Introduction to Soils

Chapter 1

Six reasons soils are important

Soils are natural bodies

Soils have s

Four general components of soil

General nutrient sources and how rootsaccess them

Give me 6 good reasons why soils are important

Plant growth

SupportWaterAeration of rootsNutrients

Water quantity and quality

Limits runoff

Recharges ground- and surface-water

Filters and purifies

Air quality

Nutrient cycling releases nutrients

Habitat for organisms

soil

Engineering considerations

problem

bigger problem

Soils as Natural Bodies

Form due to long-term pedogenicprocesses

Variable across landscape

Internally (vertically) variable

You will pick up new jargon, like pedogenic = soil forming.

This is calleda generalsoils map.

The differentcolored areasshow soilassociations.

These are notsoil types butgroups of soiltypes that occurtogether in thelandscape.

This give a senseof the lateralvariability in soils.

Overlie rock

Some form from rock in place

Others form in unconsolidated materialfrom elsewhere

Soils

Regolith is unconsolidated materialabove rock

Soils form at surface of regolith

Horizons develop

Solum is upper 1 to 2 m

Here are some more terms. All the particulate material lying aboverock is not necessarily considered soil, only that upper part that showsevidence being transformed by soil forming processes. This is called the solum, and it contains at least one (more or less) horizontally oriented zonescalled horizons that have developed due to pedogenesis.

The solum is the upper part of the regolith.

Soil Profile

Looking down from the top,you should see a dark upperhorizon, a light horizon below it,then another dark horizon, a more colorful one, etc.

Most soil profiles are not thisdistinctive and odd. The darkhorizon below the surface iswhat is called a secondaryaccumulation of organic matterand it exists due to specialconditions.

OA

E

B

C

These are the 5 masterhorizons.

They are defined on thenext slide.

Know them well.

Note that not all soils haveall five master horizons.

They will, however, have atleast an A and a C. The A isa pedogenic horizon.

O Organic horizon

A Accumulation of organic matter Darker than below

E Maximum leaching Little organic matter Lighter than below

All soils will have an A horizon. Its exists from plants growing in thesoil and depositing organic matter on and just below the surface (residueon the surface and roots). However, the A is dominantly mineral, not organic.

If there is substantial deposition of organic residue on the surface, an O(organic) horizon exists. Common in forest soils.

An E is distinguished if there is a different horizon below it. The light color justmeans there is not much organic matter. Used to be called an A2 horizon.

B Accumulation of clay or salts Bright color

C Parent material

The B is a secondary accumulation of clay or salts. It is often morebrightly colored that the other horizons. The clay or salt has been washeddown to this depth by water. Salts will be found in the subsoil like this where the climate is arid (little water for dissolving and leaching out of theprofile). A clayey subsoil is common where there has been a lot of waterdraining through the profile.

So, the A and E are depleted of clay.

The C is thought to be what existed when pedogenesis began –unalteredregolith.

General Composition of SoilThe solids are either mineral or organic and the pores arefilled with air or water.

Mineral particles vary

SizeMineralogyAggregation

.

Approximaterelative sizes ofsand, silt and clay.

You see the clayparticle?

Coarse fragments

Fine earth

Sand 2.00 to 0.05 mmSilt 0.05 to 0.002Clay 0.002 and smaller

While some soils contain solids that are larger than the largest sand,these large bodies do not strongly affect biological, chemical and physical process in soils. The fine earth particles do, especially the smallones.

Tiny particles (< 0.001 mm) are colloidal

Soil colloids are chargedSo do they adsorb cations and anions?

Soil colloids have high surface area perunit mass

Colloidal particles tend to remain in suspension. As you will see, theclay particles (and organic matter) carry electrostatic charges. These mustbe balanced by adsorption of oppositely charged ions from solution.

Primary and

secondary minerals

The ultimate origin of soil mineralparticles are rocks, and the firstrocks come from within the earth.

Thus minerals derived from igneousrocks are called primary.

Some soils contain an appreciableamount of primary minerals, whereasothers are dominated by minerals thatwere produced from primary minerals.The latter are called secondary.

Individual particles may be aggregatedinto larger units of soil structure

The small clay particlestend to form aggregatesbecause of their largesurface area to massratio. However, aggregatescontain silt and sand, too.

Organic matter is alsoimportant in aggregateformation and stability. Ittends to glue particles together.

Organic Matter

Mineral soil has about 1 to 6 %

Source of nutrients

With respect to plant nutrition, recognize that organic matter is notjust C, H and O. It may contain appreciable N, P and S that are releasedwhen the organic matter decomposes. Also, the electrostatic charges on it are sites that hold nutrient ions like Ca, Mg, and K that may be used by plants.

Soil Water

Contains various solutes (soil solution)

Soil Air

Inversely related to soil water content

Compared to the atmosphere, soil air

Higher or lower in H2OHigher or lower in CO2 Higher or lower in O2

What do you think?

Well, on the average, do you think soil air is wetter (higher humidity)than the aboveground air? I’d say so.

Recognize that the soil is full of life, especially microbial life, that isrespiring and giving off CO2. If this life is aerobic, it is also consuming O2.

Diffusion of gases through soil pores, especially when some are filled withwater, is a very slow process.

Nutrient Sources in Soil andAccess by Plant Roots

MineralsOrganic matterIons adsorbed on colloidsDissolved

These are the sources of plant nutrients in the soil.See next slide.

Minerals weather

Organic matter rots

Ions released from colloids

Mineral weathering is a slow process (geologic time scale).

Decomposition of organic matter with release of N, etc. is muchfaster but still somewhat slow.

On the other hand, the ions that are adsorbed on mineral and organicmatter in soil are in equilibrium (more or less) with ions in the soilsolution so that depletion of ion X from the soil solution by plant uptakefrom the soil solution results in release of ion X from mineral and organic colloidal surfaces (this is somewhat of a simplification).

Now, how do roots come into contact with nutrients in the soil solution?See next slide.

Mass flow

Diffusion

Root interception

Plants transpire water(into roots through stemsto leaves where it evapor-ates). The mass flowof water to roots (down thegradient, wetter to drier)carries dissolve nutrientsto the plant root surface.

Also, since nutrients are taken up at the root surface, their concentration is lowerthere, resulting in a concentration gradient, thus diffusive mass transfer to the root.

Finally, the plant root system grows and expands over time.