RHS Level 2 R2102 Soils

Question 1

What is meant by parent rock?

Answer 1

Parent rock is the underlying rock beneath the soil

Question 2

What is meant by parent material?

Answer 2

Generally pieces which have broken away from the parent rock.
In organic soil, parent material refers to the plants that created it.

Question 3

How are mineral soils formed?

Answer 3

From the breakdown products of rocks.

Affected by parent materials, climate, living organisms, topography and time.

Question 4

How does physical weathering affect soil formation?

Answer 4

Separation/cracking of rocks and mixing of particles by heat, freezing, erosion by rain/water, wind, earthquakes.

Question 5

How does chemical weathering affect soil formation?

Answer 5

Effects of the presence of water as with dissolving and leaching. PH has a strong effect (carbonic acid) as does temperature.

Question 6

How does biological weathering affect soil formation?

Answer 6

Plants and other organisms through physical (roots in cracks of rock) and chemical (organic acids) mean rocks can break up.

Question 7

What are the components of soil?

Answer 7

The major component is mineral particles, 50-60%.
Organic matter 1-10%
Living organisms less than 1%
The rest is made up of air and water, amounts fluctuate.

Question 8

Describe the mineral particles component in soil.

Answer 8

Pieces of rock of varying sizes. Produced by the weathering of the parent rock.
They are the largest ingredient in soil 50-60%
They can often hold and supply nutrients and water for plant growth.
Classified by size, larger than 2mm are stones/gravel.
Less than 2mm will be either sand, silt or clay.

Question 9

What are the sizing of sand, silt and clay and stones?

Answer 9

Stones/gravel: anything bigger than 2mm

Sand: 2mm - 0.06mm diameter

Silt: 0.06mm - 0.002mm diameter

Clay: 0.002mm and below

Question 10

Describe the organic matter component in soil.

Answer 10

Dead or decaying plant or animal matter.
1-10% in the soil make-up
Can store and supply nutrients and water.
O.M is broken down in soil to form a dark brown/black stable material called humus.

Question 11

Describe the process of mineralisation.

Answer 11

Fresh organic matter decomposes in soil through the actions of soil organisms, releasing the nutrients contained within the organic matter that plants can then use for growth. Nitrogen and carbon cycles.
This process occurs continuously in the soil as long as conditions are suitable, adequate temperature and moisture.

Question 12

Describe the living soil organisms component in soil.

Answer 12

These require a supply of organic matter to feed and multiply.
Bacteria, fungi, algae, protozoa, nematodes…
Optimum numbers require a warm, moist, aerated soul with pH around 6.5-7
They work best if soil is left undisturbed.
Most are beneficial, some are regarded as pests.

Question 13

Describe the water and air components in soil.

Answer 13

Water: held in small pores or spaces between the soil particles, and within organic matter in soil.
Required by plants for growth, cooling, the uptake and transport of nutrients within the plant and to keep the plant rigid.

Air: found in the larger pores or spaces in soil.
Oxygen is needed by roots for the release of energy through respiration.

Question 14

Stages in formation of sedentary soils:

Answer 14

Rock.
Mosses.
Plant growth.
Leaf litter.
Top soil.
Sub soil.

Question 15

List the horizons of the soil profile:

Answer 15

Horizon O - Overlaying organic matter
Horizon A - Top soil
Horizon B - Subsoil
Horizon C - Parent material
Horizon R - Underlying rock/bedrock.

Question 16

Describe the soil horizons.

Answer 16

O - Organic matter. Un-decompsed/party decomposed organic debris. Humified layer.
A - Top soil. Courser in texture than layers below. Darker due to the incorporation of organic matter. Most roots found here.
B - Subsoil. This layer has undergone significant weathering so the original parent material is no longer discernable. Often materials from the above layers accumulation here through illuviation, clay, iron, aluminium etc …
C - Parent material. Immediately above the Rock layer. Least weathering. Low biological activity.

Question 17

Properties, good and bad, of top soil.

Answer 17

Lots of incorporated organic matter compared to subsoil.
Darker due to incorporation of O.M.
Courser texture with more pore space.
More air and water due to pore space.
Higher nutrient content.
Most roots found here.

Question 18

Properties, good and bad, of sub soil.

Answer 18

Lower pore space than top soil due to illuviation of finer particles into this layer.
Less space for air and water.
Less organic matter therefore fewer nutrients for plants.
Lighter than the top soil. Usually brown-red or brown-yellow due to mineral particles.

Question 19

Define ‘soil texture’

Answer 19

Soil texture is the relative proportions of sand, silt & clay within the soil.
Clay has the most dominant effect on soil. You need less of it to make a difference.

Question 20

Describe the characteristics of Clay Loam soils.

Answer 20

1) Feels very sticky when wet. Can be moulded. They are gluey and plastic.
2) They swell when wet and shrink when dry.
3) Can hold more total water than other soil types (not all is available to plants).
4) Rarely deficient in nutrients.
5) Late to warm up in spring (water heats up slower than mineral matter).

Question 21

Describe the characteristics of Sandy Loam soils.

Answer 21

1) Easily worked. Gritty.
2) Frequently low in nutrients. Not held by soil particles so are leached out.
3) Free draining. Hold onto less water. Drought prone.
4) Warm up early in spring.

Question 22

Describe the characteristics of Silt Loam soils.

Answer 22

1) Smooth and silky.
2) Poor retention of nutrients, but better than sand.
3) Good water holding capacity which can be used by plants.
4) Bad drainage. Particles don’t bond together to form aggregates.
6) Frost has little effect.

Question 23

Definite the term ‘soil structure’

Answer 23

The arrangement of Sand, silt and clay.
Individually e.g. grains of sand
In groups/aggregates e.g. crumbs
Or a mixture.
It can be altered by weather, plant roots, cultivations etc...

Question 24

What are soil aggregates?

Answer 24

Groupings of soil particles.

Question 25

What is a good Crumb Structure?

Answer 25

Good sized. E.g. 5mm for a seed bed.
High proportion of macropores to enable good air exchange, drainage, root growth etc…
Good stability - retains its structure in the presence of water etc…
Good mix of clay and humus.

Question 26

What might destroy soil structure?

Answer 26

Traffic/compaction. Topsoil.
Water/rain. Leading to capping on the surface.
Livestock. Poaching on wet ground. Topsoil.
Plough pan. Creating a compressed layer below topsoil.

Question 27

Describe a good soil structure.

Answer 27

Crumbs - loose, aggregates.
Good draining.
Worms.
Healthy plants.
Fungi.
Warm colours.

Question 28

Describe a bad soil structure.

Answer 28

Solid layer.
Standing water.
Capping.
No worms.
Too loose.
Cold colours.
Smells off.
Moss, reeds, sedges.

Question 29

Describe chemical processes affecting aggregate formation.

Answer 29

Chemical bonding due to large number of charges that clay particles carry.
Flocculation happens when Calcium ions, with a positive charge, are attracted to adjacent clay particles, with negative charges, at the same time bringing them together.

Question 30

Describe physical processes affecting aggregate formation.

Answer 30

The removal of water from between clay particles will cause shrinkage and cracking which build up over cycles of drying and rewetting.
Natural loss of water due to sun/high temps, plant root extraction and freezing can contribute to forming of aggregates and good Crumb Structure.

Question 31

Describe biological processes affecting aggregate formation.

Answer 31

Important in sand or silt soils.
Chemical linkage between organic molecules.
Fungal mycelia can produce tangled mass wrappings around soil particles holding them together.
Gelatinous exudates from plant roots and micro organisms can glue particles together.

Question 32

Describe 3 different stages of wetting of the soil.

Answer 32

1) Infiltration. The water soaks in.
2) Ponding. Water accumulates on the surface when infiltration is slower than rainfall.
3) Surface run-off. Movement of water over the soil.

Question 33

Describe saturation.

Answer 33

When water has filled all the pore spaces to the exclusion of everything else.
If no more water is applied to the surface, the water in the soil will drain out under the influence of gravity.
After 2-3 days the rate of water movement out of a sandy soil becomes quite slow and the soil is said to be at field capacity. High clay content soil may take a long time to drain.

Question 34

What’s the difference between saturation and field capacity?

Answer 34

Gravitational Water, which represents the water that has drained from the macropores of the soil.

Question 35

Describe field capacity.

Answer 35

After drainage, water is held in soil in the form of a film around all the soil particles and aggregates, against the forces of gravity.
Some water is held more loosely in mesopores and plants can access this.

Question 36

What is Soil Moisture Deficit?

Answer 36

The amount of water needed to return a soil to field capacity.

Question 37

Describe Permanent Wilting Point

Answer 37

When the water in the soil is held in micropores and held tightly around the soil particles to be accessed by plants roots.
The plants are unable to draw out water leading to irreversible wilting and death.

Question 38

What is available water?

Answer 38

The difference between Field capacity and the permanent wilting point. Mesopores occupied by available water.
Affected by soil texture and amount of organic matter in the soil.

Question 39

What is groundwater?

Answer 39

When the lower reaches of the soil and the parent material is saturated this is groundwater and the water table marks the top of this zone.
The water table depends on the season, the area and the depth of soil to the impermeable layer.

Question 40

Describe capillary action in soils.

Answer 40

Capillary forces are at work in all moist soils, moving water in all directions.

Question 41

How can water be lost in soil between Field capacity and PWP?

Answer 41

1) Evaporation from the soil surface. - increased by higher air temperature and wind speed or lower humidity levels.
2) Evapotranspiration from the surface combined with transpiration from vegetation. As a soil becomes covered by a leaf canopy the rate of water loss from soil becomes more closely related to transportation rates.
Water lost from leaves is replaced by water drawn from soil by the roots which in turn is being replaced by water from lower down.