R2102 1.1 – 1.4 Properties of Soil COPY Flashcards
How is soil formed?
Why is it important?
Formation:
(1) Weathering of parent rock into sand, silt, clay; proportion of these three gives the texture, which influences the structure
(2) Addition of organic materials
Importance:
Interface between biosphere (life), hydrosphere (water), lithosphere (earth), air (atmosphere)
Name the 3 ways rock weathers
Physical – heat, cold, rain etc
Chemical – water dissolves minerals
Biological – roots create fissures and dislodge
Minerals also leave channels where water can travel and erode more rock.
What is soil texture?
What are the particle sizes of sand, silt, clay?
The relative proportion of the different-sized mineral particles; sand, silt and clay in a particular soil; the look and feel of a soil.
Gravel: > 2mm
Sand: 0.06mm – 2mm
Silt: 0.002mm – 0.06mm
Clay: < 0.002mm
Describe the characteristics of sandy loam.
Feels gritty
Good drainage
Little water retention capability
Little nutrient retention capability
Quick to heat up
No electrical charge
Describe the characteristics of a silty loam.
Feels silky and soapy when wet
Fair drainage
No electrical charge
Describe the characteristics of a clay 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 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
What are advantages and disadvantages of soil cultivation?
Advantages:
Prepares for planting
Improves structure
Exposes clods to winter weathering
Breaks up pans
Allows fertilisers to be incorporated
Buries crop remains and weeds
Disadvantages:
Disturbs natural structure – earthworms etc
Damages soil structure
Dormant seeds may be brought to the surface
Moisture may be lost from soil
Hard work
What are symptoms of poor drainage?
Poor plant growth
Water collecting on surface
Soil constantly wet
Mosses
Blue/black colouring
Smell
How can one increase drainage in any soil?
Add organic matter
Add lime
Why does adding lime (raising pH) improve drainage?
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
What are primary colonisers?
Describe the symbiotic relationship between algae and fungi.
Certain species of bacteria, fungi and plants that have evolved to live in water- and nutrient-limited environments. (Includes lichens.)
Algae can obtain carbon and nitrogen from the atmosphere, sharing it with the fungi, which can access nutrients from the rock by dissolving its surface with acid, and share these with the algae. Hence the symbiotic relationship.
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
What is the CAC (cation exchange capacity)?
Cation-exchange capacity is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules, but allow these to exchange with other positively charged particles in the surrounding soil water.
- Each soil will have a different capacity to hold these cations and this is called the cation exchange capacity.
- This is a measure of soil quality and increases with higher clay and organic matter content.
What are soil horizons?
O – Organic – organic matter that has not yet been added into the soil, e.g. surface mulch; may not be present when bed newly created
A – Topsoil – Darker topmost layer, mostly mineral material mixed with organic matter from O horizon
Greater number of soil organisms, where most biological activity occurs
Plants concentrate their roots in, and obtain most of their nutrients
(E – Alluviation – sand, silt)
B – Subsoil (E, B and C horizons) – Usually lighter in colour than topsoil
Minerals and nutrients pushed down
Where finer materials accumulate and where deposition occurs.
Higher levels of clay, salt and lime than topsoil.
Less organic matter and fewer soil organisms.
Pore spaces smaller and particles will compact more easily.
Should not be mixed with topsoil.
Only the deeper roots will reach this layer.
C – Parent rock (‘parent material’)
First layer of weathered bedrock (unconsolidated).
Little affected by soil-forming processes, often containing lumps or shelves of split or cracked bedrock.
Retains very similar traits to the original parent material. For example, ‘ghost’ rock structure may be present in the C horizon.
What are the benefits of organic matter?
Results in a greater number of soil organisms
Tilth (ease of tillage)
Structure (large pore spaces)
Water infiltration
Water-holding capacity
What makes a good top soil?
Good tilth (0.5 – 5mm aggregates; inside these crumbs are predominantly small pores which hold water, and between the crumbs are macropores which contain air when soil is at field capacity)
Well spaced peds – allowing good water-holding capacity and room for root growth
Warms up quicker
Contains essential elements necessary for healthy growth
Describe soil formation under headings “Physical”, “Chemical” and “Biological”.
Physical – Freeze/thaw cycles. The cracks widen – breakdown of rock particles. Heat – expansion and contraction of rock – breaks apart. Abrasion – rocks brushing against each other down a mountainside or in rivers where the rock is gradually worn away.
Chemical – Carbon dioxide in the air reacts with water to form a weak carbonic acid – erodes rock. Oxygen in the air can also directly react with chemicals in the rock, e.g. iron to form oxides which results in the mineral disintegrating or dissolving in water.
Biological – 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. The action of animals trampling/burrowing can cause rocks to break into smaller pieces.
What is good soil structure?
What are the advantages?
Large macropores to allow movement of air and water. This “ideal” structure is called granular, or crumbly.
The particles of sand and silt are held together in aggregates (small clumps) by clay, humus and calcium. The small empty spaces (micropores) hold the water the plants need.
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 can soil structure be improved?
Sandy soils:
- Organic matter – compost or composted manure
- Early spring, because working sandy soils in the fall promotes erosion
- Adding basalt improves their ability to retain water and minerals.
Clay soils:
- Organic matter in the form of compost or composted manure
- Late fall. Poorly drained clay soils can also be amended by adding sand.
- Some clay soils are very high in sodium, which prevents mineral particles from forming aggregates. In this case working in gypsum (if their pH is neutral or alkaline) or lime (if their pH is acidic).
Silty soils:
- Large amounts of organic matter in the form of compost or composted manure.
- Late fall
Also:
- Regularly add organic matter
- Encourage biological activity in the soil.
- Correct the pH as necessary.
- Avoid overworking the soil. Hoe the soil or turn it over lightly.
- Use mulch.
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.