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)
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 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
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
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?
Loam
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
ped type affects soil properties and fertility
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
