Soil Science – Chemical, Biological & Fertility Concepts

Vocabulary flashcards summarizing key terms and definitions related to soil chemical properties, colloids, ion exchange, soil reactions, biological processes, organic matter, and fertility management.

Friable Consistency

Soil condition that is soft, easily crumbled and ideal for cultivation due to good structure.

Harsh (Hard) Consistency

Soil condition that is hard and clod-forming, requiring high energy to plow.

Munsell Color Chart

Standard guide used to describe soil color by hue, value and chroma.

Hue (Soil Color)

The dominant spectral color component identified on the Munsell chart (e.g., 7.5 YR).

Value (Soil Color)

The lightness or darkness of a soil color on the Munsell scale.

Chroma (Soil Color)

The purity or intensity of a soil color on the Munsell scale.

Soil Colloids

Very small (0.2–1 µm) organic or inorganic particles with charged surfaces that drive most soil chemical reactions.

Organic Colloids

Colloids composed of humus that carry mainly negative charges from dissociated functional groups.

Humus

Stable, high-molecular-weight organic material that increases ion adsorption and CEC.

Inorganic (Mineral) Colloids

Clay particles made of sheet-structured aluminosilicates or hydrous oxides.

Silica Tetrahedron

Basic clay structural unit with one Si atom surrounded by four O atoms.

Alumina Octahedron

Clay structural unit with Al (or Mg/Fe) surrounded by six hydroxyls.

1:1 Clay (Kaolinite)

Non-expanding clay with one silica sheet to one alumina sheet; low CEC, pH-dependent charge.

2:1 Expanding Clay (Smectite/Montmorillonite)

Clay with two silica sheets sandwiching one alumina sheet; high CEC and swelling when wet.

Vermiculite

2:1 clay with limited expansion and very high CEC.

Illite

2:1 non-expanding potassium-rich clay with moderate CEC.

Chlorite

2:2 clay containing an extra hydroxide layer; non-expanding.

Allophone & Imogolite

Non-crystalline (amorphous) silicate clays common in volcanic ash soils.

Hydrous Oxides (Sesquioxides)

Amorphous non-silicate clays of Fe and Al such as goethite or gibbsite.

Isomorphous Substitution

Replacement of one ion by another of similar size in clay sheets, creating permanent negative charge.

Cation Exchange

Reversible replacement of one cation on colloid surfaces by another from soil solution.

Anion Exchange

Reversible replacement of adsorbed anions (e.g., PO₄³⁻) on positively charged surfaces.

Milliequivalent (me)

1/1000 of a chemical equivalent; used to express exchangeable ions in soils.

Centimole Charge (cmolc)

1/100 of a mole of charge; modern unit equivalent to me per 100 g soil.

Cation Exchange Capacity (CEC)

Total amount of exchangeable cations a soil can adsorb, expressed as me/100 g or cmolc/kg.

Percent Base Saturation (%BS)

Percentage of CEC occupied by basic cations (Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺).

Exchangeable Sodium Percentage (ESP)

Portion of CEC occupied by Na⁺; values >15 % indicate sodic conditions.

Active Acidity

H⁺ concentration in soil solution measured with water; determines pH reading.

Reserve (Exchangeable) Acidity

H⁺ and Al³⁺ adsorbed on colloids, revealed when soil is extracted with salt (e.g., KCl).

Buffering Capacity

Soil’s resistance to pH change; increases with higher CEC and organic matter.

Liming

Application of Ca or Mg compounds to neutralize soil acidity and raise pH.

Relative Neutralizing Value (RNV)

Strength of a liming material compared with CaCO₃ (RNV = 100 %).

Calcium Carbonate Equivalent (CCE)

Amount of CaCO₃ that would provide the same neutralizing effect as a liming material.

Gypsum (CaSO₄·2H₂O)

Calcium sulfate used to supply Ca²⁺ or reclaim sodic soils; not a liming agent.

Saline Soil

Soil with electrical conductivity >4 mmhos cm⁻¹ due to excess soluble salts.

Sodic Soil

Soil whose exchange complex has >15 % Na⁺, leading to dispersion and poor drainage.

Upland (Dryland) Soil

Aerobic soil type used for crops like maize; nutrients mostly in oxidized forms.

Lowland (Paddy) Soil

Anaerobic submerged soil used for rice; nutrients largely in reduced forms such as NH₄⁺.

CFU (Colony-Forming Unit)

Measurement unit indicating viable microbial cells per gram of soil.

HFS (Hectare-Furrow Slice)

Approximate mass (2 × 10⁶ kg) of soil to plow depth on one hectare; used for biomass estimates.

Earthworms

Macro-fauna that ingest detritus, create channels, and enhance aeration and aggregation.

Bacteria (Soil)

Diverse microbes responsible for nutrient transformations including N, S, and C cycles.

Fungi

Heterotrophic, often acid-tolerant decomposers capable of breaking down lignin & cellulose; include mycorrhizae.

Actinomycetes

Filamentous bacteria bridging bacteria and fungi; important decomposers and antibiotic producers.

Protozoa

Single-celled predators that consume bacteria, releasing immobilized nutrients.

Algae (Soil)

Photosynthetic organisms, including N-fixing blue-green algae, inhabiting moist soil surfaces.

Soil Organic Matter (SOM)

All carbon-containing compounds in soil derived from plants and animals.

C/N Ratio

Proportion of carbon to nitrogen in organic material; governs decomposition speed and N availability.

Mineralization

Microbial conversion of organic nutrients (e.g., N, P, S) to inorganic plant-available forms.

Immobilization

Microbial uptake of inorganic nutrients, converting them into organic forms and temporarily reducing availability.

Nitrification

Biological oxidation of NH₄⁺ to NO₃⁻ via Nitrosomonas and Nitrobacter under aerobic, neutral-alkaline pH.

Denitrification

Anaerobic reduction of NO₃⁻ to gaseous N₂O and N₂ by bacteria such as Pseudomonas, causing N loss.

Ammonification

Decomposition step that converts organic N into NH₃/NH₄⁺.

Symbiotic Nitrogen Fixation

Conversion of atmospheric N₂ to NH₃ by rhizobia within legume root nodules.

Non-Symbiotic Nitrogen Fixation

Free-living microbes (e.g., Azotobacter, Clostridium, cyanobacteria) reduce N₂ without plant hosts.

P Solubilization

Microbial release of phosphate from insoluble minerals through acid or chelate production.

Mycorrhiza

Mutualistic association of fungi with plant roots improving P uptake; includes ectotrophic and endomycorrhizae.

Thiobacillus

Genus of sulfur-oxidizing bacteria that convert elemental S to sulfate, producing sulfuric acid.

Desulfovibrio

Sulfate-reducing bacteria that convert SO₄²⁻ to sulfide under anaerobic conditions.

Composting

Managed aerobic decomposition of organic residues to produce stable, humus-like compost.

Cellulose Decomposition

Breakdown of cellulose by fungi (e.g., Trichoderma) and bacteria (e.g., Cellulomonas, Clostridium).

Soil Fertility

Soil’s capacity to supply nutrients in adequate amounts and balance for a crop.

Soil Productivity

Soil’s ability to produce a specified yield; depends on fertility plus physical, biological and climatic factors.

Plant Nutrition

Supply and absorption of essential chemical elements by plants for normal growth.