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.
