Soil Fertility Exam 1 Pt. 2

What are the characteristics of Ultisols?

Strongly leached, acid forest soils, low native fertility, found in humid temperate and tropical areas.

What happens to primary minerals in Ultisols?

They are leached (Ca, Mg, K).

What is present in the subsurface of Ultisols?

Accumulation of clay.

Can Ultisols support productive forests?

Yes, in favorable climates.

What do Ultisols require for continuous agriculture?

Fertilizer and lime (high acidity, low Ca, Mg, K).

Where are Ultisols dominant?

Southeastern US.

What two features do Ultisols have?

(1) Subsurface zone of clay accumulation (argillic or kandic horizon); (2) Base saturation < 35% in subsoil.

What are the characteristics of Alfisols?

Moderately leached, relatively high native fertility, formed under forest vegetation, found in temperate humid and subhumid regions.

Are Alfisols productive soils?

Yes, favorable climate and high native fertility.

What two features do Alfisols have?

(1) Subsurface zone of clay accumulation (argillic, kandic, or natric horizon); (2) Base saturation ≥ 35% in subsoil.

What do Inceptisols lack?

Features characteristic of other soil orders.

Where are Inceptisols found?

Steep slopes, young geomorphic surfaces, resistant parent materials, mountainous areas.

What are Inceptisols used for?

Forestry, recreation, watershed.

How developed are Inceptisols?

Range from weakly developed to having various diagnostic horizons/features, but not meeting criteria of other orders.

What ecosystem do Mollisols represent?

Grassland ecosystem.

How do Mollisols get their organic matter?

Long-term addition of OM from plant roots.

How important are Mollisols agriculturally?

Among the world’s most important and productive agricultural soils.

Where are Mollisols extensive?

Prairie regions (US Great Plains, Pampas of South America, Ukraine).

What two features do Mollisols have?

(1) Mollic epipedon (thick, dark surface horizon); (2) Base saturation ≥ 50% throughout subsoil.

What are Suborders?

Soil orders divided into suborders based on properties that influence soil formation or are important to plant growth.

What is an ion?

An atom, group of atoms, or compound that is electrically charged due to the loss or gain of electrons.

Which common cations are NOT plant-essential nutrients?

Aluminum and Sodium.

What is ion exchange?

Adsorption and desorption of ions on the surface of clay minerals (and OM) with another ion in the soil solution.

What does ion exchange affect?

Plant nutrient availability and soil nutrient retention.

What are the two types of exchange capacity?

Cation Exchange Capacity (CEC) and Anion Exchange Capacity (AEC).

Which is dominant in most agricultural soils?

CEC.

What governs exchange capacity?

Surface charges of soil minerals and OM.

What is Cation Exchange?

Interchange between a cation in solution and another cation on the exchange site of any active material (clay or OM).

What is CEC?

The sum total of exchangeable cations a soil can adsorb.

How is CEC measured?

In meq/100g (old unit) or cmolc/kg.

What is a meq?

Milliequivalent.

What is an equivalent?

The amount of an ion that supplies or balances one mole of electrical charge.

How many equivalents are in 1 mol of K+?

1 equivalent.

How many equivalents are in 1 mol of Ca2+?

2 equivalents.

How many equivalents are in 1 mol of Al3+?

3 equivalents.

How do you convert meq/100g to cmolc/kg?

1 meq/100g = 1 cmolc/kg.

What does a CEC of 6 cmolc/kg mean?

The soil can retain 6 centimoles of positive charge per kilogram of soil.

How do equivalents relate to each other?

K+ = 1/2 Ca2+ = 1/3 Al3+ (they all satisfy 1 negative charge).

What does AEC involve?

Exchange sites hold anions (e.g., NO3-, SO4-).

Is AEC similar to CEC?

Yes, similar processes.

What are the two sources of CEC?

Permanent charge and pH-dependent charge.

What causes permanent charge?

Cation substitution in clay layers (isomorphous substitution).

What causes pH-dependent charge?

Broken bonds and hydrogen ionizing.

What happens during weathering?

Water penetrates rock, breaking it down into smaller units composed of Si and O (building blocks of clay).

What is the Si4+ layer?

Tetrahedral (four-sided) structure.

Why do Si-O structures form?

Si-O remains unbalanced; Si shares O with other Si to create sheets.

What is the Al+3 layer?

Octahedral (eight-sided) structure.

Why do Al-O structures form?

Al+3 shares positive charge with O-2; also unbalanced, creating sheets.

How are tetrahedral and octahedral sheets connected?

They share oxygen to create balance, forming clay minerals.

Can there be more than one tetrahedral layer?

Yes.

What does isomorphous substitution result in?

An overall negative charge.

Does substitution change the soil mineral structure?

No.

What is the formula for Kaolinite?

Si4Al4O10(OH)8.

What is the Si:Al ratio in Kaolinite?

1:1.

Does Kaolinite have isomorphous substitution?

No.

Does Kaolinite swell?

No, it is non-swelling.

Why does Kaolinite have no CEC?

H bonds are strongly held together.

What are the layers in a 2:1 clay?

Tetrahedral, octahedral, tetrahedral sheets.

What is Pyrophyllite

Al2Si4O10(OH)2; no isomorphous substitution, not common in soil.

What is Smectite?

Al2-xMgx)Si4O10(OH)2; Si:Al = 2:1.

Does Smectite have isomorphous substitution?

Yes, in the octahedral layer.

Does Smectite swell?

Yes.

What is the net charge in Smectite with substitution?

Overall -0.4 charge.

What is Montmorillonite?

The most prominent smectite in soils.

What happens in Montmorillonite?

Al is substituted by Mg in the octahedral layer.

What are the characteristics of Montmorillonite?

High CEC, charges closer to the inner surface, adsorption of water between layers, significant swelling when wet.

Where does substitution occur in Vermiculite?

In both tetrahedral (Al or Si) and octahedral layers.

What is the CEC of Vermiculite?

Very high.

Where are the negative charges located?

Closer to the outside of the mineral.

How do interlayer cations act in Vermiculite?

As bridges holding units together, limiting swelling and shrinkage.

Where does substitution occur in Illite?

In the tetrahedral layer (Al for Si).

What fits into the hexagonal cavities between layers in Illite?

K+ (potassium).

Is K+ in Illite exchangeable?

No, it is non-exchangeable (but slowly available for plant uptake).

Is Illite an expanding clay?

No (unless K+ is removed).

What happens when K+ cations are removed from Illite?

It can become an expanding clay (turning into vermiculite).

Which clay group has the highest CEC?

Vermiculite.

How does Vermiculite develop?

Weathering of mica removes some interlayer K+, allowing hydrated exchangeable cations to enter.

What is the key behavior of Vermiculite?

Very high layer charge and CEC.

What ions does Vermiculite have a strong affinity for?

K+ and NH4+.

How does expansion in Vermiculite compare to Smectite?

Often limited expansion compared with smectite.

What can reduce effective CEC in Vermiculite?

Hydroxy interlayers.

What is the management implication for Vermiculite?

Applied K+ and NH4+ fertilizers can be fixed between layers, especially when drying.

What is isomorphous substitution?

Cations fit in with no change in soil mineral structure.

Does isomorphous substitution occur in 1:1 clays?

Rarely.

Does isomorphous substitution occur in 2:1 clays?

Yes.

What substitutions occur in isomorphous substitution?

Si substituted by Al in tetrahedral; Al substituted by Mg (or Fe) in octahedral.

Is all substitution part of CEC?

Only if exchangeable.

Can internally held cations become available?

Yes, through weathering.

What causes broken edge bonds?

Where mineral lattice structure terminates, ions are left with unsatisfied or + charges.

Why are broken edge bonds significant in clay but not silt/sand?

Clay has large surface area; silt/sand surface area is too small.

What is the ionization of H?

Carboxyl groups (-COOH) lose H+, leaving an exchange site.

Where is pH-dependent charge important?

In OM, 1:1 clays, and Fe/Al oxides.

Where does most OM CEC come from?

pH-dependent charge.

What is Gibbsite?

An aluminum oxide clay common in highly weathered soils.

What is Gibbsite composed of?

Two octahedral sheets hydrogen bonded together.

What other oxide-type clays exist?

Geothite (iron instead of aluminum in the octahedral position).

Which clays have the highest CEC?

Silicate clays.

Which clays have the greatest AEC and abundance of positive charges?

Oxide clays (gibbsite and geothite).

How does weathering affect AEC?

Greater weathering leads to greater AEC.

How does pH affect the balance between CEC and AEC?

pH shifts the balance.