soils midterm

what is soil?

formed by identifiable factors/processes over time

organized into a profile

variable across landscapes

living, breathing, cycling - dynamic!

what four spheres does soil sit at the intersection of? what is this zone of overlap called?

lithosphere (minerals), atmosphere (gases), hydrosphere (water), and biosphere (organisms) all overlap in the middle to form the pedosphere, or soil! this is because soil heavily involves these factors - soil air, organic matter and biomass, soil particles, and soil water. interaction between these components can occur at any scale and shapes the physical, chemical, and biological components of soil.

name all layers in the soil profile (general)

solum = the upper and most weathered part of the soil - A, E, and B horizons

C horizon = a mineral horizon largely unaltered by biological activity and pedogenesis

regolith - all the unconsolidated material above bedrock

bedrock - the solid rock underlying soils and the regolith

name the soil horizons

O - organic matter like decaying leaves, above the mineral soil

A - topsoil, minerals from parent material w/ a little organic matter

E - eluviated horizon, lightly colored and bleached

B - subsoil, rich in minerals that came down from A and E horizons

C - parent material, least-weathered material

R - bedrock

what is soil made of?

mineral material (around 45%), organic matter (5%), water (20-30%), and air (20-30%).

mineral material - largest component by volume, divided into sand/silt/clay, primary minerals are inherited directly from parent rock/unaltered (sand and coarse silt), while secondary minerals are formed through weathering (clay minerals, iron and aluminum oxides) and carry most of the soil’s chemical reactivity.

organic matter - smallest by volume but highly influential! binds mineral particles into aggregates, holds water/nutrients more effectively than minerals, sustains soil biota

water - occupies pore space, described as the soil solution b/c it has dissolved nutrients, gases, and organic compounds. this is NOT a passive filler; rather a dynamic and chemically active component

air - fills whatever pore space water does not. NOT the same as atmospheric air, typically lower in O2 and higher in CO2 because of root/microbial respiration. soil aeration is essential for root respiration and aerobic microorganisms

how does soil function as a dynamic system?

soil can cycle matter and energy through four different pathways

inputs - precipitation, organic residues, weathering products, atmospheric deposition

transformations - decomposition, mineral weathering, aggregate formation

transfers - leaching, translocation within profile, root uptake

outputs - drainage, gas emissions, erosion, harvest/removal

what is soil texture?

the relative proportions of sand, silt, and clay. clay particles are so small that they have a huge surface area compared to the same mass of sand! these size boundaries between particles are to denote a shift in behavior.

which soil properties change over time? which property doesn’t?

properties that change over time are structure (reshaped by tillage, roots, wetting/drying), biological activity (shifts with season, disturbance), and organic matter (can build or deplete with time).

the property that doesn’t change with time is soil texture! this is set by parent material/initial weathering and informs the interpretation of all other soil properties, but does not substantially shift within a human lifetime.

what are the six ecosystem services of soil?

medium for plant growth - anchors roots, supplies water and nutrients. soils provide around 98% of the calories we consume!

recycling system - decomposes organic waste, cycles nutrients

water supply/purification - stores, filters, and regulates water

habitat - home to a large diversity of organisms, likely 59% of the species on earth! this biodiversity is what drives the other ecosystem services

atmospheric modifier - regulates gas exchange, one of the largest carbon reservoirs on land - management directly determines whether soil is a carbon source or sink

engineering medium - foundation for roads, buildings, structures

how do roots get nutrients from the soil? what nutrients are they taking up?

there are 3 ways -

root interception - roots physically grow into contact with nutrients in the soil

mass flow - dissolved nutrients are carried towards roots along with water as plants draw water in

diffusion - nutrients move from high to low concentrations around the root surface

plants are getting their macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Zn, Cu, B, Mn, others) from soil solids and solutions!

explain igneous rocks?

igneous rocks form when molten rock cools and recrystallizes! the grain size reflects cooling speed - coarse from slow underground cooling, fine from rapid surface cooling.

color can serve as a proxy for composition - light rocks are quartz/feldspar rich, dark rocks are rich in iron/Mn bearing minerals (these are more easily weathered).

explain sedimentary and metamorphic rocks?

sedimentary rock (sandstone/shale/limestone) plus temperature and pressure recrystallize and equals metamorphic rock (marble, gneiss, slate).

these are classified largely by their dominant maineral - rocks with more mixed minerals don’t correlate to a single dominant type of rock easily.

what is weathering?

the physical and chemical breakdown of rocks and minerals at/near the earth’s surface. split into two types - physical weathering, where rock is broken into smaller pieces without changing composition, and chemical weathering, where the minerals themselves are transformed. these kinda work together! physical breakdown exposes surface area, which speeds chemical reactions, which weakens rock and leads to physical breakdown…

what can affect physical weathering?

temperature - repeated freezing/thawing can stress rock until it fractures, also water changing to ice expands in volume and cracks rock

abrasion - rocks carried by water/wind/ice grind against each other and wear each other down

plants/animals - roots growing into cracks pry rocks apart as they thicken, and animals loosen/displace rocks and soil

what can affect chemical weathering?

hydration - water molecules bind directly to a mineral, forming a new hdyrated mineral (ex: hydrated Fe and Al oxides)

hydrolysis - water splits into hydrogen and hydroxyl, the new hydrogen ion often replaces a cation in the mineral structure

dissolution - some minerals (calcite) dissolve directly in water, especially around acids (ex: CO2 dissolves in water to form weak carbonic acid)

oxidation-reduction - Fe, Mn, and S bearing minerals are easily oxidized (lose an electron), destabilizing mineral structure - this produces the red/black colors visible in soil

complexation - organic acids released by decomposing plants/root activity foster dissolution and form organic complexes with Fe ions, further disintegrating them

how do lichens weather rock?

the fungal partner of lichen produces organic acids that break down minerals through dissolution and complexation, releasing mineral nutrients. the resulting loose mineral material/soluble nutrients, along with organic material left by the lichen, supports the growth of higher plants.

weathering differences between climates?

chemical weathering dominates in humid climates! most chemical weathering reactions need water to proceed. humid tropical profiles will be relatively uniform and very weathered, often bright colored Fe oxides.

physical weathering dominates in dry climates. temperate profiles will have largely unweathered rock fragments visible in the soil profile.

explain mineral weathering differences?

not all primary minerals convert to secondary minerals at the same pace, even when exposed to the same conditions. quartz is really resistant and dominates sand globally, while olivine and other minerals that crystallize at high temps are less stable at earth’s surface and weather quickly.

quartz and potassium-feldspar-rich rocks like granite resist weathering longer than rocks with olivine and augite (basalt)

name all the parent materials and their sources?

organic parent material - accumulated plant remains in wet areas where plant growth outpaces residue decomposition, this is peat!

Residual parent material - rocks and minerals weathering in place, formed directly from underlying bedrock, mainly in stable landscapes with limited erosion (relatively rare)

Alluvial - floodplains - deposited by moving water (usually floods), it’s sorted (coarsest particles settle out first, then finer), floodplain soils often w/ sharp layers, these are well suited for crops because of their level topography, proximity to water, high fertility + productivity

Alluvial - fans - stream abruptly loses speed (like at base of a mountain), dumps its load (ha!) in a fan radiating outward

Alluvial - delta - same as fan, but where a stream enters standing water, clayey and poorly drained, often marshes and important wetlands

Coastal/Marine - streams deposit remaining sediment in oceans, sea level shifts expose these deposits as coastal planes, they weather, typically strongly determined by parent material properties

Colluvial - gravity transported, at the base of slopes and cliffs, poorly sorted with lots of angular rock fragments

Till - material deposited directly by ice, unsorted, like colluvial material but more rounded (by abrasion) and compacted (weight of ice), can accumulate into ridges called moraines, drumlins, and eskers

Outwash/Lacustrine - glacial meltwater sorted and deposited sediment into outwash plains, glacial lakes in the ice left behind lacustrine deposits, often fertile but not well drained

Eolian - Dune Sand and Loess - exposed dry land can supply wind-blown parent material anywhere! winds shape sand into dunes, quartz rich and nutrient poor, loess is wind-deposited silt that forms productive silt loam soils

Eolian - dust and ash - sahara-sourced dust contributing nutrients to the Amazon, volcanic ash, dust from China’s loess region, etc etc

what minerals correspond to texture?

sand and coarse silt are dominated by quartz (highly resistant to weathering), clay is dominated by secondary silicate minerals, fine silt/coarse clay have other secondary minerals like iron and aluminum oxides.

what was wrong with the teton dam?

it was built with local wind-deposited silt (loess) instead of clay, moist silt cracks instead of staying compacted and the silt was rapidly washed away once cracks began to appear/

explain soil texture classifications!

the textural triangle converts the relative proportions of sand/silt/clay into a single textural class. there are 12 classes, a representative loam is roughly 40% sand, 40% silt, and 20% clay b/c clay has such an influence on soil behavior.

the cornell soil health assessment simplifies these 12 groups into 3 groups - coarse, medium, and fine.

how did we measure soil texture in lab?

hand texturing provides a quick field method to place soil into a class - it’s a qualitative test. in the lab, the main method suspends a soil sample in water and measures change in density as larger (then smaller) particles settle out. this takes super long - so in our lab, we sieved the sand out, then separated silt and clay with timed settling.

how is soil color described? where does it come from?

the munsell system! breaks color into three components - hue (basic spectral color), value (how light or dark it is), and chroma (how intense or dull it is).

reddish soils come from oxidized iron and maganese minerals, while blue/gray/green soils come from reduced forms of Fe and Mn. in well-drained wamer climates, well-oxidized iron compounds create deep reds/browns. organic matter coats mineral particles with black or dark brown.

under prolonged waterlogging, iron reduces to gray/blue/green, also can wash away entirely and expose gray mineral grains. when oxygen reaches isolated pores/old root channels, the iron re-oxidizes and leaves reddish spots in a gray matrix - a sign of a fluctuating water table.

what are the five factors that determine a soil’s character?

ClORPT!

Cl - climate - precipitation, temperature

O - organisms - vegetation, microbes, soil animals, humans

R - relief - slope, aspect, landscape position

P - parent materials - geologic or organic precursors to soil

T - time - amount of time parent materials have been exposed to soil-forming processes

these factors are deeply independent! a shift in one often means a shift in the others… but when one factor dominates and accounts for most of the difference between a set of soils, it’s named accordingly (climosequence, biosequence, toposequence, lithosequence, chronosequence).

how does climate affect soil formation?

two climatic variables - effective precipitation and temperature - drive most of soil weathering. effective precipitation is the fraction that infiltrates the soil, can be affected by timing/temperature/topography/permeability, while temperature controls how deep weathering reaches into the rock, warmer temperatures mean deeper weathering.

climate also shapes vegetation, which can influence organic matter and soil chemistry.

how do organisms affect soil formation?

organisms drive biochemical weathering, physically mix the soil profile, drive nutrient cycling, and build soil structure. plants can have species-specific nutrient cycling, like maple and hemlock differing in their ability to cycle nutrients (maple has fast-decomposing litter, accelerates acidification deep in the profile, while hemlock needles are slow to decompose and build an acidic surface layer). in arid and semiarid rangelands, plants can accumulate soil and form islands in a feedback loop. biological soil mixing, or pedoturbation, can also shape the soil profile.

how does slope and landscape position affect soil formation?

three dimensions…

slope angle - runoff vs infiltration

landscape position - determines where water and sediment end up

slope aspect - controls solar energy, temperature and heat

a catena is a sequence of related soils that differ by their landscape position, while a toposequence is stricter where the other factors (parent material, climate, vegetation, and time) are held relatively constant.

topography also shapes parent materials, upper slopes often have residual parent material while lower slopes recieve colluvial material and the valley bottom recieves alluvial material.

how does plowing disturb soil?

repeated plowing mixes the O, A, and E horizons into one layer (Ap, for plowing). once formed, an Ap horizon can persist for decades, this is human-caused translocation/transformation (and eventually loss) triggered by machinery.

what’s the difference between soil texture and structure?

soil texture is the relative proportions of sand/silt/clay, while structure describes how those particles are organized into units. these units are called peds or aggregates - the particles in here are more attracted to each other than to the surrounding soil particles. these form along natural zones of weakness, NOT via compression or excavation!

what are the different soil structure types?

single grain - particles are seperate and unaggregated (sand)

massive - large cohesive masses w/ no internal disivision (some clay sediments)

spheroidal - smaller rounded aggregates, formed by roots and exudates, in organic-rich A horizons, most affected by management, can be granular (porous) or crumb (very porous)

platy - thin sheetlike plates, in E horizons, can be inherited from parent material or be a sign of past mechanical disturbance (in clay soils)

blocky - irregular, cube-like, each block molded by surrounding shapes, B horizons where it promotes drainage/aeration/root penetration, can be anglular blocky (sharp edges) or subangular blocky (rounded edges)

prism-like - vertical pillar-like peds, swelling clays, can be columnar (rounded tops, from sodium dispersing clay at the surface) or prismatic (angular, flat-topped pillars, in arid/semiarid subsurface horizons and humid poorly drained soils).

explain the hierarchy of aggregation? which forces dominate?

soil has a nested structure - large peds are made of smaller peds, macroaggregates are made of microaggregates, which are built from tiny packets of clay and organic matter.

different forces dominate at each level - physical/chemical forces at the smaller end of the scale (they depend mainly on clay, fine particles) and biological process at the larger end.

explain flocculation in clay particles?

clay particles have negative charges, they repel each other, cations compressed between two clay platelets bridge the negative charges and form a clay domain (small stack of platelets). multivalent cations (Ca 2+, Fe 2+, Al 3+) can also bind hydrophobic organic molecules to clay surfaces, which can bond to each other.

these build silt-size microaggregates, which stabilize larger aggregates.

explain dispersion in clay particles?

cations like Na+ and K+ and Mg 2+ have weaker flocculating ability, they can’t bridge the repulsion between clay platelets. these platelets remain dispersed and make the soil gel-like, impervious to water and air, and not able to sustain plant growth. mostly caused by Na+ in arid/semiarid soils.

why do cracks form in clayey soils?

as soil dries, the clay platelets draw closer togheter, cracks open along zones of weakness, then repeated wetting/drying can widen this network over time.

mechanisms of biological aggregation?

burrowing and molding by soil animals, enmeshment of particles by roots/fungal hyphae, and organic glues (polysaccharides) produced by microorganisms.

in most temperate soils, granular aggregate formation and stability is mainly driven by soil organic matter! in tropical soils, iron and aluminum oxides coat soil particles and cement aggregates, resisting breakdown from tillage or wetting.

consequences of low aggregate stability?

rainfall can break apart surface aggregates and create a low-permeability layer called a surface seal, and it forms a hard crust as it dries. soil can also be more vulnerable to erosion, either wind or water, this was the dust bowl!

what is tilth? what is friability?

tilth - soil’s physical condition for plant growth, shaped by aggregation, BD, moisture, aeration, and drainage

friability - a key part of tilth, where clods crumble easily into their aggregates (aggregates are stronger than the surrounding clod), this varies sharply with soil moisture!

elaborate on tillage practices and compaction in terms of soil structure…

subsoil compaction from plowing (a plow pan) is common because subsoils are typically wetter, denser, higher in clay, lower in organic matter, and less aggregated. compaction impacts are far deeper in wet soil and with heavy traffic.

long term tillage accelerates the oxidaative loss of SOM, and tillage on wet soil crushes/smears aggregates, destroying macroporosity and creating puddles. the consequences of tillage mainly depend on timing!

this can be remedied by conservation tillage - leaving surfaces covered by plant residues (at least 3-%) to maintain biological habitat, stabilize structure, conserve SOM, and physically shield the soil. you can also add things like gypsum (promotes flocculation), and polymers like polyacramide (stabilize aggregates, reduce erosion).