Soil chemistry, Lecture 29

Question 1

Key reactions and processes in soil chemistry

6

Answer 1

1. pH
2. Ion exchange and adsorption/desorption
3. Cation (anion) exchange
4. Precipitation and dissolution
5. Complexation
6. Oxidation/reduction

Question 2

Where is most plant nutrient derived from?

Answer 2

the soil

Question 3

Two types of nutrients that plants consume

Answer 3

macronutrients and micronutrients

Question 4

Microcontaminants

Answer 4

not wanted by plants

Question 5

How are nutrients present in the soil?

Answer 5

as anions and cations

Question 6

Where are micronutrient deficient soils common worldwide?

Answer 6

Middle East, Northern Africa, south-west USA and Australia

where cereals are usually grown as staple crops

Question 7

Why are many calcareous soil micronutrient deficient?

Answer 7

high pH and predominate calcium carbonate contents

Question 8

Soil pH

Answer 8

H+ ion conc. of soil water

Question 9

What is dependent on pH

Answer 9

many chemical and biological reactions

Question 10

What does pH influence

Answer 10

the solubility of plant nutrients and pollutants (particularly metals) and therefore, their availability

Question 11

Can soil pH be manipulated?

Answer 11

yes

Question 12

What does soil pH range from?

Answer 12

3-11

Question 13

pH range of UK agricultural soils?

and ideal range of them?

Answer 13

5-8

ideally: 5.5-6

Question 14

pH of peat soil

Answer 14

3

Question 15

Common pH range of soil for humid region

Answer 15

5-7

Question 16

Common pH range of soil for arid region

Answer 16

7-9

Question 17

Soil ion exchange

Answer 17

Exchange (i.e. mobility) of ions in minerals with ions in solution

Question 18

Cation exchange capcity

Answer 18

Propensity for adsorbing cations

Question 19

Anion exchange capacity

Answer 19

Propensity for adsorbing anions

Question 20

Cation charge

Answer 20

positive

Question 21

Anion charge

Answer 21

negative

Question 22

Soils which contain what act as a buffer zone between the atmosphere and groundwater

Answer 22

clay and organic matter

Question 23

Sorptive property

Answer 23

Arises from the electrical charges and large SA of clay minerals and humus

Question 24

Silicate clay particle

net charge

Answer 24

net negative charge

Question 25

Iron/aluminium oxide particle | net charge

Answer 25

net positive charge

Question 26

'Competition' between ions to...

Answer 26

reach the soil surface

Question 27

CEC

Answer 27

the sum total of cations that soil can adsorb as 'exchangeable'

Question 28

Is the amount of cations held on the soil surface higher or lower thsn in the overall soil?

Answer 28

much higher

Question 29

Cation exchange largely dependent on

Answer 29

clay and organic matter

Question 30

What does cation exchange control?

Answer 30

nutrient/pollutant availability and loss

Question 31

Characteristics of cation exchange | 5

Answer 31

``` rapid reversible exchange = cation charge (mole/valency) ratio law selective ```

Question 32

CEC | stands for

Answer 32

cation exchange capacity

Question 33

A high CEC value indicates

Answer 33

A good indicator that a soil has a high clay and/organic matter content and can hold a lot of cations

Question 34

High CEC value

Answer 34

>25

Question 35

Low CEC value

Answer 35

<5

Question 36

A low CEC value indicates

Answer 36

a good indication that a soil is sandy with little or no organic matter, and that is can't hold many cations

Question 37

Active acidity

Answer 37

due to the H+ ion activity in the soil solution

Question 38

Reserve acidity

Answer 38

Represented by the H+ that are easily exchanged by other cations (positively charged ion)

Question 39

How to measure active acidity

Answer 39

pH probe

Question 40

Soils as pH buffers

Answer 40

As protons come into soil solution they will exchange with other cations

Question 41

Why will adding a little acid have little effect on soils?

Answer 41

The pH won't drop much as the H+ is exchanged up to the maximum CEC of the soil

Question 42

Managing soil activity neutralisation of acidity 3

Answer 42

addition of: | 1. limestone (Liming 2. gypsum 3. organic matter 2. and 3. reduce Al toxicity)

Question 43

Managing soil acidity Selecting adapated plants 2

Answer 43

selection of tolerant plant species | genetic improvement of acid tolerance

Question 44

Two methods for managing soil acidity

Answer 44

neutrailisation of acidity | selecting adapted plants

Question 45

Precipitation

Answer 45

The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid.

Question 46

Precipitate

Answer 46

solid formed from a chemical reaction in a liquid

Question 47

Common precipitates | 3

Answer 47

hydroxides carbonates sulphides

Question 48

A complex

Answer 48

A structure consisting of a central atom or ion (usually metallic), bonded to a surrounding array of molecules or anions (ligands, complexing agents)

Question 49

Are substances more or less mobile when they are complexed?

Answer 49

less mobile

Question 50

Nutrient bioavailability

Answer 50

the total nutrient is not equal to the amount which is bioavailable