Learn About Soil

In this episode of the I Can't Sleep Podcast, fall asleep learning about Soil. I'm sure you could guess that this would be a boring subject, and you would be correct. The only interesting thing I got out of it was that I now know I should aerate my lawn or something.

Happy sleeping!

Transcript

Welcome to the I Can't Sleep podcast,

Where I read random articles from across the web to bore you to sleep with my soothing voice.

I'm your host,

Benjamin Boster.

Today's episode is from a Wikipedia article titled Soil.

Soil,

Also commonly referred to as earth or dirt,

Is a mixture of organic matter,

Minerals,

Gases,

Liquids,

And organisms that together support the life of plants and soil organisms.

Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

Soil consists of a solid phase of minerals and organic matter,

The soil matrix,

As well as a porous phase that holds gases,

The soil atmosphere,

And water,

The soil solution.

Accordingly,

Soil is a three-state system of solids,

Liquids,

And gases.

Soil is a product of several factors,

The influence of climate,

Relief,

Elevation,

Orientation,

And slope of terrain,

Organisms,

And the soil's parent materials,

Original minerals,

Interacting over time.

It continually undergoes development by way of numerous physical,

Chemical,

And biological processes,

Which include weathering with associated erosion.

Given its complexity and strong internal connectedness,

Soil ecologists regard soil as an ecosystem.

Most soils have a dry bulk density,

Density of a soil taking into account voids when dry,

Between 1.

1 and 1.

6 grams per centimeter cubed,

Though the soil particle density is much higher,

In the range of 2.

6 to 2.

7 grams per centimeter cubed.

Little of the soil of planet Earth is older than the Pleistocene,

And none is older than the Cenozoic,

Although fossilized soils are preserved from as far back as the Archean.

Collectively,

The Earth's body of soil is called the pedosphere.

The pedosphere interfaces with the lithosphere,

The hydrosphere,

The atmosphere,

And the biosphere.

Soil has four important functions,

As a medium for plant growth,

As a means of water storage,

Supply,

And purification,

As a modifier of Earth's atmosphere,

As a habitat for organisms.

All of these functions,

In their turn,

Modify the soil and its properties.

Soil science has two basic branches of study,

Etiphology and pedology.

Etiphology studies the influence of soils on living things.

Pedology focuses on the information,

Description,

Morphology,

And classification of soils in their natural environment.

In engineering terms,

Soil is included in the broader concept of regolith,

Which also includes other loose material that lies above the bedrock,

As can be found on the moon and other celestial objects.

Soil is a major component of Earth's ecosystem.

The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil,

With effects ranging from ozone depletion and global warming to rainforest destruction and water pollution.

With respect to Earth's carbon cycle,

Soil acts as an important carbon reservoir,

And it is potentially one of the most reactive to human disturbance and climate change.

As the planet warms,

It has been predicted that soils will add carbon dioxide to the atmosphere due to increased biological activity at higher temperatures,

A positive feedback amplification.

This prediction has,

However,

Been questioned on consideration of more recent knowledge on soil carbon turnover.

Soil acts as an engineering medium,

A habitat for soil organisms,

A recycling system for nutrients and organic wastes,

A regulator of water quality,

A modifier of atmospheric composition,

And a medium for plant growth,

Making it a critically important provider of ecosystem services.

Since soil has a tremendous range of available niches and habitats,

It contains a prominent part of the Earth's genetic diversity.

A gram of soil can contain billions of organisms,

Belonging to thousands of species,

Mostly microbial and largely still unexplored.

Soil has a mean prokaryotic density of roughly 10 to the 8th organisms per gram,

Whereas the ocean has no more than 10 to the 7th prokaryotic organisms per milliliter of seawater.

Organic carbon held in soil is eventually returned to the atmosphere through the process of respiration carried out by the heterotrophic organisms,

But a substantial part is retained in the soil in the form of soil organic matter.

Tillage usually increases the rate of soil respiration,

Leading to the depletion of soil organic matter.

Since plant roots need oxygen,

Aeration is an important characteristic of soil.

This ventilation can be accomplished via networks of interconnected soil pores,

Which also absorb and hold rainwater,

Making it readily available for uptake by plants.

Since plants require a nearly continuous supply of water,

But most regions receive sporadic rainfall,

The water-holding capacity of soils is vital for plant survival.

Soils can effectively remove impurities,

Kill disease agents,

And degrade contaminants,

This latter property being called natural attenuation.

Typically,

Soils maintain a net absorption of oxygen and methane,

And undergo a net release of carbon dioxide and nitrous oxide.

Soils offer plants physical support,

Air,

Water,

Temperature moderation,

Nutrients,

And protection from toxins.

Soils provide readily available nutrients to plants and animals by converting dead organic matter into various nutrient forms.

A typical soil is about 50% solids,

45% mineral and 5% organic matter,

And 50% voids or pores,

Of which half is occupied by water and half by gas.

The percent soil mineral and organic content can be treated as a constant in the short term,

While the percent soil water and gas content is considered highly variable,

Whereby a rise in one is simultaneously balanced by a reduction in the other.

The pore space allows for the infiltration and movement of air and water,

Both of which are critical for life existing in soil.

Compaction,

A common problem with soils,

Reduces this space,

Preventing air and water from reaching plant roots and soil organisms.

Given sufficient time,

An undifferentiated soil will evolve a soil profile that consists of two or more layers,

Referred to as soil horizons.

These differ in one or more properties such as in their texture,

Structure,

Density,

Porosity,

Consistency,

Temperature,

Color,

And reactivity.

The horizons differ greatly in thickness and generally lack sharp boundaries.

Their development is dependent on the type of parent material,

The processes that modify those parent materials,

And the soil-forming factors that influence those processes.

The biological influences in soil properties are strongest near the surface,

Though the geochemical influences on soil properties increase with depth.

Mature soil profiles typically include three basic matter horizons,

A,

B,

And C.

The solum normally includes the A and B horizons.

The living components of the soil is largely confined to the solum and is generally more prominent in the A horizon.

It has been suggested that the pedon,

A column of soil extending vertically from the surface to the underlying parent material,

And large enough to show the characteristics of all its horizons,

Could be subdivided into the humipedon,

The living part where most soil organisms are dwelling,

Corresponding to the humus form,

The coppedon,

An intermediary position where most weathering of minerals takes place,

And the lithopedon,

In contact with the subsoil.

The soil texture is determined by the relative proportions of the individual particles of sand,

Silt,

And clay that make up the soil.

The interaction of the individual mineral particles with organic matter,

Water,

Gases via biotic and abiotic processes,

Causes those particles to flocculate,

Stick together,

To form aggregates or peds.

Where these aggregates can be identified,

A soil can be said to be developed and can be described further in terms of color,

Porosity,

Consistency,

Reaction,

Acidity,

Etc.

Water is a critical agent in soil development due to its involvement in the dissolution,

Precipitation,

Erosion,

Transport,

And deposition of the materials of which a soil is composed.

The mixture of water and dissolved or suspended materials that occupy the soil pore space is called the soil solution.

Since soil water is never pure water,

But contains hundreds of dissolved organic and mineral substances,

It may be more accurately called the soil solution.

Water is critical to the dissolution,

Precipitation,

And leaching of minerals from the soil profile.

Finally,

Water affects the type of vegetation that grows in a soil,

Which in turn affects the development of the soil,

A complex feedback which is exemplified in the dynamics of banded vegetation patterns in semi-arid regions.

Minerals supply plants with nutrients,

Most of which are held in place by particles of clay and organic matter,

Colloids.

The nutrients may be absorbed on clay mineral surfaces,

Bound with clay minerals,

Absorbed,

Or bound within organic compounds as part of the living organisms or dead soil organic matter.

These bound nutrients interact with soil water to buffer the soil solution composition,

Attenuate changes in the soil solution,

As soils wet up or dry out,

As plants take up nutrients,

As salts are leached,

Or as acids or alkalis are added.

Plant nutrient availability is affected by the soil pH,

Which is a measure of the hydrogen ion activity in the soil solution.

Soil pH is a function of many soil forming factors and is generally lower,

More acidic,

Where weathering is more advanced.

Most plant nutrients,

With the exception of nitrogen,

Originate from the minerals that make up the soil parent material.

Some nitrogen originates from rain,

As dilute nitric acid and ammonia.

But most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria.

Once in the soil plant system,

Most nutrients are recycled through living organisms,

Plant and microbial residues,

Soil organic matter,

Mineral bound forms,

And the soil solution.

Both living soil organisms,

Microbes,

Animals,

And plant roots,

And soil organic matter are of critical importance to this recycling,

And thereby to soil formation and soil fertility.

Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms,

Sequester,

Incorporate them into living cells,

Or cause their loss from the soil by volatilization,

Loss to the atmosphere's gases,

Or leaching.

Soil is said to be formed when organic matter has accumulated and colloids are washed downward,

Leaving deposits of clay,

Humus,

Iron oxide,

Carbonate,

And gypsum,

Producing a distinct layer called the B horizon.

This is a somewhat arbitrary definition as mixtures of sand,

Silt,

Clay,

And humus will support biological and agricultural activity before that time.

These constituents are moved from one level to another by water and animal activity.

As a result,

Layers,

Horizons,

Form in the soil profile.

The alteration and movement of materials within a soil causes the formation of distinctive soil horizons.

However,

More recent definitions of soil embrace soils without any organic matter,

Such as those regoliths that formed on Mars and analogous conditions in planet Earth's deserts.

An example of the development of a soil would begin with the weathering of lava flow bedrock,

Which would produce the purely mineral-based parent material from which the soil texture forms.

Mineral development would proceed most rapidly from bare rock of recent flows in a warm climate,

Under heavy and frequent rainfall.

Under such conditions,

Plants in a first-stage nitrogen-fixing lichens and cyanobacteria than epilithic higher plants become established very quickly on basaltic lava,

Even though there is very little organic material.

Epilithic minerals commonly weather relatively quickly,

According to the Goldich dissolution series.

The plants are supported by the porous rock as it is filled with nutrient-bearing water that carries minerals dissolved from the rocks.

Crevices and pockets,

Local topography of the rocks,

Would hold fine materials and harbor plant roots.

The developing plant roots are associated with mineral-weathering mycorrhizal fungi that assist in breaking up the porous lava,

And by these means organic matter and a finer mineral soil accumulate with time.

Such initial stages of soil development have been described on volcanoes,

Inselbergs,

And glacial moraines.

How soil formation proceeds is influenced by at least five classic factors that are intertwined in the evolution of a soil.

Parent material,

Climate,

Topography,

Relief,

Organisms,

And time.

When reordered to climate,

Relief,

Organisms,

Parent material,

And time,

They form the acronym CROPT.

The physical properties of soils,

In order of decreasing importance for ecosystem services such as crop production,

Are texture,

Structure,

Bulk density,

Porosity,

Consistency,

Temperature,

Color,

And resistivity.

Soil texture is determined by the relative proportion of the three kinds of soil mineral particles,

Called soil separates,

Sand,

Silt,

And clay.

At the next larger scale,

Soil structures called PEDS,

Or more commonly soil aggregates,

Are created from the soil separates when iron oxides,

Carbonates,

Clay,

Silica,

And humus coat particles and cause them to adhere into larger,

Relatively stable secondary structures.

Soil bulk density,

When determined at standardized moisture conditions,

Is an estimate of soil compaction.

Soil porosity consists of the void part of the soil volume,

And is occupied by gases or water.

Soil consistency is the ability of soil materials to stick together.

Soil temperature and color are self-defining.

Resistivity refers to the resistance to conduction of electric currents,

And affects the rate of corrosion of metal and concrete structures which are buried in soil.

These properties vary through the depth of a soil profile,

I.

E.

Through soil horizons.

Most of these properties determine the aeration of the soil,

And the ability of water to infiltrate and be held within the soil.

Soil water content can be measured as volume or weight.

Soil moisture levels,

In order of decreasing water content,

Are saturation,

Field capacity,

Wilting point,

Air dry,

And oven dry.

Field capacity describes a drained wet soil at the point water content reaches equilibrium with gravity.

Irrigating soil above field capacity risks percolation losses.

Wilting point describes the dry limit for growing plants.

During growing season,

Soil moisture is unaffected by functional groups or species richness.

Soil water capacity is the amount of water held in a soil profile available to plants.

As water content drops,

Plants have to work against increasing forces of adhesion and sorptivity to withdraw water.

Irrigation scheduling avoids moisture stress by replenishing depleted water before stress is induced.

Capillary action is responsible for moving groundwater from wet regions of the soil to dry areas.

Irrigation designs,

E.

G.

Wicking beds,

Sub-irrigated planters,

Rely on capillarity to supply water to plant roots.

Capillary action can result in an evaporative concentration of salts,

Causing land degradation through salination.

Soil moisture measurement,

Measuring the water content of the soil,

As can be expressed in terms of volume or weight,

Can be based on in-situ probes,

E.

G.

Capacitance probes,

Neutron probes,

Or remote sensing methods.

Soil moisture measurement is an important factor in determining changes in soil activity.

The atmosphere of soil,

Or soil gas,

Is very different from the atmosphere above.

The consumption of oxygen by microbes and plant roots,

And their release of CO2,

Decreases oxygen and increases CO2 concentration.

Atmospheric CO2 concentration is 0.

04%,

But in the soil pore space it may range from 10 to 100 times that level,

Thus potentially contributing to the inhibition of root respiration.

Calcareous soils regulate CO2 concentration by carbonate buffering,

Contrary to acid soils in which all CO2 respired accumulates in the soil pore system.

At extreme levels,

CO2 is toxic.

This suggests a possible negative feedback control of soil CO2 concentration through its inhibitory effects on root and microbial respiration,

Also called soil respiration.

In addition,

The soil voids are saturated with water vapor,

At least until the point of maximal hygroscopicity,

Beyond which a vapor pressure deficit occurs in the soil pore space.

Adequate porosity is necessary,

Not just to allow the penetration of water,

But also to allow gases to diffuse in and out.

Movement of gases is by diffusion from high concentrations to lower,

The diffusion coefficient decreasing with soil compaction.

Oxygen from above atmosphere diffuses in the soil where it is consumed,

And levels of carbon dioxide in excess of above atmosphere diffuse out with other gases,

Including greenhouse gases as well as water.

Soil texture and structures strongly affect soil porosity and gas diffusion.

It is the total pore space,

Porosity,

Of soil,

Not the pore size,

And the degree of pore interconnection,

Or conversely pore sealing,

Together with water content,

Air turbulence,

And temperature,

That determine the rate of diffusion of gases into and out of soil.

Platey soil structure and soil compaction,

Low porosity,

Impede gas flow,

And a deficiency of oxygen may encourage anaerobic bacteria to reduce,

Strip oxygen from nitrate NO3 to the gases N2,

N2O,

And NO,

Which are then lost to the atmosphere,

Thereby depleting the soil of nitrogen,

A detrimental process called denitrification.

Aerated soil is also a net sink of methane,

CH4,

But a net producer of methane,

A strong heat-absorbing greenhouse gas,

When soils are depleted of oxygen and subject to elevated temperatures.

Soil atmosphere is also the seed of emissions of volatiles other than carbon and nitrogen oxides from various soil organisms,

E.

G.

Roots,

Bacteria,

Fungi,

Animals.

These volatiles are used as chemical cues,

Making soil atmosphere the seed of interaction networks,

Playing a decisive role in the stability,

Dynamics,

And evolution of soil ecosystems.

Biogenetic soil volatile organic compounds are exchanged with the above-ground atmosphere,

In which they are just one to two orders of magnitude lower than those from above-ground vegetation.

Humans can get some idea of the soil atmosphere through the well-known after-the-rain scent,

When infiltrating rainwater flushes out the whole soil atmosphere after a drought period or when soil is excavated,

A bulk property attributed in a reductionist manner to particular biochemical compounds such as petrichor or jasmine.

Soil particles can be classified by their chemical composition,

Mineralogy,

As well as their size.

The particle size distribution of a soil,

Its texture,

Determines many of the properties of that soil,

In particular hydraulic conductivity and water potential,

But the mineralogy of those particles can strongly modify those properties.

The mineralogy of the finest soil particles,

Clay,

Is especially important.

Large numbers of microbes,

Animals,

Plants,

And fungi are living in soil.

However,

Biodiversity in soil is much harder to study as most of this life is invisible,

Hence estimates about soil biodiversity have been unsatisfactory.

A recent study suggested that soil is likely home to 59 plus or minus 15 percent of the species on earth.

Enchitreidae,

Worms,

Have the greatest percentage of species in soil,

98.

6 percent,

Followed by fungi,

90 percent,

Plants,

85.

5 percent,

And termites,

Isoptera,

84.

2 percent.

Many other groups of animals have substantial fractions of species living in soil,

E.

G.

About 30 percent of insects,

And close to 50 percent of arachnids.

While most vertebrates live above ground,

Ignoring aquatic species,

Many species are fossorial,

That is,

They live in soil,

Such as most blind snakes.

The chemistry of a soil determines its ability to supply available plant nutrients and affects its physical properties and the health of its living population.

In addition,

A soil's chemistry also determines its corrosivity,

Stability,

And ability to absorb pollutants and to filter water.

It is the surface chemistry of mineral and organic colloids that determines soil's chemical properties.

A colloid is a small,

Insoluble particle ranging in size from 1 nm to 1 micrometer,

Thus small enough to remain suspended by Brownian motion in a fluid medium without settling.

Most soils contain organic colloidal particles called humus,

As well as the inorganic colloidal particles of clay.

The very high specific surface area of colloids and their net electrical charges give soil its ability to hold and release ions.

Negatively charged sites on colloids attract and release cations in what is referred to as a cation exchange.

Cation exchange capacity is the amount of exchangeable cations per unit weight of dry soil and is expressed in terms of milliequivalents of positively charged ions per 100 grams of soil,

Or centimoles of positive charge per kilogram of soil.

Similarly,

Positively charged sites on colloids can attract and release anions in the soil,

Given the soil anion exchange capacity.

The cation exchange that takes place between colloids and soil water buffers or moderates soil pH,

Alters soil structure,

And purifies percolating water by absorbing cations of all types,

Both useful and harmful.

The negative or positive charges on colloid particles make them able to hold cations or anions respectively to their surfaces.

The charges result from four sources.

1.

Isomorphous substitution occurs in clay during its formation,

When lower valence cations substitute for higher valence cations in the crystal structure.

Substitutions in the outermost layers are more effective than for the innermost layers,

As the electric charge strength drops off as the square of the distance.

The net result is oxygen atoms with net negative charge and the ability to attract cations.

2.

Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete.

3.

Hydroxyls may substitute for oxygens of the silica layers,

A process called hydroxylation.

When the hydrogens of the clay hydroxyls are ionized into solution,

They leave the oxygen with a negative charge,

Anionic clays.

4.

Hydrogens of humus hydroxyl groups may also be ionized into solution,

Leaving similarly to clay an oxygen with a negative charge.

Cations held to the negatively charged colloids resist being washed downward by water and are out of reach of plant roots,

Thereby preserving the soil fertility in areas of moderate rainfall and low temperatures.

There is a hierarchy in the process of cation exchange on colloids,

As cations differ in the strength of absorption by the colloid and hence their ability to replace one another ion exchange.

If present in equal amounts in the soil-water solution,

Al3+,

Replaces H+,

Replaces Ca2+,

Replaces Mg2+,

Replaces K+,

Same as NH+,

4,

Replaces Na+.

If one cation is added in large amounts,

It may replace the others by the sheer force of its numbers.

This is called law of mass action.

This is largely what occurs with the addition of cationic fertilizers,

Potash or lime.

As the soil solution becomes more acidic,

Low pH,

Meaning an abundance of H+,

The other cations more weakly bound to colloids are pushed into solution as hydrogen ions occupy exchange sites,

Protonation.

A low pH may cause the hydrogen of hydroxyl groups to be pulled into solution,

Leaving charge sites on the colloid available to the occupied by other cations.

This ionization of hydroxy groups on the surface of soil colloids creates what is described as pH-dependent surface charges.

Unlike permanent charges developed by isomorphous substitution,

PH-dependent charges are variable and increase with increasing pH.

Freed cations can be made available to plants,

But are also prone to be leached from the soil,

Possibly making the soil less fertile.

Plants are able to excrete H+,

Into the soil through the synthesis of organic acids,

And by that means change the pH of the soil near the root and push cations off the colloids,

Thus making those available to the plant.

Cation exchange capacity is the soil's ability to remove cations from the soil-water solution and sequester those to be exchanged later as the plant roots release hydrogen ions to the solution.

CEC is the amount of exchangeable hydrogen cation,

H+,

That will combine with 100 g dry weight of soil and whose measure is 1 mEq per 100 g of soil.

Hydrogen ions have a single charge,

And 1,

000th of a gram of hydrogen ions per 100 g dry soil gives a measure of 1 mEq of hydrogen ion.

Calcium with an atomic weight 40 times that of hydrogen,

And with a valence of 2,

Converts to 40 divided by 2 times 1 mEq equaling 20 mEq of hydrogen ion per 100 g of dry soil,

Or 20 mEq divided by 100 g.

The modern measure of CEC is expressed as centimoles of positive charge per kilogram of oven-dry soil.

Most of the soil's CEC occurs on clay and humus colloids,

And the lack of those in hot,

Humid,

Wet climates,

Such as tropical rainforests,

Due to leaching and decomposition respectively,

Explains the apparent sterility of tropical soils.

Live plant roots also have some CEC linked to their specific surface area.

An exchange capacity is the soil's ability to remove anions,

Such as nitrate,

Phosphate,

From the soil-water solution and sequester those for later exchange as the plant roots release carbonate anions to the soil-water solution.

Those colloids which have low CEC tend to have some AEC.

Amorphous and sesquioxide clays have the highest AEC,

Followed by the iron oxides.

Levels of AEC are much lower than for CEC,

Because of the generally higher rate of positively versus negatively charged surfaces on soil colloids,

To the exception of variable-charged soils.

Phosphates tend to be held at anion exchange sites.

Iron and aluminum hydroxide clays are able to exchange their hydroxide anions for other anions.

The amount of exchangeable anions is of a magnitude of tenths to a few milliequivalents per 100 grams dry soil.

As pH rises,

There are relatively more hydroxyls,

Which will displace anions from the colloids and force them into solution and out of storage.

Hence,

AEC decreases the increasing pH alkalinity.

Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil.

More precisely,

It is a measure of hydronium concentration in an aqueous solution and ranges in values from 0 to 14,

Acidic to basic.

But practically speaking for soils,

PH ranges from 3.

5 to 9.

5,

As pH values beyond those extremes are toxic to lifeforms.

At 25 degrees Celsius,

An aqueous solution that has a pH of 3.

5 has 10 to the negative 3.

5 moles,

H3O+,

Per liter of solution.

A pH of 7,

Defined as neutral,

Has 10 to the negative 7 moles of H2O- per liter of solution and also 10 to the negative 7 moles of OH- per liter.

Since the two concentrations are equal,

They are said to neutralize each other.

A pH of 9.

5 has 10 to the negative 9.

5 moles,

H2O- per liter of solution.

A pH of 3.

5 has 1 million times more H2O- per liter than a solution with pH of 9.

5 and is more acidic.

The effect of pH on a soil is to remove from the soil or to take available certain ions.

Soils with high acidity tend to have toxic amounts of aluminum and manganese.

As a result of a trade-off between toxicity and requirement,

Most nutrients are better available to plants at moderate pH,

Although most minerals are more soluble in acid soils.

Soil organisms are hindered by high acidity,

And most agricultural crops do best with mineral soils of pH 6.

5 and organic soils of pH 5.

5.

Given that at low pH,

Toxic metals are positively charged as cations and organic pollutants are in non-ionic form,

Thus both made more available to organisms,

It has been suggested that plants,

Animals,

And microbes commonly living in acid soils are pre-adapted to every kind of pollution,

Whether of natural or human origin.

In high rainfall areas,

Soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydronium ions from usual or unusual rain acidity against those attached to the colloids.

High rainfall rates can then wash the nutrients out,

Leaving the soil inhabited only by those organisms which are particularly efficient to uptake nutrients in very acidic conditions,

Like in tropical rainforests.

Once the colloids are saturated with H3O+,

The addition of any more hydronium ions or aluminum hydroxyl cations drives the pH even lower,

More acidic,

As the soil has been left with no buffering capacity.

In areas of extreme rainfall and high temperatures,

The clay and humus may be washed out,

Further reducing the buffering capacity of the soil.

In low rainfall areas,

Unleached calcium pushes pH to 8.

5,

And with the addition of exchangeable sodium,

Soils may reach pH 10.

Beyond a pH of 9,

Plant growth is reduced.

High pH results in low micronutrient mobility,

But water-soluble chelates of those nutrients can correct the deficit.

Sodium can be reduced by the addition of gypsum,

Calcium sulfate,

As calcium adheres to clay more tightly than does sodium,

Causing sodium to be pushed into the soil water solution where it can be washed out by an abundance of water.

There are acid-forming cations,

E.

G.

Hydronium,

Aluminum,

Iron,

And there are base-forming cations,

E.

G.

Calcium,

Magnesium,

Sodium.

The fraction of the negatively charged soil colloid exchange sites,

CEC,

That are occupied by base-forming cations is called base saturation.

If a soil has a CEC of 20 mEq and 5 mEq are aluminum and hydronium cations,

Acid-forming,

The remainder of positions on the colloids,

20-5 equals 15 mEq,

Are assumed occupied by base-forming cations,

So that the base saturation is 15 ÷ 20 × 100% equals 75%.

The complement 25% is assumed acid-forming cations.

Base saturation is almost in direct proportion to pH.

It increases with increasing pH.

It is of use in calculating the amount of lime needed to neutralize an acid soil lime requirement.

The amount of lime needed to neutralize a soil must take account of the amount of acid-forming ions on the colloids,

Exchangeable acidity,

Not just those in the soil-water solution,

Free acidity.

The addition of enough lime to neutralize a soil-water solution will be insufficient to change the pH,

As the acid-forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime.

The resistance of soil to change in pH as a result of the addition of acid or basic material is a measure of the buffering capacity of a soil,

And for a particular soil type,

Increases as the CEC increases.

Hence,

Pure sand has almost no buffering ability.

Soils high on colloids,

Whether mineral or organic,

Have high buffering capacity.

Buffering occurs by cation exchange and neutralization.

However,

Colloids are not the only regulators of soil pH.

The role of carbonates should be underlined too.

More generally,

According to pH levels,

Several buffer systems take precedence over each other,

From calcium carbonate buffer range to iron buffer range.

The addition of a small amount of highly basic aqueous ammonia to a soil will cause the ammonium to displace hydronium ions from the colloids,

And the end product is water and colloidally fixed ammonium,

But little permanent change overall in soil pH.

The addition of a small amount of lime,

CaO2,

Will displace hydronium ions from the soil colloids,

Causing the fixation of calcium to colloids and the evolution of CO2 in water,

With little permanent change in soil pH.

The general principle is that an increase in a particular cation in the soil water solution will cause the cation to be fixed to colloids,

Buffered,

And a decrease in solution of that cation will cause it to be withdrawn from the colloid and moved into solution,

Buffered.

The degree of buffering is often related to the CEC of the soil.

The greater the CEC,

The greater the buffering capacity of the soil.

Soil chemical reactions involve some combination of proton and electron transfer.

Oxidation occurs if there is a loss of electrons in the transfer process,

While reduction occurs if there is a gain of electrons.

Reduction potential is measured in volts or millivolts.

Soil microbial communities develop along electron transport chains,

Forming electrically conductive biofilms,

And developing networks of bacterial nanowires.

Redox factors in soil development,

Where formation of redoxomorphic color features,

Provides critical information for soil interpretation.

Understanding the redox gradient is important to managing carbon sequestration,

Bioremediation,

Wetland delineation,

And soil-based microbial fuel cells.

Reviews

I really like how they differentiate between soil and dirt. If that's not boring, I don't know what is. 😴😴