Soil Formation

Question 1

What is the meaning of sedentary parent materials ?

Answer 1

Parent materials where overlying soils form as a product from the weathering of underlying rocks are called sedentary parent materials.

Question 2

What is the meaning of Transported parent materials?

Answer 2

Parent materials carried in from elsewhere are called transported parent materials, for example, alluvium, where the rock has weathered elsewhere, and then carried by water to be deposited and form new soils.

Question 3

What rocks are classified as sedentary materials?

Answer 3

Igneous rocks from molten lava;
Sedimentary rocks from consolidation of particles derived from weathering of other rocks or chemical deposits;
Metamorphic rocks from changes in either of the above by heat or pressure.

Question 4

What four forms of Igneous Rocks are there?

Answer 4

plutonic acid = granite
plutonic basic = gabbro
volcanic acid = rhyolite
volcanic basic = basalt

Question 5

What forms of sedentary rocks are there?

Answer 5

mudstones/shale (from clays);
siltstones (silt);
sandstones (sand);
conglomerates (cemented gravels).

Question 6

What are metamorphic rocks?

Answer 6

These are formed when igneous or sedimentary rocks have been melted by heat or pressure near a volcano or as the earth’s crust bends. Metamorphic rocks are characterised by layering but the layers are often irregular and frequently contain clearly distinguishable crystals, for example, schist, gneiss. Hardened shales are slates.

Question 7

What are the modes of transport for Transported parent materials?

Answer 7

water
wind
gravity
ice

Question 8

What are water deposits called?

Answer 8

Water deposits are alluvium and lake deposits.The soils developed from alluvial parent materials have the following features:

Rounded stones and sand particles due to abrasion in the stream;

Layers of different texture. As you dig into the soil, you may observe many and rapid changes in soil texture, for example, sand layers between clay layers. The different layers are deposited depending on the type of material being transported by the creek or river;

Buried layers. Old soils originally at the surface which had organic matter can be covered by subsequent deposits. Hence organic rich layers exist in the subsoil.

Question 9

What are wind deposits called?

Answer 9

Wind transported materials form aeolian deposits.
Wind lifts particles from the soil surface. Only particles or aggregates that are greater than clay size (silt and sand sizes) can be lifted from the soil surface.

Question 10

What is Gravity affected soils called?

Answer 10

The material moving downslope under the effects of gravity combined with some water flow is called ‘colluvium’.

The soils that develop from colluvial parent material have:
Buried organic layers
angular broken rock fragments, and
layers of different texture.

Question 11

How does ice transport soils?

Answer 11

The material either carried on the surface or within the ice produces:

Glacial till - mixed textures but compacted, so poor drainage properties.
glacial outwash - materials deposited by the water melting from the glacier.

Question 12

What are the four soil formation types?

Answer 12

Transformation (minerals to different sizes and composition) i.e. weathering;
Translocation (or transporting materials through the developing soil); i.e. leaching;
Additions
Losses

Question 13

define the meaning of Transformation in soils.

Answer 13

Weathering is the physical or chemical alteration or transformation of the parent material.

Question 14

What are the two types of weathering that occur?

Answer 14

Physical

Chemical

Question 15

What are the 4 types of physical weathering?

Answer 15

Temperature
Water
Wind
Plant roots

Question 16

How does temperature affect rocks?

Answer 16

The different minerals present in rocks heat and cool at different rates. This heating and cooling causes expansion and contraction respectively of the minerals. Because minerals change size at different rates, stresses build up in the rock that can result in fracture (the particles get smaller). The greatest changes occur at sunrise and sunset especially in arid environments. In colder climates, the effects are associated with freezing and thawing of water in cracks in rocks.

Question 17

How does water effect rocks?

Answer 17

Water transports sand and silt particles as it flows. This suspended material abrades rock fragments reducing particle size. This action is particularly noticeable in coastal environments where wave action occurs.

Question 18

How does wind effect rocks?

Answer 18

In arid areas, wind transports particles that again abrade rocks.

Question 19

How do plant roots effect rocks?

Answer 19

Once plants become established, roots grow into cracks in rocks. As the roots enlarge during growth, the cracks are enlarged until fracture occurs.

Question 20

What is chemical weathering?

Answer 20

This type of weathering depends on the reaction of water molecules with the various rock minerals. These reactions normally result in;

The alteration of rock minerals to soil minerals;
the reduction in particle sizes;
the release of plant nutrients in soluble, that is, plant available forms.

Question 21

What are the 6 chemical forms of weathering in rocks?

Answer 21

Dissolution 
Hydrolysis
Hydration
Oxidation/Reduction
Carbonation
Chelation

Question 22

What is dissolution?

Answer 22

Some of the rock minerals dissolve in water and can be removed in water percolating through or flowing over the rock.

Question 23

What is Hydrolysis?

Answer 23

This is probably one of the more important reactions. In this reaction, the small concentration of H+ present in water reacts with the rock minerals releasing Ca2+, Mg2+, K+ and Na+. The OH- fraction of the water may react with the cations that are released

Question 24

What is Hydration?

Answer 24

Water is added to the structure of the minerals in the parent material. This is different from hydrolysis as the water does not have to be present as H+ and OH- for this reaction to occur.

As layer silicate minerals such as mica hydrate, they expand. This enables more water to enter the crystal allowing other reactions to occur.

Question 25

What is Oxidation / Reduction?

Answer 25

This occurs mainly with Fe present in minerals. The Fe in minerals is normally in the reduced Fe2+ form. On exposure to air (containing O2), the Fe is oxidised to the Fe3+ form. It is this form of iron oxide which gives soil their red, brown and yellow colours. Thus, as soils weather they tend to develop these colours especially in their subsoils.

If at some stage, the soil becomes waterlogged, the Fe is reduced to Fe2+ producing grey or mottled colours.

Question 26

What is Carbonation?

Answer 26

When CO2 released by plant or microbial activity is dissolved in water, the water becomes more acid. Thus reactions such as hydrolysis can proceed more rapidly. It can also increase the solubility of some minerals such as calcium carbonate.
i.e Carbonic Acid

Question 27

What is Chelation?

Answer 27

Soluble organic molecules released during decomposition of plant and microbial tissues, can take Fe, Mn and Cu into solution. This removal helps weathering to continue. These polyvalent metallic cations can be removed to deeper layers.

Question 28

What are the five factors affecting soil formation?

Answer 28

Climate, topography, parent material, organisms and time are known as the soil forming factors.

Question 29

What does soil classification enable?

Answer 29

Grouping soils into useful categories means that statements about one soil can apply to another in the group.
Allows for a logical examination of the soil and can become a useful tool in diagnosing soil problems.
Experienced workers can make references in relation to other soil properties not originally used in classification.
Enables communication.

Question 30

What are the three classification systems?

Answer 30

Great Soil Groups
Factual Key (Northcote)
The Australian Soil Classification

Question 31

How does the Great Soils Group work?

Answer 31

Individual soils described are grouped into great soil groups. This means that soils were grouped according to similarity in morphology and commonality of soil forming factors such as climate, vegetation and topographic relationships.

Question 32

What are the four divisions of the Northcote Factual key?

Answer 32

The divisions and their descriptive code letter are:

O- organic - the top 25 cm is dominated by organic matter
G- gradational - a gradual increase in clay content with depth
D- duplex - a sudden increase in clay content with depth
U- uniform - little change in clay content with depth