Soil chemistry, Lecture 29 Flashcards
Key reactions and processes in soil chemistry
6
- pH
- Ion exchange and adsorption/desorption
- Cation (anion) exchange
- Precipitation and dissolution
- Complexation
- Oxidation/reduction
Where is most plant nutrient derived from?
the soil
Two types of nutrients that plants consume
macronutrients and micronutrients
Microcontaminants
not wanted by plants
How are nutrients present in the soil?
as anions and cations
Where are micronutrient deficient soils common worldwide?
Middle East, Northern Africa, south-west USA and Australia
where cereals are usually grown as staple crops
Why are many calcareous soil micronutrient deficient?
high pH and predominate calcium carbonate contents
Soil pH
H+ ion conc. of soil water
What is dependent on pH
many chemical and biological reactions
What does pH influence
the solubility of plant nutrients and pollutants (particularly metals) and therefore, their availability
Can soil pH be manipulated?
yes
What does soil pH range from?
3-11
pH range of UK agricultural soils?
and ideal range of them?
5-8
ideally: 5.5-6
pH of peat soil
3
Common pH range of soil for humid region
5-7
Common pH range of soil for arid region
7-9
Soil ion exchange
Exchange (i.e. mobility) of ions in minerals with ions in solution
Cation exchange capcity
Propensity for adsorbing cations
Anion exchange capacity
Propensity for adsorbing anions
Cation charge
positive
Anion charge
negative
Soils which contain what act as a buffer zone between the atmosphere and groundwater
clay and organic matter
Sorptive property
Arises from the electrical charges and large SA of clay minerals and humus
Silicate clay particle
net charge
net negative charge
Iron/aluminium oxide particle
net charge
net positive charge
‘Competition’ between ions to…
reach the soil surface
CEC
the sum total of cations that soil can adsorb as ‘exchangeable’
Is the amount of cations held on the soil surface higher or lower thsn in the overall soil?
much higher
Cation exchange largely dependent on
clay and organic matter
What does cation exchange control?
nutrient/pollutant availability and loss
Characteristics of cation exchange
5
rapid reversible exchange = cation charge (mole/valency) ratio law selective
CEC
stands for
cation exchange capacity
A high CEC value indicates
A good indicator that a soil has a high clay and/organic matter content and can hold a lot of cations
High CEC value
> 25
Low CEC value
<5
A low CEC value indicates
a good indication that a soil is sandy with little or no organic matter, and that is can’t hold many cations
Active acidity
due to the H+ ion activity in the soil solution
Reserve acidity
Represented by the H+ that are easily exchanged by other cations (positively charged ion)
How to measure active acidity
pH probe
Soils as pH buffers
As protons come into soil solution they will exchange with other cations
Why will adding a little acid have little effect on soils?
The pH won’t drop much as the H+ is exchanged up to the maximum CEC of the soil
Managing soil activity
neutralisation of acidity
3
addition of:
1. limestone (Liming
2. gypsum
3. organic matter
2. and 3. reduce Al toxicity)
Managing soil acidity
Selecting adapated plants
2
selection of tolerant plant species
genetic improvement of acid tolerance
Two methods for managing soil acidity
neutrailisation of acidity
selecting adapted plants
Precipitation
The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid.
Precipitate
solid formed from a chemical reaction in a liquid
Common precipitates
3
hydroxides
carbonates
sulphides
A complex
A structure consisting of a central atom or ion (usually metallic), bonded to a surrounding array of molecules or anions (ligands, complexing agents)
Are substances more or less mobile when they are complexed?
less mobile
Nutrient bioavailability
the total nutrient is not equal to the amount which is bioavailable