Soils Flashcards
What is soil?
a mix of broken rocks and mineral, living organisms and decaying organic matter called humus.
- also includes air and water
“natural bodies of animal, mineral and organic constituents differentiated into different horizons of variable depth”
What is humus?
Give characteristics
the type of organic matter in the soil that is formed by the decomposition of plant and animal matter
- dark
- soft
- rich in nutrients
Why is it essential for soils to have all of their respective components?
makes it possible for plants, bacteria, fungi, small animals (e.g. earthworms) to live in soil
What is the importance of soil?
- basis of out food supply
- regulator of water supply
- atmospheric modification
- habitat for many organisms
- nutrient cycling
- medium for plant growth
How much animal life can live in an acre of soil?
5-10 tonnes
What is the percentage of:
a) pore space
b) soil solids
in an average soil?
a) 50
b) 50
What are the components and percentages of soil solids in an average soil?
Minerals 45%
Organic matter 1-5%
What are the components and percentages of pore space in an average soil?
Air 20-30%
Water 20-30%
What do the proportions of components of soil influence?
Soil physical properties:
- texture
- structure
- porosity
In turn, these properties affect air or water movement in the soil and thus the soil’s ability to function. Orgs in the soil need air and water to survive
What are the features of fertile soil?
(first 5 most important + 4)
Water content
pH
Aeration
Texture/structure
Temperature
Soluble materials
Dead organic matter
Soil biota
Soil depth
Why is water content an essential component of fertile soil?
- all orgs in soil need water to survive, incl. plants
- a fertile soil allows good drainage so it does not become waterlogged but still retains enough water for the survival of the soil biota such as plant nutrient uptake
- plant nutrients are absorbed in ionic form, dissolved in water
- where grains do not touch then water flows through easily e.g. sandy soil
What types of water can plants take up?
Gravitational/free water
Capillary water
Why are soluble materials an essential component of fertile soil?
IN FERTILE SOILS:
- contains macronutrients (e.g. N, P, K) in ionic form mainly as nitrates, phosphates, potassium ions
- also contain micronutrients, including boron, cobalt, copper, iron, manganese and magnesium
- toxic ions (e.g. aluminium, heavy metals) are adsorbed onto the surface of mineral particles (usually clay) so they cannot dissolve in the water where they could harm soil orgs
What is capillary water?
Water held by the soil particles so does not drain from the soil but can be absorbed by plants
What is hygroscopic water?
water held by adhesive forces on the mineral particle, cannot be absorbed by plants
What is gravitational water?
water that is freely present and is held loosely. It can be absorbed by plants but drains easily as it flows quickly through large soil pores
enters soil through precipitation then infiltrates through the pore spaces
Why is aeration an essential component of fertile soil?
Most living orgs in soil need O, and many processes that increase fertility are aerobic, so well aerated soils are likely to be more fertile
- air = present in pore spaces that are not completely filled with water
- if no air is present (anaerobic) and the soil is waterlogged, the the roots will be unable to grow and will therefore mean crop yields reduce and eventually cease
What is soil air content dependent on?
Soil texture/structure
Compaction
Rainfall
Why is temperature an essential component of fertile soil?
Soil is heated largely by incoming solar radiation
Temperature affects activity of all orgs in soil including plants, invertebrates and bacteria
What is soil temperature affected by?
Veg = insulates the ground (intercepts some radiation) and reduces the range of temp extremes
Air = poor conductor. Once radiation enters the soil, air traps it (insulation)
Air in the soil needs less heat to raise its temp by 1 degree compared to water which requires the most, dry soil heats up quick
How does
a) high temps
b) low temps
affect soil?
a) decomposition increases. This increases organic matter added to the soil and increases nutrient recycling
b) less growth. This means there is:
1. less org matter added to the soil
2. less interception of rainfall, so more leaching
Why is dead org matter an essential component of fertile soil?
fertile soils usually have high dead org matter content, which releases plant nutrients as it decomposes (e.g., N, P)
- also increases water retention and provides food for soil biota
Why is pH an essential component of fertile soil?
Outline the consequences of acidic/alkaline soils
fertile range usually = pH5.5 - pH7.0
- affects nutrient availability in soil
- the range tolerance for most plants and other soil biota
- acidic soils can increase the leaching of plant nutrients and damage root cell membranes
- under alkaline conditions, phosphates become insoluble
What happens in the soil if the soil is too acidic?
- less plant and invertebrate activity
leads to - less organic matter leads to
- reduced decomposition leads to
- fewer nutrients added to the soil leads to
- reduced fertility leads to
- 1
Why is soil biota an essential component of fertile soil?
Living orgs are involved in many soil processes affecting soil fertility
- detritivores (e.g. beetle larvae, millipedes, woodlice, slugs) break up dead org matter and release nutrients into the soil. Worms are detritivores, they also increase soil drainage and aeration by creating tunnels in the soil
- decomposers (including bacteria and fungi) break down dead org matter. They secrete digestive enzymes and rely on detritivores to physically break up the dead org matter and increase its surface area
- nitrogen-fixing bacteria convert gaseous nitrogen into ammonium ions
- nitrifying bacteria oxidise ammonium ions to nitrite ions then to nitrate ions
- mycorrhizal fungi form symbiotic relationships with plant roots and aid phosphate uptake by the plants
What is soil texture and how does it contribute to soil fertility?
the property controlled by the proportions of the different size categories of mineral particles present in the soil
- it regulates the accessibility of air, water and nutrients for uptake by plants
What are the different soil particle type sizes?
Clay = <0.002
Silt = 0.002 - 0.02
Sand = 0.02 - 2.0
What does the size of the mineral particles in soil influence?
How do large soil particles affect this?
- infiltration of water through soil
- amount of air entering soil
Large soil particles = larger pore spaces and greater infiltration of air that can enter the soil
What is the difference in drainage rate between clay soils and sandy soils?
SANDY SOILS:
large pore space = rapid drainage = reduced water content = increased aeration
CLAY SOILS:
low effective porosity = poor drainage
What is the difference in capillary action between clay soils and sandy soils?
SANDY SOILS:
no capillary rise
CLAY SOILS:
tiny pore space = underground water can rise towards the surface
Give characteristics of sandy soils
high porosity
high permeability
Give characteristics of clay soils
high porosity
low permeability
high nutrients
inaccessible for roots, and stunted root growth means nutrients can’t be accessed
Give characteristics of loam soils
best of both clay and sandy soils
BEST for agriculture
- sand ensures good drainage and good air supply
- clay retains nutrients
- silt holds the sand and clay together and makes the soil easier to work with
Why do sandy soils feel grittier than other soil types?
Because the grains/peds/particles are coarser/bigger
Why do some soils form longer ‘ribbons’ than other soils?
CLAY
- has platy peds that join together easier
What is the difference in aeration between clay soils and sandy soils?
SANDY SOILS:
well drained = pore space filled with air
CLAY SOILS:
low aeration rate = waterlogged
How would you rate all of silts soil properties?
Averageq
What is the difference in nutrient retention between clay soils and sandy soils?
Nutrient ions adsorb easily onto clay particles but not onto sand particles and are leached
What is the difference in thermal capacity between clay soils and sandy soils?
SANDY SOILS:
lower thermal capacities = they warm up quicker after cold weather
CLAY SOILS:
high water content = high thermal capacity so they warm up/cool down slowly
What is the difference in root penetration between clay soils and sandy soils?
SANDY SOILS:
low capillary water = easier to penetrate
CLAY MINERALS:
held together by the capillary water on the particle surfaces = hard for plant roots to force their way through the soil particles
What is the difference in ease of cultivation between clay soils and sandy soils?
The lack of adhesion between the particles in sandy soils makes it easier to cultivate them than clay soils
What is the mix of sand/silt/clay in loam soils?
Why is this the ideal mix?
40:40:20
- ideal for crop cultivation due to good drainage, water retention, and high nutrient content
What is the ideal soil for agriculture and why?
Loam
- mix of sand (aeration and drainage)
- and clay (nutrient retention)
- silt binds the clay and sand together
What is regolith?
unconsolidated loose deposits covering solid rock
How does soil form?
- weathering of rock
- colonisation by plants and humification
- translocation of particles and dissolved minerals down or out of the soil
Why can soil be described as an open system?
Because energy can be gained and lost to its surroundings through inputs, outputs, transfers, transformations, and storage
What are the inputs into soils?
- precipitation
- atmospheric gases (e.g. N)
- inorg matter/nutrients from parent rock
- org matter from decay (e.g. leaf litter)
- energy
What are the outputs from soils?
- evapotranspiration
- leaching
- soil erosion
- uptake by plants
What are the transfers in soil?
- Biological mixing
- Translocation
- Leaching
What are the transformations in soil?
- nutrient cycling
- weathering
- decomposition
What are storages in soil?
- Minerals
- Air
- Water
- Organic matter
- Organisms
- Nutrients
What is soil’s structure?
soil particles form aggregates called PEDS which determines the size and nature of the pore spaces (porosity) which in turn influences the drainage (permeability) and air content of the soil
Give 6 examples of ped shapes and zoom in on 2 of them
blocky, columnar, massive, single grain, platy, crumb
PLATY PEDS:
- large and flat
- reduces drainage, aeration, root penetration
- so produce less fertile soils
CRUMB PEDS
- small and round
- produce good drainage, aeration, easy root penetration
- so improve soil fertility
What are peds?
Units of soil described by their shape, held together by electrical charges on the surface of the minerals and organic matter
What are ped particles bound together by?
- polysaccharide gums produced by decomposition
- fungal hyphae
- roots
- the action of soil biota
- hygroscopic clay particles
What factors affect the formation of peds?
- clay and soils with lots of org matter are more likely to form strong peds
- sand soils with little org matter often have little or no ped development
Describe the first process of soil formation
- WEATHERING OF ROCK
- breaks the parent rock into pieces and forms the regolith
- clay particles then join together (process of aggregation) to form peds
- the peds stack around each other to form a soil structure
What is weathering?
Give an example
the breakdown of rocks in situ as a result of a change in temp, addition of water and/or physical prizing apart of a rock
e.g. tree roots break up cliff faces and allow water to enter
Describe the second process of soil formation
- COLONISATION BY PLANTS AND HUMIFICATION
- once the rock has been broken apart orgs (e.g. algae and lichens) can start to colonise the area, and then add org matter to the soil when they die
- as org matter decomposes it releases nutrients and organic acids - these acids attack clays, releasing iron and aluminium in a process known as chelation
- as org matter builds up, orgs such as earthworms colonise the soil. They drag and mix org dead org matter into the soil - ORG SORTING
What is humification?
the process by which dead org matter is decomposed to form humus
- most active in the O layer where it can cause acidification of the soil and form particular soil types
- if humus is slow to decompose in cold, wet, upland areas, (e.g. peat bogs) it produces a fibrous, acidic, nutrient-deficient surface horizon known as a mor
Describe the third process of soil formation
- TRANSLOCATION OF PARTICLES AND DISSOLVED MINERALS DOWN OR OUT OF THE SOIL
- translocation = the movement of soil components in any form (solution or suspension) or in any direction (downwards or upwards)
- occurs with greatest effect with either v high or v low rainfall
- 3 types: leaching, eluviation, illuviation
Describe leaching
- the carrying of elements in dissolved form
- common in areas of intense high rainfall
- where precipitation exceeds evapotranspiration and soil drainage is good, rainwater chemically weathers the clays and dissolvable salts in the soil
e.g. Ca, Na, K, Mg are dissolved by rainwater and carried down the soil profile in solution
What is an intensive form of leaching?
Podsolisation
- particularly common in cool climates where evaporation rates are low
What are the controls on leaching?
- SOLUBILITY OF AN ELEMENT
- more soluble = more susceptible to leaching
- e.g NaCl rarely found in soils as nearly always removed
- e.g. CaCO3 far less soluble, may remain in soil for thousands of years, making leaching more rapid as the percolating water becomes more acid - RAINFALL
- more = more leaching
What is eluviation?
When small, light particles are carried downwards in suspension by rainwater infiltrating the soil and picking up particles
What is illuviation?
Where both leached and suspended elements are carried downwards towards the water table where they are precipitated and deposited in horizon C
What are the main factors affecting soil formation?
Climate
Parent rock material
Time
Topography
Organisms
Human activity
How does the parent rock material affect soil formation?
CONTROLS:
- depth
- texture
- colour
- drainage
- mineral component
Give an example of a parent rock influencing soil
GRANITE
- breaks down into quartz, mica and feldspar
QUARTZ - chemically stable, mainly affected by physical weathering. Forms sand and silts.
- coarse texture
- good drainage
- shallower
- few nutrients
MICA AND FELDSPAR - chemically complex, mainly affected by chemical process such as hydrolysis. Forms clays.
- impermeable soils
- deeper soils
- more nutrients
- platy texture
- good drainage
- shallower
- few nutrients
How does climate affect soil formation?
- affects rate of weathering (fast = hot climates and high rainfall)
- high rainfall means lots of leaching, altering horizons
- hot and arid = more evapotranspiration than precipitation so salinisation occurs
What is salinisation?
as moisture evaporates from soil, salts are drawn upwards towards the surface in solution by capillary action until eventually the salts may form a hard crust called calcrete
How does topography affect soil formation?
- height of land increases = increased precipitation and decreased temp
- south facing slopes in N. hemisphere = warmer than north facing
- steeper slopes encourage drainage (faster flow-through rates and surface run-off) = accelerated soil erosion
How do organisms affect soil formation?
- plants/animals recycle nutrients to the soil
- worms and termites aerate it
- micro-orgs (e.g. fungi, bacteria) assist in decomposition and decay of dead veg thus releasing humic acids to the soil
- more earthworms = less distinct horizon boundaries
How does time affect the rate of soil formation?
- takes estimated 3,000-12,000 years to form sufficient depth for farming
- immature soils have developed few properties = little weathered fragments and org matter and have weak horizons
- the longer the soil has had to form, the more weathering and org matter breakdown has happened, hence deep soils to support a higher species diversity
What is the biggest threat to soils? Give an example
Soil erosion
e.g. Sahel, Africa - overgrazing and deforestation results in loss of topsoil, and some of the land has become biologically sterile
Give the order of soil horizons
O horizon
A horizon
B horizon
C horizon
Bedrock
What is the O horizon?
surface (leaf) litter and partly decomposed organic matter
What is the A horizon?
topsoil with humus, living orgs and inorg minerals
What is the B horizon?
Subsoil with clay, iron and aluminium compounds accumulated from leaching
What is the C horizon?
weathered parent material and partially broken down inorg minerals
What is bedrock?
The parent material of the soil
Why is soil a non renewable resource?
- takes many years to form
- being made slower than we can use it
- takes a huge amount of effort for it to even bounce back slightly
Why might a soil not have much biota?
- lots of clay - hard to cut through
- too acidic/alkalinic
- low sand - no aeration/oxygen
Which human activities affect soil fertility?
- ploughing
- fertilisers and farming
- irrigation
- soil compaction
- pH control
How do fertilisers affect soil fertility?
How can other farming affect soil fertility?
INCREASING NUTRIENT LEVELS:
- adding inorg. fertilisers or org. fertilisers (manure etc)
- supporting natural processes that increase nutrient levels such as bacterial nitrogen fixation (legume crops etc)
DECREASING SOIL FERTILITY: (other farming)
- erosion
- biomass removal
- inhibiting natural processes that increase increase nutrients OR they increase leaching of dissolved nutrients
How does ploughing affect soil fertility?
makes soil more aerobic - increases rate of N fixation, nitrification and decomp of dead organic matter
- breaks soil structure/loose/more easy to erode
How does irrigation affect soil fertility?
increases fertility of soil where water is the limiting factor on growth
- having sufficient water also allows plants to keep their stomata open and continue gaseous exchange when the soil would otherwise be dry
- water also dissolves which can then be absorbed by plants in ionic form
How does soil compaction affect soil fertility?
excess use of heavy machinery/high livestock densities can compact soil
- reduces aeration, makes waterlogging more likely, especially if the soil is wet
How does pH control affect soil fertility?
controlling soil pH helps ensure nutrients are soluble but not too easily leached
What percentage of land affected, and by what, and why?
33% moderately-highly degraded due to the erosion/salinisation/compaction/acidification/chem pollution of soils
Give some negative soil stats
- in 60 years, all topsoil will be gone
- 1/3 topsoil has been lost due to chemical agriculture
- 2/3 of the world is desertifying
- 1 billion people estimated to be climate refugees by 2050
- more nitrogen fertilisers are needed now for one bushel of wheat than 1960
What was the largest man-made natural disaster in history (coincidentally linked to soil) and how did it happen?
The Dust Bowl - farmers ploughed midwestern plains and the soil was exposed which then blown by winds
How does soil sequester CO2 from the atmosphere?
Plants photosynthesise and then the roots pass carbon to microorganisms which make pockets in the soil where it ca then be stored as Soil Organic Carbon (SOC)
- soils store 3x more carbon than forests and veg
What happens when soil is tilled?
Devoid of microorganisms, so more sprays are used, repeat etc
a) How were pesticides invented?
b) Why are most crops genetically modified?
c) give an example of a detrimental weed killer/pesticide
a) Haber - first chemical weapon
b) to resist toxic pesticides
c) Glycophosphate - health detriments e.g. baby birth defects and cancer as it is on so much of our food
What is desertification?
when CO2 and H2O leave the soil when it is ploughed and tilled, so it turns into dust
What is transpiration?
Where moisture leaves through plants stomata, which increases humidity, which increases rain, which increases fertility naturally
What happens when cropland is covered with vegetation?
CO2 is taken in by photosynthesis, so CO2 atmos concs go down
Give two organisations helping to reduce the impacts on soil
NRCS (National Resources Conservation Service) - provides technical assistance and education to farmers to reduce their impact on soils (used to be Soil Conservation Service)
Drawdown
What is Drawdown? How does it link to soil?
U.S based non-profit organisation that helps the world stop climate change. One plan is to reduce CO2 by capturing carbon and storing it in soil
What is the 4 for 1000 initiative?
initiative aiming to increase C content of soils by 0.4% a year, demonstrating the crucial role of soil in climate regulation and food security
Which 3 countries didn’t sign the 4 for 1000 initiative? Why is this worrying?
USA, China, India
Each had agricultural political aspects but they are mass soil eroders and so their impact would have been great, as they are the 3 biggest food exporters in the world, so have a lot of soil degredation
What is agroforestry and how is it beneficial?
Growing crops and trees at the same time
- maintain and enhance farm agricultural output while protecting soils, rivers, biodiversity and climate because the trees control run off, retain nutrients, and promote organic matter
What are multi-species cover crops and how are they beneficial?
Growing at least 4 different species at the same time to provide resilience as they all thrive in different conditions and all promote different soil biota
How can using livestock help reverse desertification?
By rotating them so they are never on the same patch for long periods of time allows natural regeneration and the roots sloughing off the eaten grass allows more carbon to be stored in the soil
What are the 4 principles of regenerative agriculture?
No till, cover crops, perennials and trees, pod grazing
What approximate percentage of farms today are managed for soil health?
50%
Give an example of soil regeneration
Loess Plateau - once ‘the most eroded place on Earth’
but now 14,000 sq miles regenerates
Give 2 regenerative agriculture stats
25% world’s landmass degraded by humans for industry
$100b annual increase for US farmers by switching to regenerative agriculture
How does carbon get into the soil?
- plants photosynthesis, store CO2
- grass tops eaten
- roots slough off into humus
What is a way to reduce soil and sanitation issues?
How many people around the world lack proper sanitation?
2.5 million people lack proper sanitation - diseases
COMPOSTING TOILETS
What do 70% of US croplands grow?
Animal feed - corn, soy, hay
How much do taxpayers subsidize agriculture per year to give farmers price guarantees?
$25 billion per year
How much does it cost the US per year for:
a) erosion control
b) damages to soil
a) $8.4b
b) $44b
How much of the world is grassland/savannah?
2/3
Give an example of a working composting solution
SAN FRANCISCO
- there are fines for putting things in the general waste bins (incl. food waste)
- not charged to put in the food waste or recycling bins
- 700t food scraps/plant cuttings collected per day
What is soil erosion?
a natural process as soil particles are removed and transported by natural process such as wind or water
What is soil degredation?
the loss of soil health and a reduction in soil quality due to unsustainable management involving processes making the soil less suitable for use
What is the natural soil dynamic equillibrium?
rate of soil formation = rate of natural soil erosion
What are the major types of soil erosion?
wind
water
anthropogenic
What is wind erosion?
When some soils are dry/have little clay content, they are likely to be loose with little cohesion between the particles which would hold the soil together
If it is windy and the soil is unprotected then it may blow away
What are the problems with wind erosion?
- the immediate area loses soil
- problems caused where the soil is deposited as it may cover crops/land in urban areas
What are the three types of water erosion?
Rain splash erosion
Surface runoff erosion
Slumping and landslides
What is rain splash erosion?
when soil particles are dislodged by the splash of a raindrop, soil particles are dispersed in all directions but those going downhill are likely to travel further.
- Over time, this can cause the downhill movement of large amounts of soil
What is surface runoff erosion?
erosion caused by surface runoff when the infiltration capacity of the soil has been exceeded
- this can occur when the rainfall is heavy or prolonged, or if the soil is relatively impermeable so more of the water flows over the ground surface
What is slumping and landslides? (erosion)
when soil on slopes becomes very wet, in increased mass and lubrication of the water makes the downward movement of large amounts of soil more likely
- often occurs hen deep soil on steep slopes becomes less stable following deforestation
- as the roots that held the soil together decompose, the soil is held together less strongly and landslides following landslide heavy rain become more likely
What increases all form of water erosion?
Steeper gradients
Which areas suffer with both wind and water erosion?
Give an example
AREAS WITH SEASONAL RAINFALL
e.g. Sub-Saharan Africa, wind erosion during the dry season and water erosion during the wet season
What can happen to unprotected surface soil?
washed away during heavy rain (sheet wash), or channels can develop on hillsides (gullying)
How does vegetation reduce the rate of soil erosion?
NATURAL WINDBREAK - reduces wind velocity and therefore kinetic energy to carry away soil particles
REDUCE IMPACT OF RAINDROPS ON SOIL SURFACE (veg and leaf litter) - so soil particles are less likely to be dislodged
HELPS BIND SOIL PARTICLES TOGETHER - by soil organic matter (incl. the colloidal material humus) added to by the veg
PLANT ROOTS HOLD SOIL TOGETHER
INCREASE INFILTRATION OF WATER INTO SOIL - reduces rate of runoff which reduces water erosion
a) How long does it take for one inch of topsoil to make?
b) How fast are humans exploiting it?
a) 500 years
b) 17x the rate of formation
What are the benefits of not ploughing soil?
doesn’t disturb fungal hyphae/microorgs
- leads to increased insects, seeds, birds, wildlife, flood and drought resistance
variety of crops = bacterial diversity and health
- supports wide variety of insects
Suggest the benefits of maintaining healthy soils through conservation agriculture
- costs go down (heavy machinery loss)
- higher yields: produces more food
- sustainable future
- fewer chemicals = higher biodiversity
- resilient to extreme weather
- support more wildlife
Why is conservation agriculture not an instant cure all?
no guarantee to increase biodiversity or atmospheric carbon conc the farmland can absorb and hold
Outline the causes of soil degradation (NOT EROSION)
- ploughing/tilling/turning disrupts soils/microorgs/fungal hyphae which supports wider biodiversity
- sprays and fertilisers kill insects and plants, so roots and interrelationships cannot survive
How can we increase soil organic matter?
- Don’t lose it in the first place by limiting intensive cultivation
- Grow your own by making the best of crop residues and cover crops to protect/hold soil in place
- Import it by bringing digestate, farmyard manure and composts on to the farm
Give stats of sloping field soil erosion
- 80% occurs down tramlines - CONDUIT
- low ground pressure tyres on the sprayer machine reduces erosion by 50% due to reduced compaction leading to increased runoff
What are some possible solutions to manage slopes?
- Field corner settlement ponds - where a ditch is converted into a pond, so the water velocity falls and some soil drops out
- Green swales - grass strips running down a field dip, either with or without semi permeable dams e.g. logs to regulate water flow
- Grassy margins - at the field edge to trap sediment before it goes into the ditch
- Beetle banks - run across the middle of the sloping fields to slow surface slow
What can be used to reduce wind erosion?
- crop residue cover
- companion crops - hold soil in place
- windbreaks
What do we need to consider with wind erosion mitigation?
wind direction and strength - need to avoid funnelling of soil blown across the field collecting in hedge bottoms/windbreaks
- gateways - nothing you can do to prevent soil being blown through them
