SSC Exam 2

These flashcards cover essential vocabulary and concepts related to soil science, including definitions and relationships important for understanding soil properties and nutrient management.

Soil Colloids

Extremely small soil particles (0.1–0.001 µm) with large surface area and electrical charge that adsorb water and ions.

Types of Soil Colloids

Inorganic (clay minerals, Fe/Al oxides) and organic (humus).

1:1 vs 2:1 Clays

1:1 clays (kaolinite) one tetra to one octa, are older, non-expanding; 2:1 clays (montmorillonite) two tetra to one octa, are younger, shrink–swell.

Humus

Stable, dark organic matter with high cation exchange capacity (CEC) and pH buffering ability.

• the more organic matter = darker soil color

• has positive and negatic ends

Cation Exchange Capacity (CEC)

Sum of total cations that a soil can adsorb, expressed in meq/100g soil.

Isomorphic Substitution

process in which one element substitutes another of comparable size in the crystalline structure, creates a permanent charge.

pH-Dependent Charge

Charge arising from broken edges or organic matter; increases with pH.

Effect of pH on CEC

As pH increases, CEC increases because more negative sites are available for cation adsorption.

Cation Adsorption Strength Order

Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ = NH⁴⁺ > Na⁺ > H⁺.

Soil Acidity

Soils with pH below 7.0; each 1 unit drop equals 10× more acidity.

Active Acidity

H⁺ and Al³⁺ ions in soil solution; measured by pH meter.

Exchangeable Acidity

H⁺ and Al³⁺ held on cation exchange sites of clays and organic matter negative sites.

Residual Acidity

H⁺ and Al³⁺ bound to soil particles and not exchangeable.

Buffering Capacity

Soil’s ability to resist changes in pH; increases with clay and organic matter content.

Liming

Process of adding calcium or magnesium carbonates to raise soil pH and reduce acidity.

Calcitic vs Dolomitic Lime

Calcitic lime = CaCO3; Dolomitic lime = CaMg(CO3)2 (adds Mg).

Fineness of Lime

Smaller particle size = faster reaction and greater surface area.

Essential Plant Nutrients

17 total; 14 from soil (N, P, K, Ca, Mg, S, Fe, Mn, B, Zn, Cu, Cl, Mo, Ni).

Liebig’s Law of the Minimum

Plant growth is limited by the most deficient nutrient, even if others are adequate.

Macronutrients

N, P, K (primary); Ca, Mg, S (secondary).

Micronutrients

Fe, Mn, B, Zn, Cu, Cl, Mo, Ni – required in trace amounts.

Nutrient Movement to Roots

Mass flow, diffusion, and root interception.

Mycorrhizae

Fungal associations that increase root surface area and nutrient uptake.

Nitrogen Forms

Nitrate (NO^₃–) and ammonium (NH^₄+).

Symbiotic Biological Nitrogen Fixation

Bacteria fix N₂ to plant-available forms for plants to use. In return, bacteria get

sugars.

• Microbes (Rhizobium) convert N₂ gas to plant-available NH₃ in legume root nodules.

Mineralization

Biological conversion of unavailable organic N into plant

available inorganic N.

• Conversion of organic N to inorganic N (NH4+, NO3–) by microbes.

Immobilization

Conversion of inorganic N into organic forms within microbial or plant tissue.

N Loss Pathways

Leaching, denitrification, volatilization, and plant uptake.

Ammonification

First step of mineralization: organic N → NH₄⁺.

• Conversion of organic N to ammonium by microorganisms.

• Need warm temperatures, good soil moisture, and oxygen supply.

• Makes the soil more basic

Nitrification

NH₄⁺ → NO₂^– → NO₃^– by Nitrosomonas and Nitrobacter; acidifies soil.

• Conversion of ammonium to nitrite and then to nitrate.

Phosphorus Availability

Controlled by pH and clay; binds to Fe/Al at low pH and to Ca at high pH.

Phosphorus Environmental Impact

Excess P causes eutrophication and algal blooms.

Potassium Cycle

K+ is held on exchange sites or trapped in clay interlayers; no major environmental issues.

CEC and pH Relationship

Higher pH increases CEC due to deprotonation of negative sites.

Calcium and Magnesium

Supplied by calcitic/dolomitic lime; important for cell walls and chlorophyll.

Sulfur in Plants

Component of amino acids and proteins; deficiencies show as yellowing of new leaves.

Micronutrient Availability and pH

As soil pH increases, availability of Fe, Mn, Zn, and Cu decreases.

Copper and Zinc Toxicity

Occurs at low pH and with repeated manure application; managed by increasing pH or organic matter.

Soil Sampling Depth

Field crops: 8 in; perennials: 4 in.

Composite Sample

Combination of several subsamples representing one field or soil type.

Grid Sampling

Sampling on a regular grid to map soil variability and nutrient needs.

NCDA Soil Test Index

Unitless index value representing nutrient availability or heavy metal risk.

Realistic Yield Expectation (RYE)

Estimated yield for a soil-crop combination used to set N rate (N rate = RYE × N factor).

Four Rs of Nutrient Management

Right source, Right rate, Right time, Right place.

Cation Exchange and Flocculation

Ca²⁺, Mg²⁺, Al³⁺ promote flocculation; Na⁺ causes dispersion.

Source of Soil Acidity

CO2 reaction, organic matter decomposition, NH⁴⁺ oxidation, Al hydrolysis.

Role of Lime Reaction

Ca²⁺ replaces H⁺ and Al³⁺ on exchange sites; carbonate neutralizes acidity.

Plant Macronutrient Function – N

Promotes leaf growth, protein formation, and chlorophyll production.

Plant Macronutrient Function – P

Energy transfer and root development; deficiency causes purple leaves.

Plant Macronutrient Function – K

Water regulation and stress resistance; deficiency causes leaf scorching.

Soil pH Effects

Low pH increases metal toxicity; high pH decreases micronutrient availability.

CEC Units

Milliequivalents (meq) per 100 g dry soil.

Effect of Weathering

Older soils (Ultisols, Oxisols) are more acidic and low in nutrients.

Iron and Aluminum Oxides

Non-expansive, low CEC clays found in highly weathered soils.

Soil Buffering Mechanisms

Include mineral dissolution, Al compounds, cation exchange, and carbonates.

Organic Matter Benefits

Improves aggregation, water holding, aeration, and nutrient cycling.

pH Measurement Equation

pH = –log[H+]; lower pH = higher acidity.

Eutrophication

Nutrient enrichment (especially P) of water bodies leading to algal blooms and low O2.

CEC Influencers

Clay content, organic matter, and pH level determine soil’s cation exchange capacity.

Cation

Positive ion (+1)

• Boron (depending on pH)

• Calcium

• Copper

• Hydrogen

• Magnesium

• Manganese

• Nickel

• Nitrogen (Ammonium form, NH₄⁺)

• Potassium

• Zinc

Anion

Negative Ion (-1)

• Boron (depending on pH)

• Chloride

• Molybdate

• Nitrogen (Nitrate form, NO₃^-)

• Phosphate *

• Sulfate

Clayey soil and soils with humus (organic matter) generally have ______ CEC.

higher

Highest  —————————————————> Lowest

Buffering

Humus → 2:1 clays → 1:1 clays → oxides → sand

Beneficial effects of liming

• Crop yield improvement

• Nutrient availability effects

• Improved microbial activity

• Improved legume fixation of N

• Ca & Mg addition:

  • Calcitic lime = only Ca

  • Dolomitic lime = Ca and Mg

Step 1 of Liming

Ca²⁺ from the lime replaces Al³⁺ and H+ on the cation exchange complex.

Step 2 of Liming

The carbonate reacts with the H+ ions, removing them from solution thereby raising the pH. Lime works by turning H+ to water.

Step 3 of Liming

Al³⁺ is hydrolysed to form Al-hydroxides and H⁺ ions. Then carbonate from the lime neutralizes the H⁺ generated during the Al-hydrolosis.

Calcium Carbonate Equivalent (CCE)

Neutralizing value of any liming material compared to pure calcium carbonate. (Higher CCE = Less lime)

• Buffering capacity Higher buffering capacity = more lime

• Cation exchange capacity CEC Higher CEC = more time

• Lime type More pure lime = less lime

• Lime fineness Smaller lime particle = changes pH faster

• Soil texture More clay = More lime

Volatilization

Removal of N from the soil by turning it into a gas that leaves the soil.

Denitrification

Conversion of nitrate to a gas form of N (ideally N2).

• Need warm temperatures, low oxygen, organic matter (carbon source).

Crop uptake and removal

Removal of N by plants and then removing plant residues from the field.

– Crop variety

– Soil type

– Climate

– N application (timing, source, placement, rate)

Plant Micronutrients

• Iron

• Manganese

• Boron

• Zinc

• Copper

• Chlorine

• Molybdenum

• Nickel

Non-essential but beneficial plant nutrients

• Sodium

• Silicon

• Cobalt

• Selenium

• Aluminum

Mass Flow

Dissolved nutrients move to the root in soil water that is flowing towards the roots.

Diffusion

nutrients move from higher concentration in the bulk soil solution to lower concentration at the root

Root interception

roots obtain nutrients by physically contacting nutrients in soil solution or on soil surfaces.

A bag of 10-10-10 fertilizer is:

10% Plant-Available N

10% P₂O₅

10% K₂O

A bag of 16-0-8 fertilizer is: 

16% Plant-Available N

0% P₂O₅

8% K₂O

Phosphorus Cycle

• Available forms: H₂PO₄^-, HPO₄^2-

• Strongly binds to Fe and Al oxides, so P stays where it is

placed in the soil.

• P is most likely to be lost via erosion and moves very little down the soil profile.

• Mediated by equilibrium chemistry