soils Flashcards
How do soils connect all spheres?
atmosphere: Soils play an important role in radiation budget. Sequester carbon and are at the center of controlling global climate.
hydrosphere: Soils play an important role in water budget. Soils characteristics determine rate of flow of run-off, percolation through soil column to recharge ground water. Soils are important in sustaining freshwater resources for clean drinking water.
biosphere: important to uptake of water and nutrients by plants.
human: relate to human societies (food availability, income etc)
connection: rocks and soils
rocks are the basic substrate for the majority of soils
the geologic cycle
- hot magma rises, adding igneous rock to the crust at top
- Rocks are eroded and transported to the sea
- sedimentation: Sediments form layers over time and undergo lithification to form sedimentary rocks
- Sea floor spreading and subduction of tectonic plates introduces sedimentary rocks deeper into the crust, eventually becoming metamorphic rocks.
What is the chemical composition of the Earth’s crust?
Majority consists of oxygen and silicon. Important plant nutrients: Calcium, Potassium, Magnesium.
What are the 3 rock types and what are they composed of?
- Igneous: composed of minerals formed from molten magma.
- Sedimentary rocks: Composed of minerals weathered from other rocks (eg igneous)
- Metamorphic: formed from secondary pressure and/or temperature processes.
Intrusive vs extrusive igneous rocks
Intrusive: cooled down within Earth’s crust. Intrusive rocks cool slower so have larger crystals.
ie granite, diorite
Extrusive: reached earths surface and cooled down at surface. Extrusive rocks cool faster so develop smaller grain size, making them smoother.
ie obsidian, pumice, basalt
What three minerals is granite made of?
K feldspar, quartz, mica
what is a mineral?
inorganic natural solid compounds with a specific chemical formula and crystalline structure. formed at high temperatures and pressures within earth’s crust. Composed of silicate tetrahedrons: [SiO4]4-.
what rock types are minerals found in?
igneous and metamorphic.
What is a silicate tetrahedron?
1 Si atom surrounded by 4 O atoms. Partial negative charge of oxygen atoms shared with adjacent Si atoms or with cations (eg Fe3+, Mg2+) in mineral lattice. different silica tetrahedra form clusters.
What kind of mineral is Olivine? What is it composed of?
type: island mineral.
composition: Iron and Mg form together to form tetrahedron. Silica tetrahedra are distributed as islands within a sea of other cations.
What kind of mineral is pyroxene? What is it composed of?
type: chain mineral
composition: two oxygen atoms shared by silica and 2 unshared oxygen, creating a chain.
What kind of mineral is mica? What is it composed of?
type: sheet mineral
composition: 3 shared oxygens, 1 shared oxygen in connecting tetrahedrons.
What kind of mineral is amphibol? What is it composed of?
type: double chain mineral
composition: 2 shared oxygen atoms, two unshared oxygen atoms
What kind of mineral is quartz? What is it composed of?
type: 3d structure
composition: 100% Si-O-Si.
What is isomorphic substitution? ***
Replacement of central Si atom by Al asmineral forms from molten magma. Results in aluminosilicate minerals. Results in loss of one positive charge for each atom replaced: Si4+ –> Al3+. Requires incorporation of cation (e.g. K+, Na+, or Ca2+) into mineral lattice to provide extra positive charge.
Feldspar isomorphic substitution
Substitution of 25% of Si4+ by Al3+ and incorporation of cation into mineral lattice.
examples of combos that can form to balance lost positive charge:
Albite - Na: NaAlSi3O8.
Anorthite - Ca: CaAl2Si2O8
Orthoclase - K:KAlSi3O8
The mineralogy of igneous rocks
granite = highly acidic
- mostly consists of quarts and k-feldspar
olivine = ultrabasic
- made up of iron, magnesium
The more acidic = more difficult to weather
Significance of mineral structures
- the stronger the bond between element and O in mineral –> the more resistance to weathering
Si-O-Si bonds are very strong while bonds between Na, K, Ca and O (feldspar) are very weak.
- Changes availability of nutrients
- affects ph if minerals get weathered
- soils developed from granite are more acidic compared to soils developed from ultrabasic parent material such as olivine. Some plants are better adapted to grow on acidic soils than others.
Sedimentary rocks
particles that have been weathered, eroded, transported and cemented together. formed by combination of solar energy and gravity.
processes and sources of sedimentary rocks
- weathering and erosion of exiting rocks –> sandstone
- accumulation of shells on the ocean floor –> limestone
- Accumulation of organic matter of ancient plants –> coal
- Precipitation of secondary minerals –> CaCO3
Sedimentary rock formation processes
1.weathering - generation of detritus via rock disintegration
2. erosion - removal of grains from parent rock
3. transportation
a. overland: dispersal of solid particles and ions by gravity, wind, water, and ice
b. underland - ions dissolved in groundwater flow toward water body.
4. deposition - settling out of the transporting fluid.
5. lithification - transformation into solid rock.
What is a clastic sedimentary rock? 4 clastic sedimentary rock types
Sedimentary rocks formed when particles are moved away from an area via process of erosion.
(large clast size –> very coarse)
1. conglomerate
- boulders, cobbles
- pebbles, gravel
- breccia if pieces are angular
- sandstone
- sand - siltstone / mudstone
- silt - shale
- clay
(small clast size –> fine)
what is a chemical sedimentary rock
new minerals that are formed in situ
5 types of chemical sedimentary rocks
- limestone - CaMg(CO3)2
- evaporites - Na and Ca chlorides and sulphates
- ironstone - oxides and hydroxides of Fe and Al
- hydrothermal deposits - black smokers
- organic - coal, oil
Metamorphic rocks
Harder and more resistant to erosion and weathering than original rock.
Formed by transformation of igneous and sedimentary rock by
1. heating
2. pressure
3. heating + pressure
4. compression + shear
the rock cycle processes
- magma at center of earth consisting of primary mineral material rises to crust where crystalizes (externally and internally) to form igneous rock.
- rocks are weathered and eventually form the substrate of soils (pedogenesis).
- Sediments are further eroded and transported away to rivers, lakes, and oceans where they are sedimented.
- Sediments compacted down by own weight (diagenesis) to form sedimentary rocks.
- Sedimentary rocks undergo metamorphosis via subduction into deeper areas of earth’s crust to become metamorphic rocks.
- Come back to rising magma (anatexis) to become metamorphic rocks oncemore.
What are biogenic materials?
Sediments and soils.
What determines rate of weathering?
- intensity of process
- strength/resistance of rock and minerals
What are the two major types of weathering?
- physical: physical disintegration of rocks and minerals. facilitated by freeze-thaw cycles, variations in water content and temperature, and biological activity.
- chemical: chemical transformation of minerals into new products. facilitated by high temperatures and high biological activity, the production of organic acids and co2 in the soil, and by acidity.
what is weathering?
the breakdown of rocks and minerals. crucial to soil formation.
4 types of physical weathering processes
- Freeze-thaw weathering: water fills small cracks in rocks and freezes, expanding in volume and cracking the rock apart through release of pressure.
- Thermal changes: variations in temperature lead to different rates of expansion and contraction amongst minerals and rocks. There is a larger night/day temperature amplitude in desert because of low cloud cover (less ghg effect -longwave outgoing radiation is lost into space).
- Salt weathering: formation and growth of salt crystals in pores and cracks of rocks creates strong pressure, leading to disintegration of rocks.
- Biological: root systems of plants infiltrate rocks, grow and split it apart.
6 types of chemical weathering
- Direct solution : dissolution of soluble salts
- Hydration: minerals absorb water molecules, disintegrating or forming different minerals.
- Redox: change in valency of element
- Chelation: reaction of insoluble elements with complex (eg Fe, Al), organic compounds produced by decomposition of organic matter.
- Carbonation: reaction of carbonic acid (H2CO3) with carbonates.
- Hydrolysis: reaction of H+ ions with cation in mineral. occurs with H+ from co2 dissolution in water or from acidic rain with the exchange of cation in mineral lattice with hydrogen.
Order of strengths of rocks
Igneous > metamorphic > sedimentary
Relationship between climate and weathering
hotter + wetter –> strong chemical weathering
colder + drier –> strong physical weathering (freeze thaw cycles, thermal variations between day and night)
Processes of soil formation
- bedrock is exposed to atmosphere –> physical and chemical weathering processes that disintegrate rock
- pioneering organisms, mostly lichens, have opportunity to set roots
- Accelerates physical (root systems) and chemical (exudes organic essences) weathering.
- Biomass accumulates, slowly developing parent material. C horizon forms below parent material. Organic layers grow in depth until A horizon is formed at top of ground.
- Soil horizon becomes more mature as weathering processes continue, growing downward in depth untill B horizon is formed (between A and C).
- Larger plants such as trees and brushes set roots as soils are more developed.
Soil horizons, top to bottom
- O horizon: organic material, almost black
- A horizon: mixture of mineral fragments and organic material. Darker brown/grey.
- E horizon: transition zone between A and B horizons. lighter brown in colour.
- B horizon: sub soil. often brown in colour.
- C horizon: weathered bedrock.
- solid bedrock
4 soil forming processes
- Additions
- Precipiation (with included ions and solid particles)
- organic matter - transformations
- organic matter decomposes to become humus.
- primary minerals undergo chemical rxns to form hydroxous oxides, ions, h4SiO4 and secondary minerals such as clay. - transfers
- transfers down: humus compounds, clays, ions, h4SiO4. Ocurrs via movement of rainwater down.
- transfers up: ions, h4SiO4. Occurs via transfers of capillary rise of groundwater - removals
- ions, h4SiO4
- mostly due to leaching as water percolates into soil and sometimes runs off
- erosion is the most important removal process in soil
soil forming factors
- parent material
- sediment type, minerology, ease of weathering - climate
- temperature and precipiation - time
- older soils more strongly developed than younger soils - organisms
- eg deciduous vs coniferous forest, grassland vs forest - topography
- controls water regime, wet vs dry, and rates of soil erosion - human activity
- framing practices, land degradation, tilling, deforestation
How does parent material effect soil formation?
- Different mineralogies have different chemical compositions, some more acidic, some more basic. The more acidic = the more difficult to weather.
- Geologic processes might bring different rock layers to the surface, affecting nutrients and ph.
- sedimentation processes
- fluvial sedimentation during flooding - parent material might come from far away and have been transferred downstream, not necessarily underlying. Sediments left behind are often rich in silt. - other processes that might include parent material not derived from rock
- volcanic eruptions
- glaciation –> move material across surface
-high winds –> carries dust and soil particles from one place to another
How does climate affect soil formation?
Increased precipitation + temperature –> increased rates of weathering + chemical alterations (ie decomposition) –> more developed, deeper soil profiles.
increased precipitation –> base cations which puffer pH against acidity will be leached out and soil will become more acidic.
grassland vs forest soil formation processes
grasslands
- more below-ground biomass
- thick Ah horizon, thinner B horizon
- high ph, higher fertility, higher nutrient availability
forests
- more above-ground biomass
- thinner A horizon, thicker B horizon
- lower ph, lower fertility, lower nutrient availability
- tree roots create channels in soil for water to come through
What is the effect of topography on soil formation processes?
steep slope = free drainage
flat surface = poor drainage
list the key 3 soil properties
- texture
- pH
- organic matter content
Particle sizes listed from largest to smallest
gravel > sand > silt > clay
particle vs mineral
particle - size determines how soil responds to particular hydrology
mineral - chemical composition of particle
5 impacts of soil texture
- influences soil infilitration rate, determination rates of overland flow and soil erosion.
- influences soil permeability and therefore drainage
- controls available water capacity of soil
- determines ability to supply water to plants for transpiration - influences soil structure, allowing root growth and aeration.
- provides cation exchange capacity for retention of Ca, Mg, K supply to plants and buffering against acid rain.
Soil texture impact on field capacity, wilting point and available water
optimal conditions for plant growth is between sandy loam and clay loam
sand (large particle size)= low field capacity, slow wilting point
–> less available water since soil drains quickly but plants can easily take up this water from soil
clay (small particle size)= large storage capacity, field capacity, relatively quick wilting point
–> sheet like structure = large surface area
–> more available water but holds on to water and plant nutrients very tightly
field capacity
amount of water left behind in soil after three days of drainage.
wilting point
the amount of water per unit soil volume that is held so tightly by the soil matrix that roots cannot absorb this water and a plant will wilt.
significance of cation exchange capacity
Important control of pH and soil nutrition. Plant exude base cations or respire, adding acidity into system that can be buffered by base cations.
What determines the amount of organic matter in soils
the balance between input and output establishes the amount of organic matter in soils.
input = dead plant and animal tissues
output (as co2) = from decomposition by organisms (bacteria, fungi, earthworms..)
greater npp –> greater input (more organic matter)
greater rate of decomposition –> greater output (less organic matter)
rank tropical forest, temperate forest, boreal forest, and bog organic matter content + explain the reasons for the diffrences.
higher temp + precip = higher npp and decomposition
tropical forest (higher temperature, higher rainfall)
- hold not as much as take in because rate of decompostion so high
temperate forest (colder temperatures, lower rainfall)
- low npp but slow decomposition = high organic matter content. carbon from atmosphere more efficiently stored in system
boreal forest (cold + dry)
- most efficient carbon storage
- very high organic matter content
bog
- very high water content -> low ph due to leaching of base cations + anaerobic soil –> less microbe activity –> lower rate of decomposition
-nearly 100% organic matter content in soil because have very little output
3 factors controlling decomposition rate of organic matter
- temperature and precipiation
- plant tissue type
- tissues rich in N and P decompose quickly because nutrients are used by organisms in the soil
- pine needles are acidic - soil properties
- fertility - increase
- texture
- organisms - increase
significance of soil organic matter
- Improves soil structure and porosity
- increases infiltration rate and available water capacity - supplies nutrients (Ca, Mg, K, N, P) to plants through slow decomposition.
- Humus (part of soil organic matter) possesses a high cation exchange capacity leading to retention of cations (Ca, mg, k) and buffering against acid rain.
Importance of soil pH
- determines environmental conditions
- determines availability and accessibility of soil nutrients
- determines what community life can be supported
What determines soil pH?
the comparative concentrations of H+ (more acidic) and OH- ions (more basic).
higher precipiation = lower pH
the more water that perculates through the soil column, the more base cations that buffer against acidity will be leached from the soil. Higher temperature
higher temperature = lower pH
the molecular vibrations in the solution rise resulting in the ionization and formation of H+ ions.
Importance of soil pH
- Low soil pH can lead to aluminum toxicity for plants
- Determines nutrient availability, in particular that of phosphorous.
- Influences cation exchange capacity
- Influences earth worm activity and other fauna in soils
- strong influence on microbial community in soil.
Physical properties of soil horizons
colour, depth, texture, structure, moisture
ethnopedology
how different cultures around the world describe soils. Many cultures have traditionally names for various classes of soils that help convey the people’s collective knowledge about their soil resources.
Canadian system of soil classification (CSSC) and 5 categorical levels
Hierarchal structure from orders to families. retains old European names that contain info about soil that makes communication easy.
categorical levels
1. order: properties that reflect the environment and soil forming processes.
2. great group: subdivision of order reflecting differences in dominant processes.
3. subgroup: differentiated by content and arrangement of horizons.
4. family: differences in texture, mineralogy, climate and chemistry
5. series: detailed features of the pedon differentiate subdivisions of the family.
Soil horizons in the CSSC
Soil horizons are named and standardized as diagnostic in the classification process. Several mineral and organic horizons are used.
3 mineral horizons
A = mineral horizon formed at or near the soil surface
B = accumulation of material from ae horizon or by alteration of parent material
C = horizon with little evidence of pedogenic activity
4 organic horizons:
O = organic material
L = litter litter, readily recognizable
F = partially decomposed leaf and twig material (folic material)
H = humic material, decomposed organic materials with no original structures.
world reference base
provides a global vocabulary for communicating about soils since many countries have their own system. consists of 32 soil reference groups, differentiated mainly by the pedogenic process (eg accumulation of clay) and the parent material (eg volcanic ash).
Nutrient inputs
- weathering of minerals from rocks (rock derived nutrients ca, mg, k, and p)
- death and decay of vegetation (litter, soil organic matter provides N and P)
- atmospheric deposition dry and wet (N and particulates containing Ca, Mg, and K)
Nutrient Outputs
- plant uptake and harvest
- erosion
- atmosphere (loss of gaseous forms of N)
- ground water table and streams through leaching and erosion.
basic nutrients
c, h, o
macronutrients
primary: n, p, k
secondary: ca, mg, s
micronutrients
fe, mn, zn, cu (copper), b, mo, cl, si, co
nutrient cycle in undistributed vs disturbed systems
undisturbed system: cycling is ‘tight’ with inputs relatively equal to outputs
disturbed system: cycling can be “loose”, with outputs > inputs
ease of retention/loss of nutrients N, K, Ca, P
ease of loss : N > K > Ca > P
N as NH4+ on clays but No3- is very mobile in soils
base cations (ca, mg, k) on cation exchange complex
(from clay and organic matter)
phosphgate retained most tightly due to negative partial charge. strongly absorbed on clays, Fe-Al oxides/hydroxides, and Ca.
importance of soil pH in base cation supply to plants
Nitrification and P sorption control
plants prefer to take up nitrate because it is an anion, meaning it is mobile in the soil, and translocates quickly.
ammonium is a cation so it is retained in the soil.
control of retention of nutrient in soil matrix is controlled by the soil pH and the base cation supply to plants.
Nitrogen cycle process ***
- nitrogen fixation: incorporation of N2 from atmosphere.
- mineralization: decomposition of organic matter to mineralize N as ammonium (NH4+)
- soil microorganisms fix atmospheric nitrogen and form ammonia (NH3). NH3 is a precursor of amino acids that later becomes mineralized to form NH4, a cation that is very reactive with soil organic matter and clay surfaces. - nitrification: conversion of NH4+ to nitrate (NO3-)
- bacteria use ammonium as energy source and form nitrite as waste product (=toxic to plants and a potent GHG). Another strain of bacteria oxidizes nitrite to form nitrate, a nutrient that plants can take up. Over leaching into groundwater of nitrate can make water toxic but humans have enzyme that ensures doesn’t become toxic. - denitrification: conversion of NO3- to N2 and nitrous oxide (N2O) by microbes.
- leaching: loss of soluble NH4+ nad NO3- to surface and ground water.
Phosphorous cycle processes
- phosphros enters cycle through weathering of minerals, dust deposition, decomposition (mineralization) of organic matter, usually as organic / inorganic phosphate (PO43-).
- solubilization: dissolution of phosphorous from soil minerals.
- sorption: chemical process by which p becomes attached to surfaces of soil particles. Due to high negative charge, phosphates sorb strongly to iron and aluminum oxides and clay minerals.
- Mineralization: decomposition of organic matter (enzymatic hydrolysis of organic P compounds) to release inorganic phosphate (PO4 3-)
- Immobilization: uptake of inorganic P by plants and microorganisms and subsequent transformation to organic P.
- Leaching: loss of soluble organic and inorgic P to surface and ground water.
Relationship between P availability and soil pH***
Ph controls the availability of phosphate in soil nutrition.
pH < 6
As ph becomes lower (ex through acid rain), phosphates will react more with iron and aluminum and become chemically fixed (occlusion). Less phosphate is available to plants because it is transferred into chemical species that plants can access.
ph 6-7
phosphates are relatively available.
ph > 7
As pH becomes higher, phosphates react strongly with calcium, forming calcium phosphate minerals that are not available to plants but not strongly occluded.
How does farming impact phosphate availability?
Takes out a lot of phosphates fixated in pools faster than can be replenished by natural pathways from other pools.
the potassium cycle processes
- input via weathering and dust deposition
- solubilization: dissolution of potassium from soil minerals.
- sorption: chemical process by which K+ becomes attached to surfaces of soil particles.
- Immobilization: uptake of K+ by plants and microorganisms
- Leaching: loss of soluble K+ to surface and groundwater
deficiency symptoms of N, P, K
N - nitrogen deficiency causes pale, yellowish green corn plants with spindly stalks. symptoms appear on leaves as a v shaped yellowing, starting at the tip and progressing down the midrib toward the leaf base.
P - deficiency is usually visible on young corn plants. it readily mobilizes and translocates in the plant. Plants are dark green with reddish-purplish leaf tips and margins on older leaves.
k - deficiency is first seen as a yellowing and necrosis of the corn leaf margins, beginning on the lower leaves.
Anthropogenic additions to nutrient cycle
Rich countries use more fertilizer than poor countries. As china started to farm more intensively, they needed more fertilizer. States that don’t regulate fertilizer –> more fertilizer applied than needed.
issue =
- fertilizer feeding soils rather than plants because more is applied than plants can take up
- P and N (as nitrate bc mobile) carried overland via runoff or leaches into groundwater.
- leads to transfer of nutrients from land to aquatic ecosystem
- primary pathway for nutrient pollution, other than atmospheric nitrgous deposition (amonious gasses volatize into atmosphere, form ammonium, which is deposited downwind in ecosystems).
eutrophication
excessive nutrient concentrations –> excessive plant growth (algal blooms / cyanobacteria) –> dead plant material accumulates in water column –> decomposes and depletes dissolved oxygen in water collumn –> aquatic organisms die
cyanobacteria produce younger toxins which make drinking water unpotable and restrict recreational activities –> large economic damage
how ocean dead zones form
- freshwater runoff creates layer of freshwater in gulf, cutting off saltier water below from contact with oxygen in the air.
- nitrogen and phosphorous from fertilizer and sewage in the freshwater layer ignite huge algae blooms. when the algae die, they sink into the saltier water below and decompose, using up oxygen in the deeper water.
- starved of oxygen and cut off from resupply, the deeper water becomes a dead zone. fish avoid the area or die in massive numbers. tiny organisms that form the vital base of the gulf food chain also die. winter brings respite, but spring runoffs start the cycle anew.
Solutions to dead zone issues
- reduce fertilizer application rates and increase efficiency in application (ex precision agriculture)
- improve wastewater treatment technology and ban nutrient additions where unnecessary (ex phosphates in detergents)
- nature-based solutions ie vegetated riparian buffer strips, and wetland restoration.
vegetated riparian buffer zones
- intercept nutrients before reach stream. If roots are deep enough, they can intercept subsurface flow nutrients.
- promote denitrification in streams and rivers
proximate vs ultimate nutrient limitation
addition of proximate limiting nutrients stimulate biological processes and growth.
ex nitrogen - moves quickly so impact kicks in faster but solutions will also have immediate impacts.
addition of ultimate limiting nutrients leads to transformation of whole ecosystems and or species composition
ex phosphorus - accumulates in soil, held tightly and released slowly. residence time is much longer so difficult to solve. have legacy p in soils from years ago. P changes whole ecosystem because allows plants to fix nitrogen from atmosphere efficiently such that blue green algae can grow rapidly.
soil erosion
the wearing away of the topsoil by the natural physical forces of water and wind or through forces associated with human activities (e.g. tillage, cattle grazing, tree harvest).
top soil
soil which is high in organic matter, fertility, and soil life
Impact of soil erosion on croplands
reduces the productivity of croplands and natural ecosystems and contributes to the pollution of adjacent watercourses, wetlands, and lakes.
types of soil erosion
- soil creep: slow downslope movement of soil particles in response to disturbances (expansion/ contraction with wetting/drying, soil faunal activity etc). very slow rates (< 0.05 tons ha-1 yr-1) that increases with slope angle.
- landslides and earthflow: rapid mass movements when soil strength is exceeded by gravity, usually follows heavy rainfall.
- fluvial: by running water. By far the most important form of erosion.
- Aeolian: wind removal of surface layer; requires absence of protective vegetation cover, dry soils and strong winds.
processes of fluvial erosion
- raindrop impact and splash
- detaches soil particle and destruction of soil structure. making it easier for mobilization by overland flow - creation of overland flow
- transport particles in downward direction - deposition
- deposited in small hollows along hill slope or at bottom of hill.
- fertile soil may accumulate at downslope but might suffocate young seedinglings and has no structure (poor aeration –> poor redox potential)
3 types of fluvial erosion
- sheet erosion
- whole areas erodes in fairly homogenous manner - rill erosion
- steeper hillslopes have small channels where water flows faster ex plowing lines in agricultural fields. - gully erosion
- enhanced rill erosion. if continues to occur for many years it becomes a gully (deep and wide crevace). can occur very quickly with large rainfall events.
what is the eurasian loess belt?
Largely consists of silt that was transported away during ice age through wind erosion and deposited in areas where there was a windshield. Productive, fertile region where food production is occurring.
ex china loess plateau
tibetan plateau served as wind shield to slow down westerly winds. eroded material transpoted downstream to small channels and rivers, entering into the Chinese sea and depositing material in the yellow river delta.
Characterized by deep gully erosion all around farming systems on steep terraces.
forest vs agricultural land impact on soil erosion
In forest, a large part of rainfall is intercepted and infilitrated and a small part becomes overland flow.
In unimproved pasture, all rain hits the ground. because of cattle, soil has been compacted and the structure is destroyed, severely reducing infiltration and increasing overland flow –> increasing erosion.
Sheet erosion on agricultural fields depsoit material at far end of field, suffocating the crops located there. topsoil eroded away from field –> infertility.
2 methods of measuring soil erosion
- collect soil downslope in troughs and measure the amount of soil that has been eroded over distance and area.
- simple but imprecise - measure loss of soil against stable existing surfaces ie tree roots
- tree roots sticking out of ground indicate that in the past there was soil much higher.
universal soil loss equation
A = R x K x LS x C x P
where,
A = erosion rate (tons ha-1 yr-1)
R = rainfall erosivity
K = soil erodibility
LS = combination of length and slope of field
C = crop type (bare soil=1, crops=<1, forest=near 0)
P = conservation measures applied (no conservation=1, contour ploughing is high, terracing is low)
A, R, K defined by farmers can alter all factors to limit erosion.
What determines rainfall erosivity?
- rainfall intensity: affects the creation of overland flow by exceeding soil infiltration rate.
- rainfall energy: higher kinetic energy + mass –> increased ability to splash soil particles downslope and destroy soil aggregates
what determines soil erodibility?
- soil infilitration rate (sand > loam > clay)
- ease of detachment of soil particles (silt > clay > sand)
- silt easier to detach than clay even though larger and heavier because of ease of detachment from soil particles vs clay sticks together with soil organic matter. - ease of transport of soil particles
(clay > silt > sand > gravel)
What determines the slope length factor ?
- length of slope
- longer slopes = greater cumulative overland flow from upslope, leading to faster movement, thicker layer and greater capacity to erode and carry soil. - slope angle (larger effect)
- steeper slopes mean faster overland flow and greater capacity to erode and carry soil.
on-site environmental effects of accelerated rates of soil erosion
- reduced water availability
- increases surface runoff (reducing water for plant growth)
- removes finer particles (clay, organic matter), leaving sand and gravel particles which have lower available water capacity, and thus ability to store water and supply crops. - reduced soil fertility
- loss of fertilizers, especially N and P, by overland flow
-removal of smallest particles which have highest available nutrient content and cation exchange capacity - reduces rooting depth for plants (shallower soils)
- gully creation
- loses agricultural land and increases accesibility costs of farming - greater energy costs
- more plowing, tillage, seeding, and fertilizer needs - leads to positive feedback
exposure of lower soil horizons by erosion –> decreases infilitration rates –> less vegetation –> increases overland flow –> more erosion –> exposure of lower soil horizons
Strategies to reduce soil erosion rates in agricultural systems
- change crop type or planting/harvest schedules (C)
- protect soil surface and increase infilitration rate - add organic matter (mulch) or reduce tillage practices
- change length and angle of field by creating terraces or ploughing parallel to slope (LS)
- reduce grazing densities to allow grass cover: > 25% grass cover is critical in reducing erosion rates (C)
strategies to reduce soil erosion rates in forest systems
- reduction of itensity of timber harvest (clearcutting only gentle slopes)
- suitable methods of tree removal
- scheduling of harvest during dry season or when ground is frozen
- design and management of roads and skidding trails to ensure doesn’t become flow path for erosion / rill creation
- buffer strips along stream channels
off-site environmental effects of accelerated rates of soil erosion
- sedimentation of eroded soil in river channels, dams, reservoirs –> eutrophication, pesticide pollution –> deteriorates water quality
- increased frequency of flooding
- increased surface runoff
- channel volume may be reduced by deposited soil
3 types of wind erosion
- creeping - large particles creeping on land surface
- saltation - smaller particles jump
- suspension - airborne particles in the wind
controls of wind erosion
- soil moisture
- soil cover
- tillage
- barriers
one method of reduce wind erosion
shelterbelts - lines of trees that slow down wind before hit field
major microorganisms
- bacteria: important for biogeochemical cycling of elements, especially nutrients.
- algae: ability to fix nitrogen to provide N to other organisms.
- protozoa: regulate microbial population. can be parasites and predators.
- fungi: thrive in moist, acidic, nutrient poor soils because good at decomposing dead biomass.
impacts of mycorrhizal***
- enhance nutrient and water uptake
Mycorrhizal fungi colonize the root network called the rhizosphere. symbiotic relation= take carbohydrates from the plant, efficiently taking up nutrients through large surface area of hairlike rootings and provide that to the plant. - help with nematode pest (invasive species that damages harvest)
- stabilize soil
exude organic acids and a certain kind of protein rich gel that helps with root growth and plant uptake. Sticks cay minerals nad small organic compounds together, helping with development of soil structure. - important evolutionary role in helping plants adapt to the terrestrial environment
How to promote mycorrhizal
- organic farming
- cultivation of certain crops
- continuous soil cover
- after harvesting an additional plant is seeded that covers soil - mild soil tillage practices, mild pesticide applications
Impacts of earthworms on soil properties
- ingest and excrete large amounts of top soil
- affect soil structure and nutrient availability, by mixing of organic and inorganic (mineral) fractions and stimulating decomposition of organic matter, further developing A horizon.
- improve soil structure
- crumblke –> allows for good aeration and penetration by plants - potential to increase pH and concentrate soil nutrients
= ecosystem engineers bc change soil environment that live in
What determines distibution of earthworms
- last glacial maximum
mostly beneath line of last glacial maximum (US), mostly in east. - acidification and application of pesticides gets rid of earthworms
- water availability
- higher amount in forest soil than arable (cropping) soil
Impact of Termites
Earth mounts = sandier, more moist, more organic matter
make earth mounts that have more sandy soil stuck together with mucus. higher water availability in mounds because mucus helps with retention of water. Organic matter brought in through transport system of termites. decomposed litter enriches the nutrients of mounds.
Abondoned termite mounds can become islands of soil fertility in savannah landscapes and allow photosynthetic activity to persist even in drought.
Impact of prairie dogs
Ph and soil nutrients is higher closer to entrance of prairie dog colony because of excretion of droppings and mixing of soils.
