Earth’s Resources: SoilEarth’s Resources: Soil

What is soil?What is soil?

The outermost layer of the earth’s crustThe outermost layer of the earth’s crust Made up of both organic (from living things) Made up of both organic (from living things)

and inorganic materials.and inorganic materials. It’s divided into layers according to the ratio of It’s divided into layers according to the ratio of

organic to inorganic material at various depthsorganic to inorganic material at various depths It’s classified according to what kind of It’s classified according to what kind of

material you can find in a samplematerial you can find in a sample

Layers of the soilLayers of the soil

The layers vary as to The layers vary as to contentcontent

The closer to the top, The closer to the top, the more organic the more organic material you’ll findmaterial you’ll find LeavesLeaves SticksSticks Dead animals (insects as Dead animals (insects as

well as larger critters)well as larger critters) Finally, at the bottom, Finally, at the bottom,

you will hit BEDROCK!you will hit BEDROCK!

Layers = HORIZONSLayers = HORIZONS The topmost layer, or The topmost layer, or OO

horizon, includes “horizon, includes “litterlitter” or ” or detritus from formerly living detritus from formerly living things. things.

The The AA horizon is the most horizon is the most valuable – It is the valuable – It is the topsoiltopsoil where plants get most of where plants get most of their nutrients. The decaying their nutrients. The decaying organic matter in this layer is organic matter in this layer is called called humushumus..

(litter)

HumusHumus

As living things (both dead plants and dead As living things (both dead plants and dead animals) decompose, their molecular animals) decompose, their molecular components become available for new plants components become available for new plants to take up as to take up as nutrientsnutrients..

This decayed material is called This decayed material is called humushumus.. Humus is mostly insoluble, so it remains in Humus is mostly insoluble, so it remains in

these upper layers of soil.these upper layers of soil. It is very dark, nearly black in color.It is very dark, nearly black in color. Human created Human created humushumus is called is called compostcompost!!

TOPSOILTOPSOIL

Five tons of topsoil spread over an acre would Five tons of topsoil spread over an acre would only be as thick as a dime!only be as thick as a dime!

It takes over 500 years for organic material to It takes over 500 years for organic material to collect and decompose naturally to form only collect and decompose naturally to form only ONE INCH of topsoil!ONE INCH of topsoil!

5 to 10 tons of animal life can exist in an acre 5 to 10 tons of animal life can exist in an acre of topsoil!of topsoil!

The The EE horizon is horizon is where the nutrients where the nutrients from the organic from the organic material (humus) material (humus) begin to dissolve.begin to dissolve.

The deepest of The deepest of plants’ roots plants’ roots sometimes extend sometimes extend this far.this far.

Layers = HORIZONSLayers = HORIZONS

(litter)

LeachingLeaching

After rain or other sorts of precipitation, water After rain or other sorts of precipitation, water moves into the soil. This is called moves into the soil. This is called infiltrationinfiltration..

As it moves from the topsoil layer into the As it moves from the topsoil layer into the subsoils and deeper, it dissolves inorganic subsoils and deeper, it dissolves inorganic minerals from the upper layers. This is called minerals from the upper layers. This is called leachingleaching..

Minerals and other substances may be carried Minerals and other substances may be carried into groundwater supplies and thus into into groundwater supplies and thus into drinking water in this manner.drinking water in this manner.

The The BB horizon horizon This layer contains a This layer contains a

smaller amount of smaller amount of organic material.organic material.

It has a heavy It has a heavy concetration of the concetration of the minerals leached minerals leached from the above from the above layers. layers.

Layers = HORIZONSLayers = HORIZONS

(litter)

The The CC and and RR horizons horizons are the underlying rock.are the underlying rock.

CC horizon is weathered horizon is weathered and broken rock or and broken rock or regolithregolith. . LeachedLeached minerals may be found minerals may be found here.here.

The The RR horizon is horizon is bedrockbedrock (almost solid (almost solid crustal material.)crustal material.)

Layers = HORIZONSLayers = HORIZONS

Layers = HorizonsLayers = Horizons

Soil in different locations may or may not have Soil in different locations may or may not have all of these horizonsall of these horizons

For instance, in a construction zone, all of the For instance, in a construction zone, all of the topsoil may have been removed.topsoil may have been removed.

Topsoil may have eroded through wind or Topsoil may have eroded through wind or water effects.water effects.

Heavy earthquake or storm damage may have Heavy earthquake or storm damage may have disrupted the existing layersdisrupted the existing layers

Examples of Soil ProfilesExamples of Soil Profiles

Limestone soil in the USVI (organic limestone)

Loamy sand from Kansas

Other components of soilOther components of soil

Water – after rains or floods, water infiltrates Water – after rains or floods, water infiltrates the soil, carrying the nutrients to plant roots the soil, carrying the nutrients to plant roots and into lower horizonsand into lower horizons

Air – soil is composed of approximately 25% Air – soil is composed of approximately 25% air, aided by the movement of worms and air, aided by the movement of worms and insects in the topsoil layersinsects in the topsoil layers

5% living things – plant roots, worms, 5% living things – plant roots, worms, microorganisms make up part of the soil.microorganisms make up part of the soil.

Types of SoilsTypes of Soils

Soil is classified according to the type of Soil is classified according to the type of mineral basemineral base

SandSand SiltSilt ClayClay LoamLoam

Sandy SoilSandy Soil

Sandy soil has large Sandy soil has large particles which don’t fit particles which don’t fit together welltogether well

Water (and dissolved Water (and dissolved nutrients) seep right nutrients) seep right through and are not usable through and are not usable by plants’ rootsby plants’ roots

This type of soil dries out This type of soil dries out quicklyquickly

Particles feel grainyParticles feel grainy

Silty SoilsSilty Soils

Particles are smaller than sandParticles are smaller than sand Particles are very light, they Particles are very light, they

erode easily with both wind erode easily with both wind and waterand water

With water, this is what we With water, this is what we think of as mud.think of as mud.

Particles feel like flourParticles feel like flour Dusty texture when dryDusty texture when dry

Clay-type soilsClay-type soils ClayeyClayey soil has very small soil has very small

particles, and gums together. It particles, and gums together. It holds water and nutrients well, holds water and nutrients well, but plants find it hard to root in but plants find it hard to root in clay soil.clay soil.

While it holds water well, it While it holds water well, it takes a long time for water to takes a long time for water to permeatepermeate a clay soil. a clay soil.

Brief, heavy rains may not Brief, heavy rains may not reach plant roots.reach plant roots.

Particles feel slipperyParticles feel slippery

Loamy SoilLoamy Soil

The perfect mix of The perfect mix of particle size and texture particle size and texture to hold water and to hold water and nutrients!nutrients!

About 40% sandAbout 40% sand 40% silt40% silt 20% clay20% clay

Types of soilsTypes of soils

The ideal type of soil for plant (crop) growth is The ideal type of soil for plant (crop) growth is partway between the three types.partway between the three types.

LoamyLoamy soil has neither too small nor too large soil has neither too small nor too large particles (It’s just right!)particles (It’s just right!)

Able to drain, but also hold nutrientsAble to drain, but also hold nutrients LoamyLoamy soil has a good percentage of decayed soil has a good percentage of decayed

plant material or plant material or humushumus which provide the which provide the nutrients that plants need.nutrients that plants need.

Further ResourcesFurther Resources

National Resources Conservation Service National Resources Conservation Service website. Try out the Web Soil Survey tool to website. Try out the Web Soil Survey tool to see what’s under your feetsee what’s under your feet

US Environmental Protection AgencyUS Environmental Protection Agency Digging to China?Digging to China? Introduction to SoilsIntroduction to Soils