Soil

Question 1

What is soil?

Answer 1

• Formed by pedogenesis over 1000s of years
• An open system with flows going in and out
• Total soil compartment is not finite

Question 2

What are the inputs of soil?

Answer 2

• Gas inputs
• Plant and animal decomposition
• Solar energy
• Weathering of bedrock
• Accumulation of particles
• Mineral precipitation

Question 3

What are the outputs of soil?

Answer 3

• Wind and water erosion

- Leaching

Question 4

Why are soils important?

Answer 4

Soils are the interface between the lithosphere, biosphere, hydrosphere and atmosphere

Question 5

What does soil produce?

Answer 5

• Soils produce gases such as methane and carbon dioxide as carbon rich plant material decomposers
• Important for global carbon cycle and nitrogen fixation

Question 6

How much carbon is stored in earths soil?

Answer 6

The amount of carbo stored in earths soil is estimated at 1500 PgC – 1000 billion grams

Question 7

How does carbon get into the soil

Answer 7

• Photosynthesis, then plant death and then plant consumption by animals and microbes

Question 8

How does carbon leave the soil?

Answer 8

• Microbial respiration
• Eating larger carbon containing molecules, like sugars and breaking them into smaller components = carbon dioxide and methane

Question 9

What ecosystem functions and services does soil provide?

Answer 9

• Provides medium for food and fuel production
• Store water and modify flood run off
• Infiltration and and storage in soil pore space
• Provides habitats and important for biodiversity
• recycles plant and animal waste - cycling carbon and nitrogen

Question 10

Describe soil composition

Answer 10

• Composition by volume
• Highly variable
• Solid - minerals and organic matter takes up 40-60%
• Pore space - water 20-50% or gas 10-15%
• Organic material - 10%

Question 11

What are the geological controls on soil chemistry?

Answer 11

• Mineral component of soils is strongly dependent on the underlying parent mineral

Question 12

What are the impacts of soil composition on human health and food security?

Answer 12

• The goitre belt

- Selenium deficiencies

Question 13

What is the goitre belt?

Answer 13

• Goitre (hypothyroidism), perinatal mortality – caused by iodine deficiency – needed for the production of thyroxine in the thyroid gland
• Main source is dietary – crops, sea weed, shellfish
• Soil iodine concentrations dependent on coastal proximity and chemical mobility

Question 14

What is a selenium deficiency?

Answer 14

• Essential trace element: healthy immune systems, sperm motility, production of thyroid hormones
• Deficiency – linked to mood swings, cancers and diabetes
• Dietary intake is dependent upon soil chemistry
• Concentration in soils is directly dependent upon the concentration in underlying geology

Question 15

Why is soil water important?

Answer 15

• Important for delivering nutrients, removing toxins and controlling aeration of soils

Question 16

What are the 3 key water states of soil?

Answer 16

1) Saturation - no gas present
2) At field capacity
3) Permanent wilting point - plants cannot access it

Question 17

What is the relationship between soil water and texture?

Answer 17

• Wilting point increase as particle size decreases

- Finer grain particles, more surface area, more water being held onto particles/sediments

Question 18

What is soil waters holding capacity?

Answer 18

• Varies globally - depends on physical characteristic of soil
• This has consequences for food security and reliability of crop yields

Question 19

What is the typical soil profile?

Answer 19

• Distinctive horizontal layers due to vertical translocation of materials by water
• Gravity and water flow moves material up and down
• Generally water moves down

Question 20

What is eluviation?

Answer 20

The movement of material

Question 21

What are the different layers of soil?

Answer 21

• O - organic horizon
• A - mineral horizon
• E - mineral horizon
• B - mineral horizon
• C - unconsolidated material
• R - bedrock

Question 22

What are environmental factors affecting soil formation?

Answer 22

• Climate – temperature, rainfall, vegetation, rates of weathering
• Underlying geology – some minerals more easily weathered
• Relief and topography – hillslopes/floodplains
• Biota – bioturbation, decomposition, trampling and compaction

Question 23

Outline the physical properties of soils texture

Answer 23

• Particles - clays, silts, sands, gravels - defined by diameter
• Solution - anything passed through sieve
• Texture - distribution fo particle size determines soil texture

Question 24

Outline the chemical properties of soil

Answer 24

• A major component of soils is colloidal and fine- grained clay minerals
• Phyllosilicates or platy minerals comprising layers of Si and Al bound to oxygen

Question 25

What is the sheet structure of soil?

Answer 25

• Minerals slip over each other
• Sorption capacity
• Isomorphic substitution – negative charge – chemically reactive

Question 26

What is cation exchange?

Answer 26

Negative sites of clay minerals are every balanced with positively charged cations

These are absorbed and exchangeable - can be displaced by other cations in solution

Question 27

What is pH?

Answer 27

A logarithmic method of expressing the concentration of hydrogen

Question 28

What is the pH of most soils

Answer 28

pH is typically between 3.5 - 9

Question 29

What processes result in hydrogen becoming a predominant cation in soil?

Answer 29

• Leaching of wet cations
• Carbon dioxide from reparation dissolves into carbonic acids
• Crop harvest removes base cations taken up by plants