The Formation of Soil Chapter 8

The formation of Soil

• It literally takes hundreds to thousands of years for soil to form.

• Soil is the Result of weathering of rocks and gradual accumulation of detritus from the biosphere.– We can determine the specific properties of a soil

if we know its parent rock type.• The amount of time during which it has been forming• And its associated biotic and abiotic components.

The Formation of Soil • The processes that form soil work in two directions

simultaneously. • The weathering of rocks and primary minerals

provides the raw material for soil from below.• The deposition of organic matter from organisms

and their wastes contributes to soil formation from above.

• What we normally think of as “soil” is a mix of these mineral and organic components.

Differences in poor and developed soil

• A poorly developed soil has substantially less organic matter and fewer nutrients.– This is usually young soil

• More developed soil can be nutrient poor.– Over time, plants remove many of the essential

nutrients.– Others are leached away by water. • Erosion

Factors that determine the properties of soil

• There are 5 Factors that determine the properties of soils:– Parent Material – Climate– Topography– Organisms– Time

• None of these factors alone can determine soil properties

• They must work simultaneously!

Parent material• A soil’s parent material is the rock material

underlying it from which its inorganic components are derived. – Different soil types arise from different parent material – Ex. Quartz sand (Beach sand) parent material will give

rise to a soil that is nutrient poor.– Soils that has Calcium carbonate as its parent material

will contain an abundant supply of calcium.• Will have a High Ph• May support high agricultural productivity.

– This soil is found in areas surrounding Lake Champlain in Northern New York and Vermont.

Climate • Soils do not develop well when temperatures are

below freezing. – This is because decomposition of organic matter and water

movement are both extremely slow in Frozen or nearly frozen soils. • So, Soils in the northern hemisphere are composed largely of

organic material in an undecomposed state.

• Soil development in the humid tropics is accelerated.– This is because of the rapid weathering of rock and soil

minerals.– Leaching of Nutrients and decomposition of organic

ditritus.

Climate and vegetation

• Climate is indirectly effects the soil formation through its influence on the type of vegetation that develops. – This also effects the type of detritus left after the

vegetation dies.

Topography

• Topography is the surface slope and arrangement of a landscape.

• Soils that form on steep slopes are constantly subjected to erosion – Occasionally, have more drastic mass movements

of material such as landslides.

• Soils that form at the bottoms of steep slopes may continually accumulate material from higher elevations and become quite deep.

Organisms

• Plants remove nutrients from soil and excrete organic acids that speed chemical weathering.– Animals that tunnel or burrow mix the soil which

distributes organic and mineral matter uniformly throughout. • Earthworms, gophers and voles

– Humans also have dramatic effects on the soils.

Time• As soils age, they develop a variety of

characteristics. – The grassland soils that support much of the food crop

and livestock feed production in the U.S. are relatively old soils.• This is because they have had continual inputs of organic

matter for hundreds of thousands of years from grassland and prairie vegetation growing above them. – They have become deep and fertile because of this.

• Other soils that are as old can become relatively infertile.

• This is from less productive communities above them and perhaps greater quantities of water moving through them.

Soil Horizons and Properties• The five factors that determine soil formations

lead to different soil types at different locations.– This is because of the different environments in the

world today.

• Soils with different properties serve different functions for humans. – Ex. Some soil are great for crops, others for house

developments.

• In order to understand and classify soil types, we need to understand both soil horizons and properties.

Soil Horizons: O Horizon

• As soils form, they develop characteristic Horizons, AKA Layers. – The specific composition of those horizons depends

largely on climate, vegetation and parent material. – As the surface of many soils is a layer of organic

detritus such as leaves, needles, twigs and even animal bodies.• These are all in stages of decomposition. • This is called the O horizon• O = Organic • It is more prominent in forest soils and in some grasslands.

A horizon and E Horizon• A horizon is the horizon that is mixed, either

naturally or by human agricultural practices – This is also known as the Top Soil. – In other words, it is the zone of organic material and

minerals that have been mixed together.

• The E Horizon is soils that are acidic and is a zone of leaching or eluviation– It forms under the O horizon or, less often, the A horizon– It is always above the B horizon.– When an E Horizon is present, Fe, Al, and dissolved organic

acids from the overlying horizons are deposited in the B Horizon.

B horizon

• The B Horizon is composed primarily of mineral material with very little organic matter.– This is also known as subsoil and every soil has

this horizon. – If there are nutrients in the soil, it will be in the B

horizon!!

C horizon

• The C horizon is always under the B horizon and is the least weathered soil. – This horizon is similar to the Parent material.

• Each Horizon is important in the understanding the role of soils in evniromental science.

Physical Properties of Soil• Texture of a soil is determine by the percentages of

the particles with in it. – These particles are sand, silt and clay.

• There are a lot of ways in find the texture out but one of the ways is to plot it on a triangle shaped diagram called the soil texture chart.

• The first thing to do before we plot it is to determine the percentages of Sand, Silt and Clay. – The ideal percentage is 40% sand, 40% silt and 20% clay

• By doing this, we can determine what type of soil we have.

Porosity-Sand• Another property of soil is porosity. – Porosity is how quickly soil drains water. – This is important to know to find out what soil can be

great in planting certain plants.

• The largest of the three particles is sand.– Sand drains water very quickly and becomes dry the

quickest.– This is because the particles within sand are packed

loosely. • This would be easy for roots to penetrate.

Porosity-Clay• Clay particles are the smallest of the soil particles.• This creates less pores for water to drain from– So the Porosity of it is low.– If there is a clay-dominated soil then water and roots

cannot easily move through it.

• Silt is half of sand and clay…in other words it is the middle between the two.

• Soil that is only comprised of one particle is horrible for agriculture, you want a mixture of the three.

Particles of soil and Pollutants• Soil usually serves as a filter that removes

pollutants from the water moving through it. – Depending on the percentage of sand, clay and silt

depends how good the soil is at this.

• If the soil is mainly comprised of sand, the water will move through it quickly, making it a poor filter.

• Clay is very useful with a contaminant that needs to be contained. – Because the particles won’t allow the pollutants to filter

into the water below it. – This is why most landfills are lined with clay.