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Environmental processes and systems, LEC 103 > Soil chemistry, Lecture 29 > Flashcards

Soil chemistry, Lecture 29 Flashcards

1
Q

Key reactions and processes in soil chemistry

6

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

Where is most plant nutrient derived from?

A

the soil

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3
Q

Two types of nutrients that plants consume

A

macronutrients and micronutrients

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4
Q

Microcontaminants

A

not wanted by plants

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5
Q

How are nutrients present in the soil?

A

as anions and cations

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6
Q

Where are micronutrient deficient soils common worldwide?

A

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

where cereals are usually grown as staple crops

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7
Q

Why are many calcareous soil micronutrient deficient?

A

high pH and predominate calcium carbonate contents

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8
Q

Soil pH

A

H+ ion conc. of soil water

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9
Q

What is dependent on pH

A

many chemical and biological reactions

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10
Q

What does pH influence

A

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

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11
Q

Can soil pH be manipulated?

A

yes

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12
Q

What does soil pH range from?

A

3-11

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13
Q

pH range of UK agricultural soils?

and ideal range of them?

A

5-8

ideally: 5.5-6

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14
Q

pH of peat soil

A

3

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15
Q

Common pH range of soil for humid region

A

5-7

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16
Q

Common pH range of soil for arid region

A

7-9

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17
Q

Soil ion exchange

A

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

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18
Q

Cation exchange capcity

A

Propensity for adsorbing cations

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19
Q

Anion exchange capacity

A

Propensity for adsorbing anions

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20
Q

Cation charge

A

positive

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21
Q

Anion charge

A

negative

22
Q

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

A

clay and organic matter

23
Q

Sorptive property

A

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

24
Q

Silicate clay particle

net charge

A

net negative charge

25
Q

Iron/aluminium oxide particle

net charge

A

net positive charge

26
Q

‘Competition’ between ions to…

A

reach the soil surface

27
Q

CEC

A

the sum total of cations that soil can adsorb as ‘exchangeable’

28
Q

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

A

much higher

29
Q

Cation exchange largely dependent on

A

clay and organic matter

30
Q

What does cation exchange control?

A

nutrient/pollutant availability and loss

31
Q

Characteristics of cation exchange

5

A 
rapid
reversible
exchange = cation charge (mole/valency)
ratio law
selective
32
Q

CEC

stands for

A

cation exchange capacity

33
Q

A high CEC value indicates

A

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

34
Q

High CEC value

A

> 25

35
Q

Low CEC value

A

<5

36
Q

A low CEC value indicates

A

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

37
Q

Active acidity

A

due to the H+ ion activity in the soil solution

38
Q

Reserve acidity

A

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

39
Q

How to measure active acidity

A

pH probe

40
Q

Soils as pH buffers

A

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

41
Q

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

A

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

42
Q

Managing soil activity
neutralisation of acidity
3

A

addition of:

1. limestone (Liming
2. gypsum
3. organic matter
2. and 3. reduce Al toxicity)

43
Q

Managing soil acidity
Selecting adapated plants
2

A

selection of tolerant plant species

genetic improvement of acid tolerance

44
Q

Two methods for managing soil acidity

A

neutrailisation of acidity

selecting adapted plants

45
Q

Precipitation

A

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

46
Q

Precipitate

A

solid formed from a chemical reaction in a liquid

47
Q

Common precipitates

3

A

hydroxides
carbonates
sulphides

48
Q

A complex

A

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

49
Q

Are substances more or less mobile when they are complexed?

A

less mobile

50
Q

Nutrient bioavailability

A

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

Environmental processes and systems, LEC 103 (10 decks)

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