SOILMan, despite his artistic pretensions, his sophistication, and his many accomplishments— owes his existence to a six-inch layer of topsoil and the fact that it rains.

We know more about the movement of celestial bodies than about the soil underfoot.

-Leonardo da Vinci

Dirt: A Natural Resource

DefinitionSolid earth material that has been altered by physical, chemical and organic processes so that it can support rooted plant life.

Engineering definition: Anything that can be removed without blasting

Soil Texture

There are three noticeable partsSand (.05 – 2 mm)Silt (.002 - .05 mm) Clay (.002 mm)

Dirt: A Natural Resource

Physical weathering breaks rocks into small mineral particles

Chemical weathering dissolves and changes minerals at the Earth’s surface.

Decomposing organic material from plants and animals mixes with accumulated soil minerals.

Soil CompositionSoil, on the

average, consists of 50% mineral and 5% organic matter.

The remainder is pore space taken up by air or water.

This is just an average!

Soil CompositionDuring a precipitation event or snowmelt, water infiltrates into soil through the pores space and pores become filled. As the soil begins to drain or dry, air replaces water in the pores. On average, soil contains 50% pore space which can be evenly divided between air and water.

Soil CompositionParticle size

determines the amount of air & water contained in soil

Soil CompositionPores between the

solid materials hold liquids and gases that are essential for plant and microorganism growth, and solid materials hold or provide nutrients that can become accessible for organism growth.

Five Factors that Influence Soil Formation

• Climate• Organisms• Parent Material• Topography• Time

Parent MaterialThe earthen materials that make up the soil. rocks volcanic deposits sedimentsmineralssandstone will form coarse sandy soil.

soft shale will turn into heavy clay soils

granite bedrock produces a sandy loam.

Climate More moisture from rain will speed up soil development

Too much can wash away nutrients.

Dry climate soils become very fertile the first time they are irrigated, but if the evaporation rate is high, salts will rise to the surface and reach a toxic level to plant life.

A freezing and thawing cycle will speed up the crumbling of rocks.

Warmth increases organism activity and chemical processes will speed up.

Living OrganismsAnimals and plants are the source of organic matter and the nutrients.

These are released into the soil either by physical decay after death or from animal wastes like manure. 

The organic remains from the surface are worked by microorganisms into material that is incorporated into the soil.

There is also the direct altering by sub-soil animals like moles and earthworms.

Topography

Soils on the side of hills tend to be shallow, due to erosional losses.

Soils on the tops of hills and in valleys tend to be deepThere is material accumulation from downward leaching

from the tops of hills, and the collection of greater quantities of water in the low lying areas.

Soils that get more sun exposure will be drier. The natural vegetation and growth will also affect

composition.

Time

Time is the encompassing factor that binds all of these materials and interactions together. Soils can go through many changes over the course of years.

It takes hundreds of years to form one inch of soil from parent material.

Time

This is why soil conservation is so importantU.S. Department of Agriculture estimates that it

takes 500 years to form an inch of topsoil. That’s less than 0.01 mm yr-1

Soil Profile

A cross-section of soil is called a soil profile

The distinct layers of soil are called soil horizons

Soil HorizonsO Horizon

Organic consists of leaf litter and

other organic material lying on the surface of the soil

A Horizon Top Soil the most productive layer of

the soil. Conservation efforts are

focused here!

E Horizon Eluviated (leaching layer) Where minerals and organic

matter leave this layer and move into the next lower layer

Soil HorizonsB Horizon

Subsoil Usually lighter in color,

dense and low in organic matter.

More minerals

C Horizon Consists largely of

weathered parent material.

R Horizon Rock (Bedrock) Un-weathered

sediment of pure parent material.

Soil Development

Soil DevelopmentADDITIONS Inputs from outside

ecosystem Atmospheric inputs

Precipitation, dust, deposition

Horizontal inputsFloods, tidal

exchange, erosion, land-water movement

Inputs from within ecosystem Litterfall and root

turnover

Soil DevelopmentTRANSFORMATIONS Decomposition of organic matter Humification to form complex

organic matter Weathering of rocks

Physical weatheringFragmentation of rock

Freeze-thaw; drying-wetting; fire

Physical abrasionAbrasion by glaciers

Chemical weatheringDissolves primary mineralsForms secondary minerals

Soil DevelopmentTRANSFERS Translocation of materials

within the soil profile is primarily due to gradients in water potential and chemical concentrations within the soil pores.

Soluble minerals, colloidal material, organic compounds, and iron may move up or down the profile, between horizons, with water movement.

Biological activity may cause gradient in the chemical composition of the water and air-filled pores of the soil.

Soil DevelopmentLOSSES Modern rates of soil loss are 100 to

1000 times rates of soil formation (typically mm yr-1 to cm yr -1 in agricultural settings).

Most losses occur by leaching (fertilizers.) Water moving through the soil (rainfall or irrigation) dissolves certain minerals and transports them into deeper layers.

Carbon dioxide is consumed by growing plants, but lost to the soil as fresh organic matter decays.

When soil is wet, nitrogen can be changed to a gas and lost to the atmosphere. 

Solid mineral and organic particles are lost by erosion

A nation that destroys its soils, destroys itself.

– President Franklin D. Roosevelt, Feb. 26, 1937.