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?
A specific layer in the soil 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