Soil Science

O Horizon

A master soil horizon on the topmost layer of the soil. An “organic” horizon. Not always present, and is made up of organic matter or plant debris.

Usually not present in grasslands or agricultural areas

A Horizon

A master soil horizon that forms under the O Horizon or at the surface. A “mineral” horizon. Always present but could be eroded away. Mostly made up of dark colored humus, or humus coatings on mineral particles. Considered topsoil.

Along with E horizon, considered an eluvial horizon.

E Horizon

A master soil horizon between A and B. May or may not be present (mostly in older soils). Called the zone of eluviation because Fe and Al oxides and clays tend to be pulled from this horizon downward, giving the this horizon a bleached appearance.

B Horizon

A master soil horizon that forms below A or E. May or may not be present (mostly in younger soils). Called the zone of illuviation because Fe and Al oxides and clays tend to be deposited here through water from higher layers. It has a reddish appearance. Considered subsoil.

C Horizon

A master soil horizon that forms below A and B. Usually present, especially in mature soils, but could be replaced by hard rock. It is made up of unconsolidated parent material from which the soil formed.

R Horizon

A master horizon that is located under the parent material. It is made up of bedrock under the parent material. Not considered true soil.

Ochric

Diagnostic horizon. Characterized as thin epipedon and lacking in organic material, and a category for soils that do not fit into more organic material-rich horizons.

Mollic

Diagnostic horizon. Characterized by a thick epipedon rich in organic material, often dark in color. Comes from the latin word for “soft”.

Argillic

Diagnostic horizon. Subsoil, corresponds to B or Bt. Refers to a layer rich in silicate clay from illuviation.

Cambic

Diagnostic horizon. Subsoil, corresponds to B or Bw or Bg. Characterized by weathered B Horizon and possible parent material. Comes from latin word for “to change”.

Albic

Diagnostic horizon. Corresponds to E Horizon. Characterized by the layer being stripped of clay, organic matter, and Fe and Al oxides through eluviation. Not fertile. Comes from latin word for “white”.

Calcic

Diagnostic horizon. Subsoil, corresponds to B or C Horizons, or Bk and Ck. Refers to a buildup of calcium carbonate in the layer, or a buildup of lime.

Gypsic

Diagnostic horizon. Subsoil, corresponds to B or C. Refers to a buildup of gypsum in this layer.

Oxic

Diagnostic horizon. Subsoil, corresponds to B horizon. Refers to a very weathered layer with a lot of Fe and Al oxides. Low fertility.

Salic

Diagnostic horizon. Subsoil, corresponds to B horizon. Refers to an extreme buildup of salts in this layer.

Spodic

Diagnostic horizon. Subsoil, corresponds to B horizon. Refers to organic matter and Fe and Al being deposited into this layer because the top layers are very sandy. Also quite acidic.

Aquic

Moisture regime. Soil is saturated with water for long periods of time, long enough to be depleted of oxygen.

Udic

Moisture regime. Soil is moist with a lot of rainfall but not constantly saturated for long periods of time. Soil is never really dry, often a humid or subhumid climate.

Ustic

Moisture regime. Typically moist with rainfall during the growing season but faces prolonged dryness other parts of the year.

Xeric

Moisture regime. Soil of a Mediterranean climate, often moist and cool in the winter but dry other parts of the year.

Aridic or Torric

Moisture regime. Dry for most of year with little rainfall, even when plants are in growing season.

Gelisols

Soil order. Characterized by permafrost in the top 2 meters of soil. Soil development is quite stalled by the permafrost.

Histosols

Soil order. Characterized by deep accumulation of organic matter and wet conditions. Fertile and productive.

Andisols

Soil order. Characterized by formation from volcanic ash and maybe weathering.

Vertisols

Soil order. Characterized by swelling clays and dry seasons, giving soil a cracked appearance when dry.

Aridisols

Soil order. Characterized by dry periods, little plant growth, little rainfall, and little leaching. Can have some subsoil development (cambic, calcic, or salic) and can be productive when irrigated. Must have a B horizon.

Entisols

Soil order. Characterized by very young soils, extremely young specimens may have an A but no B Horizon. Usually are considered to have ochric epipedon over C and R horizons.

Inceptisols

Soil order. Characterized by the inception of a B horizon. Young, but more weathered/developed than entisols. More developed specimens can have an A horizon over a Bw horizon, ochric epipedon over cambic subsoil.

Alfisols

Soil order. Characterized by illuvial clay rich subsoil (argillic horizon), low acidity, and good fertility. Warm and humid climates. Generally form under forest vegetation.

Ultisols

Soil order. Characterized by illuvial clay rich subsoils (argillic horizon) and high acidity, with low to moderate fertility. Strongly leached. Some weathering. Warm humid climate and forest soils.

Mollisols

Soil order. Characterized by thick, dark, non acidic A horizon (mollic epipedon). Associated with grasslands. Subsoil can vary. Very fertile.

Spodosols

Soil order. Characterized by being acidic, sandy, and having an organic matter and Fe and Al oxide rich B horizon (spodic horizon). Cool climate and associated with coniferous forests. Genereally sandy parent material, and very poor soil.

Oxisols

Soil order. Characterized by hot, wet tropics and extreme weathering (oxic horizon). A lot of Fe/Al oxides and low natural fertility. Must be limed and fertilized for good yields.

Five Soil Forming Factors

1. Parent material

2. Climate

3. Biota

4. Topography

5. Time

Parent material

The material from which soil forms. Can be one or a combination of the following:

1. Minerals in consolidated rock

2. Minerals in unconsolidated sediments

3. Organic matter (drained peat bogs)

Igneous rock

Formed by cooling magma. Has randomly distributed mineral grains.

Sedimentary rock

Formed by “cemented” sediments

Metamorphic rock

Formed by modifications from heat and pressure. Has reoriented mineral grains.

Minerals

Naturally occurring, solid, inorganic substance with a definite chemical composition and an ordered internal structure, typically forming crystals. Can differ in composition, physical properties, and weathering rates.

Primary minerals

Formed by cooling of magma. Light colored primary minerals are more weather resistant (oxides), and darker ones are less resistant (carbonates)

Secondary minerals

Formed from primary minerals

Granite

A coarse grained, light colored igneous rock. More K than Ca or Fe/Mg. Resistant to weathering. Made up of micas, feldspars, muscovite, plagioclase, quartz, hornblende.

Basalt

Fine grained, dark colored igneous rock. Susceptible to weathering. More Fe/Mg than Ca or K.

t modifier (ex Bt)

Indicates illuvial accumulation of clay. Comes from the german word “ton”, meaning clay.

w modifier (ex Bw)

Indicates weak development of color or structure to show how a layer has changed or eroded without collecting much illuvial material.

Number modifier (ex C, 2C)

Indicates a lithologic discontinuity. Numbers for layers show which “part” the different horizons are in when it comes to the discontinuity, and where the break is.

Double letter modifier (ex AB)

Indicates a transitional horizon instead of a hard break. The letter that comes first indicates the more dominant horizon.

Lithologic discontinuity

A significant change in soil from the horizon above, could be change in particle size, mineral makeup, or chemical composition. Shows two or more different parent materials stacked on top of each other.

Organic parent material

Parent material formed from plants deposited in wet areas.

Residual parent material

Parent material formed from rocks and minerals that stayed in the same place.

Alluvial parent material

Parent material formed from rocks and minerals transported by water, specifically by streams.

Colluvial parent material

Parent material formed by rocks and minerals transported by gravity.

Till parent material

Parent material formed by rocks and minerals transported and deposited by ice.

Eolian parent material

Parent material formed by rocks and minerals transported by wind. Can be associated with lithologic discontinuity.

Four soil forming processes

1. Additions

2. Losses

3. Translocations

4. Transformations

Pedon

“Unit” of soil. 1 × 1 × 1 meters in size chunk of soil. Several contiguous pedons with similar characteristics can be grouped into polypedons or soil individuals.

Taxonomic Hierarchy

1. Order

2. Suborder

3. Great group

4. Subgroup

5. Family

6. Series

Physical properties of soil

1. Soil color

2. Texture

3. Structure

4. Bulk density

5. Porosity

6. Tilth

Soil color

Described using the Munsell system. Notation is “Hue Value/Chroma”. Has little direct effect on soil properties, but can give an indication of soil conditions. Moisture affects color greatly.

Hue

Spectrum of color of the soil. Part of Munsell system

Value

Lightness/darkness of the soil. Part of Munsell system

Chroma

Color intensity of the soil. Part of Munsell system

Gleyed

When soil is waterlogged from clay, causing a dull grayish appearance

Soil Separates

The different size classes of soil particles. Sand, silt, and clay.

Sand separate

Largest particle. 2.0-0.05 mm.

Dominant primary minerals.

Low water holding capacity,

good aeration,

drainage rate high,

OM level low.

Low attraction to other sand particles.

Very low ability to hold plant available chemicals and nutrients.

Can see w/ naked eye.

Silt separate

Middle size particle. 0.05-0.002 mm.

Dominant primary and secondary minerals.

Medium attraction to other silt particles.

Water capacity med-high,

aeration medium,

drainage rate slow-med,

OM level med-high.

Low ability to hold plant available chemicals and nutrients

Can see with a microscope

Clay separate

Smallest soil particle. >0.002 mm.

Has a larger impact on soil and more clay increases surface area and microporosity.

High water capacity,

poor aeration,

very slow drainage rate,

OM level high-med.

If bulk density decreases, pore space increases.

High attraction to other clay particles

High ability to hold plant available chemicals and nutrients

Loam

“Ideal” soil. Can’t be measured because it is a mix of the particle classes where each particle’s properties are equally expressed. (40% sand 40% silt and 20% clay)

Soil textural classes

Clay, silt loam, sandy loam, loam (12 textural classes). Refer to the breakdown of particles in a soil, not the separate classes!

Soil structure

Soil aggregates are uniformly sized and shaped units of sand, silt, and clay. Granular structure is ideal. Provides macroporosity, particularly in clay soil, can reduce erosion, and improve root penetration.

Well Developed Granular structure

Ideal, most common in A Horizon. Open nature allows for water infiltration/drainage and aeration.

Well Developed Blocky structure

Tighter than granular. More common in B Horizons.

Well Developed Prismatic structure

Most common in B Horizons in drier climates.

Platy structure

Compacted and undesirable structure.

Structureless states of soil

1. Massive (clods)

2. Single grained (loose soil)

Aggregates

Synonym for structures. Holds water in micropores like a sponge, while macropores allow for movement of water. Aggregates are like sponges.

Formation of soil structure (Physical/chemical processes)

1. Flocculation (clumping)

2. Volume changes of clays (shrinking and swelling)

Formation of soil structure (Biological processes)

1. Activities of soil organisms

2. Enmeshment by roots and fungi

3. Production of organic “glues” by bacteria and fungi

4. Production of soil matter (humus)

Flocculation

Divalent cations act as electrostatic “bridges” allowing negatively charged clays to come together. Monovalent cations, like Na⁺, are unable to effectively neutralize the negative charges leading to clay dispersion. Organic matter humus also stabilizes soil structure.

Organic matter promotes good aggregation

1. Provides raw materials for production of humus by microbes, binding soil particles

2. Stimulates activities of soil organisms that improve soil structure

3. Surface residues and mulches help reduce raindrop impact and help prevent soil crusting, runoff, and erosion

Fungi promote soil aggregation

1. “Typing” particles together

2. Producing humus

3. Making glomalin, or GRSPs

Glomalin-related Soil Proteins (GRSPs)

Account for 27% of carbon stored in soils and is important for soil structure.

Bacteria promote aggregation

1. Producing bacterial glues

2. Producing humus

Earthworms promote soil aggregation

1. Burrowing (other burrowing animals also contribute)

2. Worm castings are quite stable

Tillage

Tilling, or ripping up and overturning topsoil is bad for soil structure. Cultivation decreases organic matter, stability, and infiltration rates. Can cause erosion and runoff. Can cause nutrient loss and gaseous losses of N.

Particle density

The weight of solid particles per unit of volume (no spore space). Generally assumed to be 2.65 g/cm³ (density of quartz)

Bulk density

The oven dry weight of the solid particles in a standard volume of field soil (includes pore space) Bulk density is always less than particle density. Plant root growth generally inhibited at bulk densities >1.6. Inverse relationship to porosity.

A bulk density of 1.3 g/cm³ = ~50% pore space

Macroporosity

External to soil aggregates. Allows for water movement. Tillage decreases macropores.

Microporosity

Smaller pores inside soil aggregates. Tend to hold onto water. Tillage actually increases micropores.

No Till Agriculture

Aims to preserve aggregates and macroporosity. Tries to not to disturb the soil but sometimes it is necessary.

Soil compaction

Causes decreased drainage and aeration. More susceptible to compaction when wet, and puddling can happen. Farm equipment can decrease this by using wide wheels or standing on boards to distribute weight, or work when soil is drier.

Soil crusting

Typically caused by raindrop impact or sprinkler irrigation. Decreases porosity and makes it hard for seedlings to emerge. Can be prevented or remediated by leaving plant residues or mulch, or no till practices.

Poorly aerated soil types

1. Poorly drained (high water table)

2. Soils of high clay content shortly after rainfall or irrigation

3. Deep subsoils in clayey soils, especially if wet

4. Highly compacted soils of fine texture

5. Deep portions of structureless clayey soil (massive)

Tilth

Physical condition of a soil in relation to plant growth

Tilth factors

1. Aggregate formation and stability

2. Bulk density

3. Soil moisture characteristics

4. Aeration status

5. Rate of water filtration

6. Drainage

7. Workability

Mineral densities

1. Quartz: 2.65g/cm³. Serves as baseline for particle density

Bulk density percent soil equation

Bulk density = (mass of soil)/(volume of soil)

Physical Phases of Soils

1. Solid phase (sand, silt, clay, (45%) and organics (5%)) ideal 50%

2. Liquid phase (water/soil solution) ideal 20-30%

3. Gaseous phase (air in pores, could be O, N, C, CO2) ideal 20-30%

Soil Solum

Includes A, E, and B soil horizons. Sometimes O

Soil Regolith

Layer of loose material on top of solid rock. All layers NOT bedrock