Lecture 29- Soils

Question 1

What are the three key macro nutrients for plant growth

Answer 1

Nitrogen, potassium, phosphorous

Question 2

Why are micronutrients important?

Answer 2

To help plants carry out key enzyme functions

Question 3

Why are carbon, oxygen and hydrogen not considered major soil macro nutrients?

Answer 3

They are predominantly gathered from atmospheric and water sources by plants, and not the soil

Question 4

What is a cation and what is an anion?

Answer 4

A cation is a negatively charge ion, a cation is a positively charged one

Question 5

Saplings the yield response to micronutrient contamination?

Answer 5

Stage 1. Deficiency. This is where increasing concentrations cause an increase in plant weight/ growth
Stage 2. Adequate. The concentrations are in sufficient quantities that any increases do not have an impact on plant growth
Stage 3. Toxicity. Concentrations get too high and cause toxic conditions, causing a decrease in plant weight

Question 6

Explain the yield response to micro contaminants

Answer 6

As the plants don’t rely on them for growth, increases in the concentrations of micro contaminants have no impacts until reaching toxicity levels

Question 7

Why are cancerous soils often micronutrient deficient?

Answer 7

The high ph and cancerous content cause micronutrient deficiencies. These micronutrient deficiencies result from less h+ ions biding to colloid surfaces, meaning micronutrients can

Question 8

Where are alkaline soils and associated micronutrient deficiencies most common?

Answer 8

Middle East, North Africa, south west USA, Australia

Question 9

What is PH?

Answer 9

A measure of the H+ ion concentration in solution. More hydrogen atoms means a more acidic solution

Question 10

Why is ph considered a master variable in chemical reactions?

Answer 10

Many chemical and biological reactions are dependant on it

Question 11

What is the ph range for mineral dominated soils?

Answer 11

4.0 to 10.0

Question 12

What is the common ph range for humid regions?

Answer 12

5-7

Question 13

How do soils act as a buffer zone?

Answer 13

They act as a buffer between the atmosphere and groundwater, retaining nutrients and supply them in a steady flow

Question 14

Within soil, what absorbs ions, molecules and gas?

Answer 14

Clay minerals and organic matter

Question 15

What causes soil to have its sorptive properties?

Answer 15

Large surface area of clay and hummus, electrical charges

Question 16

What properties cause greater sorptive properties?

Answer 16

1. Divalent ions are more preferable than mono charged ones

2. Large cations are preferred over smaller ones of the same charge, due to their greater surface area

Question 17

What is cation exchange capacity?

Answer 17

A sum of the total cations that a soil can absorb

Question 18

What is cation exchange capacity dependant on?

Answer 18

1. Negative charge density
2. Number of exchange sites
3. Soil ph

Question 19

How does cec impact a soils buffering capacity?

Answer 19

Soils with a greater cec have a greater buffering capacity

Question 20

What are the three types of acidity in soils?

Answer 20

1. Active acidity
2. Reserve acidity
3. Exchangeable acidity

Question 21

What is active acidity in soils?

Answer 21

Quantity of H+ ions in soil water solution

Question 22

What is the reserve activity in soils?

Answer 22

The amount of H+ ions held on soil surfaces via adsorption. This has a greater capacity than soil water.

Question 23

What is the exchangeable acidity of soils?

Answer 23

The proportions of H+ and aluminium ions held within the soils diffuse layer. These can easily move into solution

Question 24

How does the addition of acid to soils impact ions on cation exchange sites? Refer to active and reserve acidity

Answer 24

The addition of little acid will cause displacement of the cations bound to the soil surface (by H+ ions). This means there is no change in the soil water PH (which is the measured part) as the H+ ions are held on colloid surfaces and not in solution. When the reserve acidity is full, H+ ions are then encore rates into active acidity, altering PH.

Question 25

Name the three ways of neutralising soil acidity , and one mitigation technique

Answer 25

1. Liming. This is commonly the addition of limestone and Dolomites to soils to replace H+ ions with ca and mg on colloid surfaces.
2. Addition of gypsum to reduce al toxicity
3. Addition of om, as this increase the cec of soil (more surfaces) and increases base saturation which decreases acidity.
4. Simply plant plants that are better adapted to acidic conditions

Question 26

Explain the difference between adsorption, desorption, and absorption

Answer 26

Adsorption is where molecules adhere to the surface of a phase.
Desorption is when molecules are released from the surface of a phase.
Absorption is when the molecules are drawn into the bulk of the phase.

Question 27

What is complication and how does it impact movement in soils?

Answer 27

A complex is a structure consisting of a central atom bound to surrounding molecules (e.g a ligand). Complex substances have decreased mobility, reducing the bioavailability of nutrients within the soils

Question 28

How do reduced soils impact soil chemistry?

Answer 28

They alter the form of metallic minerals present, and their subsequent reactivity

Question 29

Regarding all the nutrients within soils, which store can plants access?

Answer 29

Plants can ONLY access macro and micro nutrients available in soil water. This is due to the buffering capacity of the soil

Question 30

What is bioavailability of nutrients a function of? (9 factors)

Answer 30

1. Total concentration of the nutrient in soils
2. microbial factors (influencing whether nutrients are broken out of bonds)
3. Plant factors
4. Redox potential of soil
5. salinity
6. soil ph
7. Organic matter content (influxes cec)
8. clay content
9. Oxide content

Question 31

Which factors control the cec of soils?

Answer 31

2. soil ph
3. Organic matter content
4. clay content
5. Oxide content

Question 32

Why are agricultural soils typically more acidic than natural ones?

Answer 32

When plants die, they “replace” cations that counter acidity (eg. Ca2+) however, when plants are taken away for harvest they do not return these ions to the soil. This means there is nothing remaining to counter acidity, increasing acidity of soils

Question 33

What are colloids and why are they important in soils?

Answer 33

The most chemically active part of the soil, due to their large surface area. These large surface areas increase the cec, attracting soil nutrients and ions. This governs physical and chemical properties in the soil

Question 34

Why are anions often less present in soil than cations?

Answer 34

Cations are able to bond to particle surfaces due to differences in charge, whereas anions can’t. This makes them substantially more vulnerable to leaching processes

Question 35

What does a high cec value indicste > 25?

Answer 35

This indicates a high clay or organic matter content that can old a lot of cations

Question 36

What does a low cec value (<5)indicate?

Answer 36

A Sandy soil it’s limited organic matter that cannot hold many cations