Essential soil science Flashcards
Ashman, M, & Puri, G 2013, Essential Soil Science : A Clear and Concise Introduction to Soil Science, John Wiley & Sons, Incorporated, Hoboken. Available from: ProQuest Ebook Central. [14 July 2020].
What are soils formed from?
Soils are made from a combination of mineral material and organic matter from plants and animals.
What percentage of soil is made from minerals and organic matter? And what is the rest made from?
50% minerals and organic matter
50% pore space which is usually made from either water or air.
What is one of the main differences that changes the type of rock formed
the composition and quantities of different minerals can significantly change the rock type and their properties.
What is a significant difference between limestone and chalk and coal?
limestone and chalk are formed of prehistoric marine organisms
coal is formed from prehistoric plant matter
What is silica formed from
oxygen and silicon
forms silicates
Does a lower silica content cause rocks to break up more easily?
Yes. A lower silica concentration causes rocks to disintegrate easier than those with a higher silica content
What percentage of silica do ultrabasic rocks have?
less than 45% `
What percentage of silica do basic rocks have?
between 45-55%
What percentage of silica do intermediate rocks have?
55-65%
What percentage of silica do acidic rocks have?
more than 65%
Example of ultrabasic rocks
serpentinite and peridotite
Example of basic rocks
basalt, gabbro, dolerite
Example of intermediate rocks
amphiobolite and andesite
Example of acidic rock
granite and pegmatite
What causes rocks to become soil
Weathering - physical and chemical
Types of physical weathering
thermal
freeze shattering
plants
Type of chemical weathering
Hydrolysis Carbonation Hydration Dissolution Oxidation and reduction
What is thermal weathering
Different minerals within rocks have a different rate of expansion when heated. This causes stress within the rock which eventually leads to fracturing.
Thermal weathering can also be caused by there being different temperatures within the inner and outer part of the rock- exfoliation weathering
What is exfoliation weathering
Exfoliation is a type of thermal weathering caused by there being different temperatures within the inner and outer part of the rock leading to layer of the rock fracturing off, similar to an onion skin
Frost shattering
Water in the cracks of rocks can freeze and causing the volume to expand by 10% which exerts pressure on the crevices of the rock.
Water can also cause shrinking and swelling of the rock. The rates of this will depend on the minerals present in the rock
How do plants cause weathering
Plant roots can grow in the crevices of rocks which as they grow creates a larger surface area which can be exposed once the plant dies. The larger surface area means that more of the rock is exposed to other forms of weathering.
Hydrolysis
- the most common form of weathering
- separation of water into H+ and OH- . This ions can be used to break the bonds in molecules
Carbonation
CO2 is released from respiration dissolves in water to form carbonic acid and is an accelerated form of hydrolysis. Sugars are also excreted which has similar effects as tooth decay. Both have a particularly detrimental affect on plant roots.
Hydration
Minerals absorb water, however unlike hydrolysis do not form ions and water remains intact. Physical and chemical composition of the rock can be altered such as weakening of the rock.
Dissolution
Minerals dissolve in water
e.g. sodium chloride and potassium chloride are both completely soluble
Oxidation and reduction
Oxidation- loss of electrons= take up of oxygen
Reduction - gain of electrons= ability to lose oxygen
What is soil organic matter (from a scientist point of view) and how is this different to the general definition?
all the remains of plants and animals
covers all living and dead organisms contained within the soil
What is measured to calculate the amount of organic matter in soil
carbon
What percentage is typically formed from carbon compared to the total weight?
58%
Aggregate
binding of soil particles together into a discrete unit
What are plants made from
Cellulose and hemicellulose
proteins and amino acids
lignin
fats and waxes
What is the term for the breakdown of plants and animals into their simple chemicals
mineralisation
What is the most difficult component of plants to breakdown
lignin
There’s around 15% in plants and 25-30% in woody plant tissues.
Nitrogen fixation
being able abstract nitrogen from the air rather than soil- good adaptation for primary colonisers
example of primary colonisers
bacteria, fungi and some plants
relationship between algae and fungi
- algae obtain carbon and nitrogen from the air via a combination of photosynthesis and N fixation.
- once there is a surplus of nutrients these are passed onto fungi
- fungi release organic acids into the rock which releases minerals for algae
What type of relationship do algae and fungi have
symbiotic
What type of soil do upland areas of the UK usually have
acidic
Example of an acid tolerant plant
Ericaceae
Difference between acidic and neutral to alkaline soils
Acidic soils have a lower level of microbial activity which when coupled with difficult to decompose organic matter causes an accumulation of residue- usually a black coloured layer at the top of the surface
Neutral to alkaline soil- have a much greater rate of microbial activity and easily composible material leading to a more uniform distribution of organic matter throughout the soil
What are the 3 particle sizes of fine earth soil
sand
silt
clay
which is the largest particle size of fine earth soil
sand
0.06 -2 mm
Which is the smallest particle size of fine earth soil
clay
less than 2μm
What is the size of a silt particle
0.02 - 0.002 mm
Which type of soil particle has already undergone some part of chemical weathering
clay which has altered its physical and chemical composition
Humus
Formed from organic residues gradually breaks down plant residue (mineralisation) to form humus (humification). Humus has a variable composition and compared to plant residue is very stable. It can be present for over 1000 years and are often a dark brown colour.
Transportation mechanisms
wind- fine silty material- loess deposits
water- river terraces or flood plains
slip down slopes- solifluction terraces
Are the soils at the top of slopes usually thinner or thicker than those at the bottom of slopes
Thinner, depleted soils are usually present at the top of slopes
Soil catena
well defined sequence of soil type relative to location on a slope
What is the term for the process of involving the movement of soluble ions in soil
leaching
What happens when leaching continues for a long period of time
soluble compounds are completely washed out of the soil
What happens in extreme and prolonged weathering such as in humid tropics
clay minerals are completely washed away, leaving oxides of aluminium and iron
Eluviation
the movement of insoluble particle such as clay minerals down the profile
What is another term for eluviation in the field
lessivage
How is lessivage detected in the field
usually detected by a relative increase in clay in the B horizon compared to A or E horizon
sometimes can be detected by a thin coating of clay on the surface soil aggregates known as clay skins
soil horizons
O- top layer composed of organic matter
A- minerals and organic matter mixed together
E- where soil is depleted of material
B- accumulation zone
C- unconsolidated parent material
R- consolidated parent material (bedrock)
How large are stones/gravel
more than 2.0mm
How would you find the size of soil particles in a lab
- place soil in suspension
- the rate at which they settle will be determined by their size as all soil particles have a similar density of around 2.65g/cm3 (same as silcia)
- sample will be taken at increasing time intervals- this will change from containing all fine earth particles, then silt and clay to finally just clay.
- sample then placed in a watch glass until the water evaporates.
- the dry residue is then weighed for each sample.
- calculate the percentage composition for each sample by difference - ‘mechanical analysis’
Do soils with large pores or small pores drain quicker
larger pores
Which type of pores do sandy soils have
large pores
What does an increase of clay particles have on sandy soils
clay particles can increase the number of smaller pores in the soil which can lead to more waterlogged conditions as there is worse drainage consequently leading to problems with gas exchange etc
Soil structure
describes the way in which sand, silt and clay particles are bonded together in larger units called aggregates.
Refers to on a large scale to the size, shape and stability of soil aggregates
Single- grained
soils that do not form aggregates
massive
Soils that exist as one solid block
How are aggregates formed
formed when the soil is subjected to shrinking, swelling, plant roots penetration or freezing, which all generally cause the soil to break into discrete units
What are the 2 stages of aggregate formation
aggregate formation
aggregate stability
When are aggregates described to be stable
when soils are able to resist pressures caused by processes such as compaction and sudden wetting.
How can rapid wetting affect the stability of soil aggregates
Rapid wetting cause pores near the surface to become filled with water which traps air inside the aggregates, increasing the pressure which can sometimes be enough to break the aggregate apart.
Slaking
disintegration of aggregates
What are the two types of aggregates
microaggregates and macroaggregates
How do the 2 aggregates differ in size
microaggregates- less than 250μm
macroaggregates- more than 250μm
What are aggregates that are larger than 1 cm usually referred to as
clods
What needs to happen before microaggregates can form
microscopic clay minerals need to be grouped together in small stacks called ‘domains’.
Whats it called when clays are bonded together in domains
flocculated
disflocculated
when clay minerals are dispersed into the soil
What is the most important factor which influences flocculation process.
the presence of ions with more than one charge (e.g. Ca 2+ and Mg2+ and Al3+). these ions are attracted to the surface of the clay and allow clay minerals to bond together to form domains.
What do domains bond to
organic matter - these form mircoaggregates.
How are macro aggregates formed
collections of microaggregates bound together with organic matter- aggregate hierarchy model. Most of the time they don’t form one large object, but several larger aggregates. This is partially due to environmental factors such as drying and freezing tend to exploit planes of weakness
In soils which have a low concentration of clay, what is the stability of macroaggregates dependent on?
organic matter
What are the 2 types of macroaggregate organic matter
stablising agents- referred to as temporary- consist of microbial and plant-by- products- most important are polysaccaride gums
- transient stablising agents- include fine plant roots and fungal hyphae
- these are both vulnerable to attack from microbial so need to be replenished regularly
Why is the arrangement of aggregates a dynamic process
shrinking, swelling, plant root extension, tillage and freeze-thaw effects continually alter the arrangements of pores and aggregates.
slow mineralisation of transient or temporary binding agents lead to slower disintegration f macroaggregates
bulk density formula
bulk density (g/cm3) = mass of soil (g) / volume of cylinder (cm3)
Do soils with less pore space have a high or lower bulk density
higher bulk density
Which type of pore holds water more tightly
smaller pores
What are the 3 types of pore
macropores (transmisson pores)
mesopores (storage pores)
micropores (residual pores)
What are Storage pores
they hold water against the force of gravity and are particuarly important for storing water for plants
What are Transmission pores
allow the rapid movement of water and gas through the soil. Larger pores often allow water to drain within 48h
Residual pores
hold water very tightly. often too tightly for plant use.
What happens when soil is completely saturated
All the pores (of all sizes) are filled with water. Under these conditions the soil water tension is zero. this usually occurs after prolonged heavy rainfall or under waterlogged conditions.
hydraulic conductivity
the speed at which water percolates through the soil is determined by hydraulic conductivity. The speed of water movement down the profile will depend on: the pressure generated by water in the soil and the resistance to water flow caused by soil particles
What factors would affect th pressure component of hydraulic conductivity
elevation
pressure generated by the weight of water column overlying the wet front
Factors affecting resistance to flow
- how well the pores are connected
- the pore size distribution
- water content of the soil
Who described the factors affecting hydraulic conductivity mathematically in 1856
Henri Darcy
developed Darcy’s Law - discharge is directly proportional to the loss in head - used to describe water movement in various media including ground water
Field capacity
once the largest transmission of pores have emptied (usually after 48 h) is referred to as the field capacity
Gravitational water
water lost to reach field capacity
Residual or hygroscopic water
When the soil drains, drying causes larger pores to empty and fill with air as water is confined to smaller pores (micropores) where it is very tightly packed and remains.
How can cultivation affect the aggregate stability
- fewer plant residues are returned to the soil leading to a decline in the amount of soil organic matter.
- cultivation causes aggregates to break, exposing temporary or transient organic matter to microbial attack.
are soils that have a high proportion of sand or silt stable or unstable?
unstable as the aggregates can slake under the mildest of wetting pressures. As sand and silt are released they can fill pores preveneting water infiltration leading to soil crusting
Soil crusting
sand and silt released into pores preventing water infiltration. This leads to water being collected on the surface as puddles where they are evaporated
