R2102 - Understand the physical & chemical properties of soils Flashcards
1.1 What is weathering in soil formation?
Weathering is the breaking down of the parent rock by physical, chemical and biological means.
This produces a mineral soil.
1.1 Describe the natural processes of soil formation: Physical weathering (moving water, wind, freeze/thaw, heat, glaciers)
Moving water - streams, rivers or sea can carry rock particles and as the mix of water and rock flows over other rocks, fragments get broken off and these are further ground up by the water into smaller particles.
Wind - wind can carry abrasive particles that ‘sandblast’ exposed rock, common in hot countries where sand particles are readily blown about in the wind.
Freeze/thaw - rain water enters cracks/ joints within the rock, it freezes, forms ice which expands, then repeatedly thaws and re-freezes over time, which continuously widens the crack/ joint until the rock shatters into angular fragments.
Heat - In hotter regions rock surfaces exposed to the sun expand, whilst the rock within remains cool and this can set up strains within the rock causing the surface to crack and over time small fragments of rock to break away.
Glaciers - Ice in glaciers tends to stick to the adjacent rock and as it moves downhill the large rocks embedded in the glacier scour away any rocks they come in contact with causing the rocks to be fractured into smaller pieces.
1.1 Describe the natural processes of soil formation: Chemical weathering
Chemical weathering is where weak acids gradually dissolve rock.
Rain water combines with carbon dioxide in the air to form weak carbonic acid and this gradually dissolves chalk and limestone.
It can also react with many other minerals in rock, resulting in their gradual breakdown. .
1.1 Describe the natural processes of soil formation: biological weathering and addition of organic matter
Biological weathering is brought about by living organisms.
Biological weathering occurs when seeds are blown into rock crevices, take root and grow. The roots of plants exert great pressure and break up the rock.
Lichens and mosses produce CO2 through respiration and this forms weak carbonic acid which dissolves rock such as chalk and limestone.
When plants/ animals/organisms die their decomposition products, contribute to enrich the soil further.
The actions of micro-organisms decompose plant/ animal life to form humus.
1.1 What does loam mean?
- soil with roughly equal proportions of sand, silt, and clay
- an idealised soil for growing
1.1 What is a soil horizon?
The specific layers in the soil shown by digging a soil pit.
1.1 Describe the development of main soil horizons (4)
Organic: Leaf litter and other undecomposed plant material. Provides food for small animals such as earthworms, which are important in the decomposition process.
Topsoil: Uppermost layer dug during cultivation. Most important layer for gardeners. Between 10 and 40cm deep. Dark in colour due to the decomposed organic matter or humus it contains. Richer in plant nutrients as a result of decomposed organic matter. Contains most of the fibrous plant roots and soil organisms such as earthworms.
Subsoil: less fertile and much lighter in colour because it contains very little decomposed organic matter. Contains large tree and shrub roots that anchor the plants in the ground and also search out water.
Parent rock: original rock from which the soil is formed.
1.3 Define the term ‘soil texture’.
The relative proportions of the sand, silt and clay (mineral) particles in the soil.
1.3 What are the particle sizes of stones, sand, silt, clay (using Soil Survey England and Wales classification).
Stones: larger than 2mm in diameter
Sand: between 0.06mm & 2mm in diameter
Silt: between 0.002mm & 0.06mm in diameter
Clay: less than 0.002mm in diameter
1.3 Describe the characteristics of a sandy loam.
feel: gritty
nutrients: particles have no charge so nutrients are easily leached out of the soil
water retention: low water holding capacity but good water movement
temperature: large pore spaces containing air so will be quick to warm up in spring
1.3 Describe the characteristics of a silty loam.
feel: silky or soapy when wet
nutrients: plant nutrients will be held by the electrical charge on some of the particles but will also be available to the plants
water retention: large number of small pore spaces so will hold on to water but a high proportion of this water will be readily available to the plants
temperature: Slower to warm in Spring because of the large number of small pore spaces
1.3 Describe the characteristics of a clay loam.
feel: sticky when wet
nutrients: fertile because it has good nutrient retention due to the small negative charge on the particles
water retention: good water retention but not all the water is available to the plants
temperature: slow to warm up in spring because of all the water in the small pore spaces
1.4 Define the term ‘soil structure’.
The arrangement of particles in the soil.
How the particles (sand, silt and clay) are built, glued, cemented, or aggregated together to ideally form a “crumb structure” (peds).
1.4 Describe crumb structure and its influence on plant growth: Crumb formation
Soil crumbs are small rounded aggregates made up of mineral particles (sand, silt, clay) and organic matter.
Organic matter breaks down in the soil and forms humus. Humus coats the soil particles and allows them to aggregate or come together as crumbs.
A good crumb structure allows:
- Free water movement
- Gaseous exchange
- Thorough root exploration
1.4 Describe crumb structure and its influence on plant growth: Destruction.
- over cultivation too much digging or raking can destroy the structure
- cultivating at the wrong time of year for e.g digging a clay soil in winter when it is wet and sticky will cause compaction and damage the crumb structure
- failure to add organic matter regularly especially on a light sandy soil where it is broken down very quickly.
- over-watering can lead to the crumbs collapsing and forming a soil cap (a crust) when the soil dries out.
1.4 Describe the term saturation point.
Soil pores (large and small) are full of water and no oxygen is present. The soil is waterlogged.
1.4 Describe the term field capacity.
The amount of water the soil can hold against the pull of gravity. i.e the amount of water remaining in the soil a few days after having been wetted, and after free drainage has ceased.
The large air spaces are now filled with both air and water while the smaller pores are filled with water.
Field capacity is said to be ideal for plant growth.
1.4 Describe the term available water content.
Available water is the water that plants can actually take out of the soil.
It is the amount of water held by the soil between Field Capacity and the Permanent Wilting Point.
1.4 Describe the term permanent wilting point.
PWP is the water content of the soil when a wilted plant does not recover (retain turgor) overnight.
The point when water can no longer be extracted from soil.
1.4 Describe the term soil moisture deficit.
Soil Moisture Deficit is the amount of water required to return the soil to field capacity.
1.4 Describe the importance of an appropriate balance between air and water for the healthy growth of plants.
- A well-structured soil will have a good balance between water and air
- with water draining out of the large pore spaces by gravity, leaving them occupied by air
- but water still held in the smaller pore spaces and around the soil particles
- healthy growth requires a constant supply of water; too little rain and the plants will need irrigating whilst
- too much water can lead to the soil becoming waterlogged and plants dying, due to lack of oxygen around their roots
1.5 Describe irrigation methods - watering can.
- Plants need water at their roots which gets down into the soil rather than just damping down the top layer
- Use collected rainwater wherever possible
- It’s best to water cool of the evening or the very early morning, so that less water is lost immediately to evaporation
- well-aimed can of water will usually get down to the roots of the plant far better than a spray from a hose
- care should be taken to avoid damaging the crumb structure of the soil
- avoid disturbing the surface of the growing medium in containers by pouring water on too quickly
- watering seeds and cuttings, use a fine rose turned upward to minimize surface disturbance by large droplets of water
1.5 Describe irrigation methods - hose.
- Plants need water at their roots which gets down into the soil rather than just damping down the top layer
- It’s best to water cool of the evening or the very early morning, so that less water is lost immediately to evaporation
- a hose fitted with a trigger lance that can be adjusted for flow rate and fineness of spray is a good choice
- make sure you direct the water to the roots rather than spraying the tops of the plants and the foliage.
- avoid using a powerful jet of water as this can disturb the soil and cause damage to both soil and plants
- watering seeds and cuttings, use a fine mist turned upward to minimize surface disturbance by large droplets of water
1.5 Describe irrigation methods - sprinkler.
- a sprinkler is useful when you want to water a large area rather than just individual plants
- on bare soils the structure can easily be damaged leading to the formation of a surface cap.
- on slopes, runoff can cause erosion of the soil and loss of seeds and fertilizer.
- sprinklers are wasteful, watering everything within their range rather than just those plants that actually need it
1.5 Describe irrigation methods - seep hose.
fairly efficient method of applying water as delivers water very slowly to the soil, leaving plant foliage and most of the soil surface dry
a hose that is porous or has holes, laid on the soil allowing water to seep out
delivery rate can be adjusted to suit plant growth and temperature
made from recycled materials and biodegrade safely after a number of years
1.5 What is single digging?
- Digging down to the depth of a spade, known as a spit (30cm)
- in shallow soils where topsoil may be less than a spit, dig to this level even if it’s less than a spit, to avoid bringing up subsoil.
- Dig out the first trench and move the soil close to where the last trench will be.
- Work backwards along the plot a second trench is then dug and the soil taken out of this second trench is inverted into the first trench.
- This process is repeated until the last trench on the plot is filled with the soil from the first trench.
- Annual weeds are buried. Remove deep rooted perennial weeds.
- Organic matter can either, be spread on the surface of the soil and turned into the trench with the soil, or it can be incorporated into the bottom of each trench.
- Dig heavy clay soil in Autumn so large clods are broken down by frost and snow to improve soil structure.
- Sandy soils are best dug in Spring.
- Never during frost or when covered with snow as this can harm soil organisms and the soil structure
1.5 Why single dig?
Why do it?
- Exposes the top ‘spit’ or spade depth to weathering, helping provide plant nutrients, improving drainage and burying weeds
- Improves soil structure (raises the soil surface, air is trapped and this increases total pore space, allows the incorporation of organic matter)
- Important in heavy clay soils (and compacted soils) in order to increase the porosity of the soil
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1.5 What is double digging?
Remove the soil from the upper and lower spits of the first trench and from the upper spit of the second, laying it aside in three separate clearly marked piles.
Soil may then be transferred from the lower spit of the second trench to the bottom of the first trench, and from the upper spit of the third trench to the top of the first. In this way topsoil and subsoil remain completely separate.
Continue digging further trenches in the same way, and at the end of the bed use soil saved from the first two trenches to fill the appropriate spits of the final two.
If the topsoil is more than two spits deep there is no need to keep the soil from the upper and lower spits separate.
Organic matter can be incorporated into the bottom spit.
1.5 Why/when double dig?
Improves uncultivated ground
if the soil is heavily compacted
if is a soil pan below the surface and drainage needs to be improved.
It is very important not to mix the topsoil and subsoil when double digging.
1.5 Describe rotary cultivation.
Rotary cultivators are used to create soil crumbs on uncultivated or roughly prepared ground instead of digging, forking and raking.
A rotary cultivator can be used to:
- prepare a tilth for seed sowing
- break up a soil cap
- incorporate green manures
- control annual weeds when preparing a stale seed bed
1.5 Describe forking.
Used on areas where loosening or breaking up the soil is the major requirement
Forking is less harmful to the soil structure than using a spade because it tends to break the clods along the existing natural fracture lines rather than along artificial ones created by the spade.
Suitable for rough digging and for clearing weeds from the soil surface.
Also used to improve the structure of soil that cannot be dug over, such as borders with established shrubs and herbaceous plants
1.5 Describe raking.
Raking is used on soil that has been roughly prepared by digging or forking.
It is used to level the soil, remove stones and produce a fine tilth ready for planting or seed sowing.
Fertiliser can also be raked in.
1.5 What are the advantages of bed systems?
Improving drainage – the soil is raised above the surrounding ground level
Increasing soil temperature - Soil in raised beds is better drained, so warms up faster in spring
Improving access - higher raised beds can be used for people who are less mobile or need to use a wheelchair
bulky organic manures are concentrated on a smaller area so it is easier to build up high levels of fertility
1.5 Describe ‘no dig’ method.
In most cases initial deep cultivation of the soil, incorporating organic matter to improve the structure is necessary before this method can be fully adopted.
Clear any weeds from the bed by hoeing, hand weeding
In the autumn add a 5cm layer of organic matter such as garden compost or well- rotted farm yard manure to the soil surface.
Within a short time this organic matter becomes incorporated into the soil by the action of earthworms.
In spring, plants and seedlings go directly into the soil.
This ‘no dig’ method means that there is less interference with the soil structure as it keeps soil disturbance to a minimum.
1.5 What is primary cultivation?
Involves the turning over of the soil
- Single and double digging by hand
- Mechanical ploughing and subsoiling
- Rotovating
1.5 What is secondary cultivation?
- secondary cultivation to produces a fine seedbed, ready for sowing seed or planting.
- Forking
- Raking
- Hoeing
- Consolidation
1.5 Describe consolidation.
Once the ground has been dug and raked level it needs to be consolidated by lightly treading.
This is done by walking over the area using the flats of the feet and small steps.
This action gets rid of the larger air spaces and allows for water to be drawn up through the very small pore spaces by capillary action.
It also ensures that the seeds will be in contact with the soil particles.
The soil is then raked to a fine tilth and the seeds are sown either into a shallow drill or broadcast.
1.5 Describe methods of improving drainage (getting rid of excess water).
Normal cultivation methods to improve soil structure and drainage
Soakaway – deep holes filled with rubble that penetrate to porous rocks, allowing water to filter through it
French drain – a gravel-filled trench lined with landscape fabric to keep soil and silt out of the gravel.
Raised bed
Appropriate planting – plants that can cope with waterlogged and marginal conditions
1.5 What causes excess water? (Identify causes of excess water)
soil texture eg a clay soil with a high proportion of small particles
poor soil structure caused by compaction, surface capping or a soil pan
surface run off when large areas are covered by roads, driveways, patios and car parks and water is unable to soak into the ground.
a high water table When the soil lies over impervious rock or very heavy clay, the water cannot drain away and much of the soil becomes saturated. The top of this level of saturated soil is called the water table.
a soil pan which is an impermeable layer that can prevent water draining away and water can back up above the soil pan forming a perched water table.
1.5 How can you identify poor drainage?
areas of standing water on the soil surface, this is called puddling
surface run off after heavy rain because the soil is already saturated
indicator plants such as mosses, rushes and sedges are growing in the area. Weed problems such as creeping buttercup, particularly in lawns.
permanently wet soil even in dry periods
smell of hydrogen sulphide – bad eggs
plants don’t grow well, stunted growth and a yellowish tinge to the leaves because they have poor or restricted roots
gleying - there may be a blue-grey colouring in the soil, this is due to a lack of oxygen
1.5 What are soil pans?
An impermeable layer in the soil which acts like a barrier to plant roots and as water is unable to drain away the topsoil is likely to become waterlogged.
may develop on a clay soil if there is repeated cultivation to the same depth with a rotary cultivator
or when iron particles are washed down through the topsoil and combine with the clay particles to form an impermeable layer
can be broken up by double digging or subsoiling
a soil pan should be visible if you dig a soil profile pit
1.5 Why add inorganic materials to soil?
- Lime – used to in a clay soil to improve soil structure by forming smaller aggregates
- Grit – for poorly draining soils to improve aeration
- Sand – can also be added to improve soil structure through improved drainage and aeration
3.2 Describe what is meant by the pH terms: acidic, neutral and alkaline.
Soils are classified as acid, neutral, or alkaline and this factor influences the plants that can be grown in a particular soil.
The pH scale is used to measure the acidity or alkalinity of a soil.
The pH scale goes from 1 to 14.
Acidic: below 7 is acid
Neutral: 7 is neutral
Alkaline: above 7 is alkaline
3.2 Identify materials used to influence the soil pH, lime. State the benefits and limitations (environmental, health and safety issues, timing of application, effectiveness of the material).
More lime has to be added to clay soil than sandy soil. See table below.
When applying more than 0.5kg per sqm, it is best to dig half into the soil and sprinkle the rest on the surface after digging.
When applying less than 0.5kg per sqm, dig the entire amount in, but you can sprinkle it on the surface if digging is not practical.
Benefits: Raises the pH, provides the nutrient calcium, encourages worms and beneficial bacteria, discourages diseases such as clubroot in brassicas, makes clay soils more workable by flocculating the particles, can make other nutrients more available
Limitations: Does not have an immediate effect best applied in autumn and combined with digging. Over-liming may cause nutrient deficiencies. Can’t apply at the same time as manure as the two can react, causing loss of nitrogen in the form of ammonia. Lime and manure on alternate years.
Health and safety: Wear gloves and goggles as can irritate the skin or eyes
