Soil Fertility Exam 1 Pt. 3

What are the major controls of cation selectivity?

Concentration, charge and hydration, exchange site composition, and clay mineralogy.

How does concentration affect exchange?

More of a replacing ion drives exchange through mass action.

How do charge and hydration affect retention?

Higher-charge and less strongly hydrated ions are retained more strongly (Lyotropic series).

How does exchange site composition affect replaceability?

Replaceability changes with the ions and saturation levels already present.

How does clay mineralogy affect selectivity?

Different minerals show different selectivity patterns.

Is exchange reversible?

Yes, but cations are not retained equally.

What does the Lyotropic series represent?

The strength of cation adsorption.

What is the Lyotropic series based on?

Charge density.

How does charge density change?

Increases with charge, decreases with increasing size.

What is the consequence of stronger adsorption of H+ and Al3+ than other cations?

Leaches out sodium, potassium, etc., leaving behind Al3+ and H+.

What does heavy leaching leave in the soil?

Al and H (non-essential nutrients that can be toxic in high concentrations).

What type of soil environment does this create?

A very acidic environment.

Which ions form inner sphere complexes?

Phosphates, arsenates, molybdates, sulfates.

How do they react(ions)?

They react with the particle surface.

What is the effect on net positive charge?

Reduces net + charge in soil.

How tightly are inner sphere complexes held?

Very tightly, not readily available.

Which anions are retained more strongly?

Phosphate, arsenate, fluoride.

Which anions are retained more weakly?

Nitrate and chloride.

What is the management implication of AEC?

AEC influences phosphorus availability and the mobility of nitrate and contaminants.

Where is AEC important?

Highly weathered acidic soils, Fe and Al oxide-rich surfaces, variable-charge minerals at low pH.

What are the three ways to determine CEC?

1. Sum exchangeable cations, 2. Displace and measure, 3. Estimate from soil properties.

What must be reported with a CEC value?

The extraction method and soil pH conditions.

How do you calculate CEC using the summation method?

Add Ca2+, Mg2+, K+, Na+, exchangeable acidity (H+), and Al3+ from soil test reports.

Example: If K+=0.2, Mg2+=2.0, Ca2+=8.0, Na+=0.3, H+=1.0, Al3+=1.0, what is the CEC?

12.5 cmolc kg^-1.

What is important before adding components in the summation method?

Keep units consistent.

How does the displacement method measure CEC?

Leach soil with NH4 acetate (pH 7) to saturate sites, remove excess with alcohol, then displace NH4 with Na or K and titrate to find CEC.

How is CEC estimated?

Use clay amount, clay mineralogy, organic matter, and a calibrated equation.

Does texture alone determine CEC?

No. Clay type, organic matter, and measurement pH can shift the value.

Does high CEC always mean high fertility?

No. Exchange sites may be occupied with H, Al, or other non-nutrient cations.

How are soil cations separated?

Into basic and acidic cations.

What is Base Saturation?

The percentage of CEC occupied by basic cations.

What does % acidity represent?

The percentage of CEC occupied by acidic cations.

How does high BS affect soil pH?

Higher soil pH results from higher BS.

Why are soils with high BS generally more fertile?

They contain greater amounts of essential plant nutrient cations (K+, Ca2+, Mg2+).

What toxic cation is absent in high BS soils?

Al3+ (aluminum).

How does high BS buffer the soil?

More buffered against acid cations from plant roots and soil processes (nitrification, acid rain, etc.).

What happens to Al3+ at lower soil pH?

Greater amounts of toxic Al3+ are present.

At what pH is Al3+ present in significantly high concentrations?

When soil pH < 5.4.

How does Al3+ affect plants?

Hinders growth of most plant species.

How is % Base Saturation calculated?

%BS = [(Total of basic cations) / CEC] x 100.

What is the formula for pH?

pH = -log[H+].

What does [H+] represent?

mol/liter, or M.

How much does one pH unit represent?

A tenfold change.

What is the pH of a solution with 0.01 g of H per 1000 liters?

pH 5. (0.01 mol / 1000 L = 0.00001 M; -log(0.00001) = 5).

What is the pH of a 0.1 M HCl solution?

1.

What fraction of global soils are acidic?

About 1/3.

Where are acid soils found in the US?

SE and NW regions.

Where are acid soils found globally?

A northern and southern belt.

In what climate do acid soils form?

Where precipitation is greater than 24 inches (60 cm) per year.

What is the pH of H2O in equilibrium with atmospheric CO2?

~5.7.

What is the chemical reaction for CO2 in water?

H2O + CO2 ⇄ H2CO3 ⇄ H+ + HCO3-.

Why do areas with greater precipitation result in more acidic soils?

More precipitation leads to more leaching and acidification via this reaction.

What is Source 1 of soil acidity?

Formation of carbonic acid from CO2 (atmospheric CO2 dissolved in rainwater results in greater acidity in soil than in the atmosphere).

What is Source 2 of soil acidity?

Natural occurrence of many organic acids in soil (low molecular weight organic acids produced by roots and soil microbes, such as citric and malic acids).

What is Source 3 of soil acidity?

Oxidation of N, S, and Fe compounds.

Besides N oxidation, where else do you get NH4+ from regarding oxidation?

Sulfur and Iron oxidation.

What is Source 4 of soil acidity?

Acid Precipitation.

What produces sulfur and nitrogen gases for acid precipitation?

Lightning, volcanic eruptions, forest fires, and combustion of fossil fuels.

Where are the impacts of acid precipitation stronger?

Downwind from industrial areas.

How do plants uptake cations and anions regarding pH?

By altering pH to maintain electrical neutrality.

What happens when plants absorb cations?

They extrude H+ (or uptake anion) to maintain electrical neutrality.

What happens when plants absorb anions?

They extrude OH- or HCO3- (or uptake cation) to maintain electrical neutrality.

What happens when cation uptake exceeds anion uptake?

Excess H+ is released (acidifying the soil).

Do most plants uptake more cations or anions?

More cations.

What is the result of most plants uptaking more cations?

The soil is acidified.

Why do legumes add H+ to the soil?

Most N comes from N-fixation (little NO3- uptake), so H+ is added to balance charge.

Where does deprotonation of pH-dependent charges occur?

From edges of clay minerals, Fe and Al oxides, and OM.

What is the result of deprotonation?

A reduction in pH.

What happens to Al3+ concentration as pH decreases?

It increases due to the dissolution of Al(OH)3 (gibbsite).

What does Al hydrolysis generate?

H+ and lowers pH.

Why is Al considered an acid cation?

Its dissolution generates H+ and lowers pH.

How does Al buffer pH?

pH won’t increase until enough base is added to decrease soluble Al3+.

At what pH does Al(OH)3 precipitate?

pH 6.5 (decreasing Al3+ and increasing pH).

How does Al3+ on exchange sites buffer pH?

When neutralized with a base, Al3+ precipitates as Al(OH)3. Exchangeable Al3+ is released to replace Al3+ in solution, resulting in a gradual pH increase.

What is the reverse reaction of Al buffering?

Addition of acid neutralizes OH-, Al(OH)3 dissolves to release Al3+ into solution, resupplying exchangeable sites.

How do pH-dependent CEC sites buffer pH?

When a base is added, deprotonation occurs from pH-dependent sites.

What determines buffering capacity?

Clay mineral and OM content.

How does CEC relate to buffering capacity?

Higher CEC = greater buffering capacity.

If a sandy loam and a clay loam both have pH 5.0 and receive the same lime, which shows the larger pH increase?

The sandy soil (lower buffering capacity).

What is Active Acidity?

H+ activity in the soil solution; measured as soil pH; small pool; immediate root environment.

What is Exchangeable Acidity?

Readily displaced H+ and Al species held on clay and OM surfaces; often extracted with an unbuffered salt.

What is Residual Acidity?

Less readily neutralized solid-phase acidity associated with OM mineral edges and hydroxy-Al forms.

What is Total Acidity?

Active acidity + Potential acidity (Exchangeable + Residual).

How do carbonate/bicarbonate inputs consume H+?

Weathering of calcium carbonate (CaCO3) and bicarbonates consumes free protons.

How does anion protonation consume H+?

Functional groups on organic matter consume H+ during reverse association (RCOO- + H+ → RCOOH).

How do reduction reactions consume H+?

Biological reduction of N, S, and Fe compounds consumes H+.

How does mineral weathering consume H+?

Dissolution and weathering of primary silicate and non-silicate minerals consume protons and release base cations.

How does anion uptake by plants consume H+?

It removes acidity from the soil system.

What are two human causes of soil acidification?

Acid rain and nitrogen fertilization.

Are CaCO3 equivalents of N fertilizers usually accurate?

No, they usually overestimate acidity.

Why do CaCO3 equivalents overestimate acidity?

Root absorption of anion fertilizers (NO3-, SO42-, H2PO4-) neutralizes some acidity.

By how much is the acidity from N fertilizers usually reduced?

By 50%.

How does nitrate leaching accelerate soil acidification?

NO3- is a product of nitrification of NH4+ (from fertilizer, OM, manure). If all NO3- is absorbed, no H+ is produced. But crop recovery is incomplete, so leaching occurs.

Where is nitrate leaching greater?

Under legumes than unfertilized grass pastures.

What factors determine the net effect of crop growth on soil acidity?

Plant species, proportion of NO3- and NH4+ uptake, quantity of NO3- leached, total yield, and quantity of plant material harvested.

How does increasing biomass left in the field affect acidity?

It increases acidity produced by microbial degradation of crop residues.

How does soil type affect pH decline from fertilizer?

Shorter period of decline on sandy soil; longer period on clayey soil.

How does duration of fertilizer application affect pH?

Longer period of application = greater pH decrease.

What happens during liming regarding exchange sites?

Ca2+ replaces acids on exchange sites.

What happens to Al3+ and H+ during liming?

They leave exchange sites to replace acid neutralized in solution.