2. Properties of Soil Flashcards
How are mineral soils formed?
- Weathering of parent rock into sand, silt, clay (proportion of these three gives the texture, which influences the structure)
- Addition of organic matter
- Starts as a single layer but develops over time. Initially mosses will grow, die and add to the layering, this is repeated with gradually increasing plants until the soil is deep enough to support tall tree’s
Name the 3 ways rock weathers
Physical – movement of water (rain/stream), heat, frost, glacier’s
Chemical – water/ other chemical such as weak carbonic acid dissolve minerals (chalk & limestone)
Biological – Action from living organisms (plant roots, animals)
What is soil texture?
What are the particle sizes of sand, silt, clay?
The relative proportion of the different-sized mineral particles; gravel, sand, silt and clay in a particular soil (the look and feel of a soil/ its characteristic) - The characteristic can not be changed
Gravel: > 2mm
Sand: 0.06mm – 2mm
Silt: 0.002mm – 0.06mm
Clay: < 0.002mm
Describe the characteristics of sandy soil & loam.
Soil
- Easy to dig
- Ideal for crops/plants from hot regions
- Organic matter is broken down fast
Loam
- Feels gritty when wet
- Good drainage
- Little water retention capability
- Little nutrient retention capability
- Quick to heat up
- No electrical charge
Describe the characteristics of a silty soil & loam.
Soil
- Mostly inert and non porous
- Fertile
- Can dry out at surface - soil cap
Loam
- Feels silky and soapy when wet
- Fair drainage
- Good water retention capacity
- Fair Nutrient levels
- Warms up well in spring
- Electrical charge but nutrients are available to plant
Describe the characteristics of a clay soil & loam.
Soil
- Flat platelets that pack closely together, can be easily compacted and can make it hard to dig
- Fertile with good supply of nutrients
- On hot day plants may wilt as they can not extract water from small pore space
Loam
- Feels sticky when wet and hard when dry
- Poor drainage
- Good water retention
- Good nutrient retention
- Slow to heat up
- Negative electric charge
Describe key things a good crumb structure allows.
Free water movement
Gaseous exchange
Thorough root exploration
How can root environment be improved and protected?
Cultivation – single and double digging, forking, raking, rotavating
Addition of organic and inorganic material
Managing soil water content – drainage and irrigation
Why does adding lime improve drainage in a clay soil?
Lime causes flocculation of clay particles
Clay particles group together to form larger particles, improving structure of soil and making it less dense.
What is erosion?
The breakdown of rocks by means of transportation e.g. by movement by wind, waves, streams, rivers, or glaciers
What is weathering?
The break-down of rocks by mechanical, chemical or biological means
Why are soil organisms important?
Why is humus important?
They transform the plant and animal residues of soil into humus.
Humus:
- holds nutrients in forms that break down slowly and can be made available to plants
- helps bind soil together and give good structure
Describe 2 methods of soil formation under headings “Physical”, “Chemical” and “Biological”.
Physical –
Frost - Water gets into cracks and when frozen the cracks widen and sections of rock break off into smaller particles.
Heat – expansion and contraction of rock in high temperatures cause the rock to break apart.
Abrasion – rocks brushing against each other down a mountainside or in rivers where the rock is gradually worn away.
Chemical –
Water - Carbon dioxide in the air reacts with water to form a weak carbonic acid this erodes rock.
Oxygen - Oxygen in the air can directly react with chemicals in the rock, e.g. iron to form oxides which results in the mineral disintegrating or dissolving in water.
Biological –
Roots - Plant roots and seeds can grow deep within the cracks in rocks and cause them to widen. The growth of moss and lichen on the surface of rocks can also cause cracks etc.
Animals - The action of animals trampling/burrowing can cause rocks to break into smaller pieces.
Describe what makes a good soil structure?
What are the advantages?
- “Ideal” structure is called granular, or crumbly.
- The particles of sand and silt are held together in aggregates (small clumps) by clay and humus
- Small pores hold water against the pull of gravity
- Large pores are filled with air
Advantages:
- holds water and nutrients well
- good drainage
- good aeration
- good plant-root-system development
- soil is easy to work
- soil warms up quickly in spring
- good biological activity in the soil
- soil resists erosion and compaction
How is humus created in the soil?
Microorganisms in the soil break down organic matter, quickly releasing nutrients (mineralization) and forming humus (humification).
Humification: The process by which organic matter decomposes to form earthy-smelling dark brown to black material, called humus. Humus is mineralized in turn, but very slowly. This makes small amounts of nutrients available to plants over a very long time.
What can destroy crumb structure?
Overcultivation can lead to the formation of compacted areas such as soil pans, and soil caps.
Cap is created when the surface is exposed to heavy rain and the crumb structure collapses. These caps interfere with gaseous exchange and prevent seedlings emerging.
Waterlogging can cause aggregates to collapse due to the formation of a slurried cap.
A decline in organic matter will impact soil structure.
Having large areas of bare earth subject to rainfall batter.
What is saturation point?
- No air is available and the pores are full of water
- Only oxygen available to plants is in the water
- Often surface water pooling (water logging)
- Plants may start to die within 5 days
What is field capacity?
- The amount of water the soil can hold against the force of gravity
- This is ideal for plant growth.
- Water is present in the micro and mesopores, while air files the macropores
- Usually in spring
What is the permanent wilting point?
- The soil’s water content when a plant growing in it does not regain turgor overnight
- Uptake of water is not efficient to meet plant needs
- In sandy soil no water is available in the pores only air
- In clay soil there is water but it is not available to the plant only air fills the accessible pores 15% - not available to plant
What is the available water content?
- The water that lies between field capacity and permanent wilting point
- The amount of water that is available to the plants
- May be increased by cultivation or adding organic matter
What is soil moisture deficit?
- The amount of water needed to bring the soil moisture content back to field capacity
Name 4 main issues of over-watering
- Nutrients are leeched out into the soil
- Excessive leaf growth to increase stoma and the expense of fruit and flowers
- Reduced flavour in fruit and veg
- May cause fruit to split
Describe temporary wilting and why it is dangerous for the plant
- Plants wilt in hot weather due to the lack of water in the plant cells but will often recover overnight
- Wilted leaves are less effective at intersecting sun light
- Plant may need to close its stomata to prevent water loss therefore photosynthesis has stopped, this inhibits plant growth
What are the key functions soil provides to a plant?
- Physical support/anchorage - soil needs to be prepared before planting to ensure roots can explore and grow
- Nutrients - Vital for plant growth (N,P,K)
- Water - Plants need a constant supply, too little restricts plant growth, too much restricts oxygen
- Oxygen - Needed for respiration
- Allow drainage
What temperature is required for root growth?
- Temperate climates = above 5’C
- Tropical regions = above 10’c
Describe the 4 key soil profiles/horizons
- Leaf litter - Un-decomposed plant material, food for earthworms
- Top soil - 10-40cm, most important for plant growth (fibrous roots), dark in colour, rich in nutrients, earth worms live here
- Sub-soil - Less fertile, lighter in colour, little or no composted material, large tree roots (Tap roots)
- Parent rock - Everything else
How can gardeners see the soil profiles/horizons within their soil?
- Dig soil profile pit
- Large hole 60cm deep
State key differences between top and sub soil
Top soil
- High percentage of organic matter 2-5%
- Large numbers of living organisms
- Large quantity of dead organisms
- Hummus is present
- Large pore spaces
- Good aeration
- High nutrient content
Sub soil
- Low percentage of organic matter less than 1%
- Few of living organisms
- Few dead organisms
- No Hummus present
- Small pore spaces
- Limited aeration
- Low nutrient content
% of Soil composition ?
Composed of organic material, weathered rock, water and air
- Mineral particles 50-70%
- Organic matter 1-5%
- Air & water - makes up the rest. Fluctuates due to temperature/weather
What is a loam, state the differences between them?
- Loam (Ideal soil) - It rolls into a ball but has no noticeable grittiness. Sand, silt, clay particles are proportioned so none is overly evident
- Sandy soil - Gritty and won’t roll into a ball
- Sandy loam - Gritty, rolls into a ball but doesn’t hold shape
- Silty loam - Feels silky, soapy will roll into a ball but doesn’t hold shape
- Clay soil - Rolls easily into a ball, feels sticky but has a shine
- Clay loam - Rolls easily into a ball and has a shine
Key roles of water from the soil
- Carries sugars and nutrients around the plant in the xylem and phloem
- Provides structural stability to non-woody parts keeping them turgid
- Participates in chemical reactions such as photosynthesis
