soil science

A natural mixture of organic and inorganic materials formed by weathering and conditioned by climate, organisms, topography, parent material and time; serves as a medium for plant growth.

Soil

The basic sampling unit in soil surveys: a hexagonal column of soil 1–10 m² at the surface representing a soil individual.

Pedon

A group of contiguous, similar pedons that together form a distinguishable soil body in the landscape.

Polypedon

The study of soils focusing on their genesis, morphology and classification, with minor emphasis on use.

Pedological Approach

The study of soils emphasizing their practical use, especially their relation to plant growth and productivity.

Edaphological Approach

Branch of soil science dealing with the ability of soil to supply essential nutrients for optimal plant growth.

Soil Fertility (Field)

Study of physical properties and processes of soil such as texture, structure, water and heat movement.

Soil Physics

Examination of chemical reactions and interactions among solid, liquid and gaseous soil phases.

Soil Chemistry

Study of biochemical reactions in soil driven primarily by microorganisms.

Soil Microbiology

Protection of soil from erosion or chemical deterioration and the set of practices (tillage, cropping, fertilization) used for sustained production.

Soil Conservation and Management

Discipline that maps, describes and systematically arranges soils according to their properties and genesis.

Soil Survey and Classification

Study of the structural chemistry of the solid mineral components of soils.

Soil Mineralogy

Allocation and planning of land areas for broad purposes such as agriculture, forestry, settlement or military aims.

Land Use (Field)

The 45 % inorganic solid fraction of most mineral soils derived from weathered rocks; primary source of most plant nutrients except N, C and O.

Mineral Matter

About 5 % of mineral soils; decomposed plant and animal residues that supply N, P, S and micronutrients and improve aggregation and cation storage.

Organic Matter

Gas phase occupying 20–30 % of pore space; richer in CO₂ and poorer in O₂ than atmospheric air and vital for root respiration.

Soil Air

Liquid phase in pores (20–30 %); dissolves and transports nutrients and participates in mineral weathering and plant metabolism.

Soil Water

Resistant minerals (e.g., quartz, feldspars, micas) that persist from parent rock and supply sand and silt fractions.

Primary Minerals

New minerals (e.g., kaolinite, montmorillonite, gibbsite) formed from chemical alteration of primary minerals, contributing mainly to clay fraction.

Secondary Minerals

Disintegration of rocks into smaller particles without chemical change, e.g., freeze–thaw, unloading.

Physical Weathering

Alteration of mineral chemistry producing simpler products; includes hydrolysis, hydration, oxidation, carbonation and solution.

Chemical Weathering

Chemical weathering process where water reacts with minerals, destroying original structure and producing an acid and a base.

Hydrolysis

Addition of water molecules to a mineral without structural breakdown, e.g., hematite → limonite.

Hydration

Conversion of reduced ions (often Fe²⁺) to oxidized forms (Fe³⁺), weakening mineral structure.

Oxidation

Reaction of carbonic acid with minerals, forming soluble bicarbonates, e.g., calcite → calcium bicarbonate.

Carbonation

Direct dissolution of minerals by carbonic acid or H⁺, separating constituent ions.

Solution (Weathering)

Rocks formed by solidification of molten magma; examples: granite, basalt, andesite.

Igneous Rocks

Rocks formed from transported and recemented fragments or precipitates; e.g., limestone, sandstone, shale.

Sedimentary Rocks

Rocks recrystallized under heat and pressure from pre-existing rocks; e.g., gneiss, schist, marble.

Metamorphic Rocks

Igneous rocks that cool rapidly on the surface, fine-grained; e.g., basalt.

Extrusive (Volcanic) Rocks

Igneous rocks that cool slowly within the crust, coarse-grained; e.g., granite.

Intrusive (Plutonic) Rocks

Partly weathered mineral or organic matter from which a soil develops.

Parent Material

Parent material that forms and remains in place over the underlying rock.

Residual Parent Material

Material moved by water, ice, wind, gravity or volcanic activity before soil formation.

Transported Parent Material

Transported parent material deposited by running water such as rivers and streams.

Alluvium

Fine sediments laid down in former lake beds.

Lacustrine Deposits

Materials accumulated in ancient sea environments.

Marine Deposits

Unsorted material deposited directly by moving glaciers; also called moraine.

Glacial Till

Parent material transported and deposited by wind, e.g., dune sands, volcanic ash.

Aeolian Deposits

Lithified volcanic ash layers serving as parent material for soils like Lipa or Tagaytay series.

Volcanic Tuff

Material moved downslope by gravity and deposited at slope bases.

Colluvium

Acronym for Climate, Organisms, Relief, Parent material and Time – the five soil-forming factors.

CLORPT

A roughly parallel layer within a soil profile that differs in properties from layers above or below.

Soil Horizon

Vertical cross-section of soil exposing all horizons from surface to unweathered rock.

Soil Profile

The A and B horizons together – the true soil zone influenced by plant roots.

Solum

All unconsolidated material above bedrock, including A, B and C horizons.

Regolith

Surface layer dominated by organic litter (Oi, Oe, Oa).

O Horizon

Topsoil rich in organic matter and minerals; zone of maximum biological activity.

A Horizon

Eluvial mineral layer where clay, Fe and Al have been leached out; typically light-colored.

E Horizon

Subsoil where materials such as clay, Fe or carbonates accumulate via illuviation.

B Horizon

Relatively unaltered weathered parent material beneath the zone of soil development.

C Horizon

Consolidated bedrock underlying the soil.

R Horizon

Movement and deposition of soil constituents (clay, Fe, organics) into a lower horizon.

Illuviation

Removal of materials from an upper horizon through leaching or translocation.

Eluviation

Accumulation of calcium carbonate in a soil horizon.

Calcification

Build-up of soluble salts (Ca, Mg, Na, K sulfates/chlorides) in soil layers.

Salinization

Accumulation of exchangeable sodium ions in the soil profile.

Alkalinization (Solonization)

Leaching of Al, Fe and organics downward, leaving silica-rich upper horizons.

Podzolization (Silication)

Intense leaching that removes silica and concentrates Fe and Al oxides.

Laterization (Desilication)

Downward movement of fine clay particles, enriching lower horizons in clay.

Lessivage

Relative proportion of sand, silt and clay particles in a soil; a stable physical property.

Soil Texture

Largest soil separate (0.05–2 mm USDA); gritty, low nutrient and water holding capacity.

Sand

Intermediate soil separate (0.002–0.05 mm); smooth, moderate properties.

Silt

Smallest separate (<0.002 mm); sticky, plastic, high water and nutrient retention.

Clay

Ideal textural class with nearly equal contributions of sand, silt and clay.

Loam

Arrangement of soil particles into aggregates of defined shapes and stability.

Soil Structure

Small, rounded aggregates common in surface soils rich in organic matter; best for plant growth.

Granular (Crumb) Structure

Cube-like aggregates with sharp (blocky) or rounded (sub-angular) edges, common in subsoils.

Blocky Structure

Vertical, pillar-like aggregates with flat tops, typical of subsoils.

Prismatic Structure

Prismatic aggregates with rounded tops; often in arid, sodium-rich soils.

Columnar Structure

Thin, horizontal, disk-like aggregates that impede water movement; may form in compacted layers.

Platy Structure

Mass of dry soil per unit total volume (g cm⁻³); indicator of compaction and porosity.

Bulk Density

Mass of soil solids per unit volume of solids (excluding pores); usually ~2.65 g cm⁻³ for mineral soils.

Particle Density

Percentage of total soil volume occupied by pores (air + water).

Porosity

Portion of pore space filled with air after drainage; essential for root respiration.

Aeration Porosity

Force with which water is held in soil, expressed in bars or atmospheres; increases as soil dries.

Soil Moisture Tension

Soil water content at 0.3 bar (⅓ atm) tension after gravitational drainage; upper limit of available water.

Field Capacity

Soil water content at 15 bar tension where plants cannot extract water and wilt irreversibly.

Permanent Wilting Point

Water film tightly adsorbed to particle surfaces at ~31 atm tension; unavailable to plants.

Hygroscopic Coefficient

Difference between moisture at field capacity and permanent wilting point.

Available Water

Water that drains rapidly from macropores after saturation; unavailable to plants.

Gravitational Water

Downward entry of water through the soil surface.

Infiltration

Deeper downward movement of water through the soil profile under gravity.

Percolation

Upward or lateral flow of water through small soil pores driven by adhesion and cohesion.

Capillary Movement

Degree of cohesion and adhesion in soil at varying moisture contents, affecting workability.

Soil Consistency

Laboratory measures defining soil consistency states: liquid, plastic and shrinkage limits.

Atterberg Limits

Standard reference for describing soil color using hue, value and chroma.

Munsell Color Chart

Very fine organic or inorganic particles (<1 µm) with large surface area and reactive charges.

Soil Colloids

Total surface area per unit mass of soil particles; higher in clays and colloids.

Specific Surface Area

Small pores within aggregates that store water against gravity.

Micropores

Large pores between aggregates that facilitate drainage and aeration.

Macropores

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

Friable Consistency

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

Harsh (Hard) Consistency

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

Munsell Color Chart

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

Hue (Soil Color)

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

Value (Soil Color)

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

Chroma (Soil Color)

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

Soil Colloids

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

Organic Colloids

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

Humus