers of soil

http://www.nrcs.usda.gov

Soil horizons

It shows the various layers or horizons, beginning at the surface with the “O” horizon and continuing downward through the “A”, “B”, “C”, and “R” horizons to the parent material.

A soil profile is a vertical cross-section of soil.

The layers in a cross-section of soil are easy to spot here but they are usually much harder to find.

Photo coutesy of USDA

The horizons are named O, A, B, C and R.

It's about an inch thick and is made up of dead plants, animals and some humus.

“O” is the top horizon.

The A horizon is alive with plant roots and tiny microorganism like bacteria, fungi, and other decomposers such as nematodes, mites, pill bugs and worms.

The A horizon called topsoil is made up of humus (decomposed organic matter) mixed with mineral materials.

It is generally dark-colored because it contains large amount of humus.

This layer, between the A and B Horizons is light in color and made up mostly of sand and silt.

E Horizon -- the Eluviaton Layer

Due to a process known as leaching in which water with dissolved minerals drips through the soil above, Horizon E loses most of its minerals and clay.

It contains clay and mineral (such as iron, aluminum oxides, and calcium carbonate).

Horizon BThe B Horizon is commonly called “subsoil”.

Horizon B is formed when clay and minerals migrate down through Horizon A and E so sub-soils generally have more clay than top-soils.

Horizon C

.

That layer beneath the B Horizon and above the D Horizon consists of broken-up bedrock that is only slightly weathered .

Plant roots do not grow down into this layer and organic material is pretty much nonexistent.

Layer R is the unweathered rock (bedrock) layer that is beneath all the other layers.

Layer R

Layer R is the crust, the outermost layer of the Earth, that lies below the soil. NASA image

In discussing soil texture, one of the main characteristics we discussed was how each was able to hold or pass through water.

Which brings us to the water table, a phenomenon dependant upon water supply but also affected by soil texture.

Surface runoff water seeps through empty spaces in the soil, sand, or rocks until it reaches an impermeable layer of rock.

The water then fills the pores, empty spaces and cracks above that rock layer.

The upper “surface” of the water filling those spaces in the soil, sand, or rocks is called the water table.

Water table

Again, the upper “surface” of theground water is called the water table.

usgs

It is in this unsaturated zone above the water table that plants absorb the moisture through their roots and return it to the atmosphere through transpiration. The dirt and rocks in this zone contain air and usually some water. In dry spells, this layer sometimes dries out.

A large amount of surface water can result in water seeping through the upper soil layers and accumulating underground as ground water.

Ground water is the water trapped in the empty spaces under ground.

A large body of ground water is called an aquifer.

Ground water is recharged from rain water and snowmelt or from water that leaks from lakes and rivers.

usgs

Thinking long term about soil• Over long periods (thousands of years) the soil

horizon textures change.• As soils get older, sand weathers to silt and silt

weathers to clay. Consequently, old soils have more clay than younger soils.

•Soil, along with oxygen, water, plants and animals, is considered a renewable natural resource.

This is because soil can be replaced in a relatively short period of time.•”Relatively” short because it may take a 1,000 years to form one inch of soil as compared to the 350 millionyears for fossil fuels and around 100 million years for most rocks on the surface of the Earth.

So there it is, the dirt or soil.