Soil
2.2.1 Chemical properties of soil
Specifications
2.2.1 (a)
examine the chemical features of soil including plant-available nutrients, pH
and liming, cation exchange, and flocculation
2.2.1 (b)
conduct an investigation into the chemical properties of soil to
• demonstrate cation exchange capacity (CEC)*
• show flocculation*
• determine the pH*
Examine
consider an argument or concept in a way that uncovers the
assumptions and relationships of the issue
Conduct
to direct in action or course; manage
Ion
An atom or a molecule that has a net electric charge
Anion
Negatively charged ion
Cation
Positively charged ion
Soil colloids
Small particles of soil clay and soil humus
Flocculation
The clustering together of soil particles to create larger
structures called floccules
Cation exchange
The ability of soil particles (clay and humus) to attract, retain
and release cations
Cation exchange capacity (C.E.C.)
The capacity of a soil to exchange cations between the soil
surfaces and water (soil solution)
pH
A measure of the concentration of hydrogen ions in a solution
Liming
Spreading ground limestone on soil
Milliequivalent (mEq)
Amount of a substance that will react with a certain number of
hydrogen ions
Soil
2.2.1 Chemical properties of soil
Chemical properties that are important in determining the characteristics of a soil:
1. Flocculation 2. Cation exchange 3. pH 4. Plant-available nutrients
Ion
Cation
Anion
An atom or a molecule that
has a net electric charge
Positively charged ion
Negatively charged ion
Soil colloids: smallest particles, soil clay and soil humus, most chemically active
Flocculation
The clustering together of soil particles to create larger structures called floccules
1.
Chemical reactions form negative charges on soil colloids
2. Cations are adsorbed onto the surface
3. The colloids are linked together by polarized water to form a floccule
4. Floccules trap larger particles such as sand to form aggregates
An aggregate is a substance formed by combining
several separate elements
Cations that promote flocculation and that are abundant in Irish soils:
Al3+, Fe3+, Ca2+, Mg2+, H+, K+, Na+
Soil
2.2.1 Chemical properties of soil
Cations are attracted to negatively charged humus and clay particles
They are released into the soil through weathering
Cation exchange:
The ability of soil particles (clay and humus)
to attract, retain and release cations.
Constantly taking place.
A cation with 2 +ve charges can replace two
separate cations with 1 +ve charge.
Cation exchange capacity (C.E.C.)
The quantity of cations that a soil adsorbs or the capacity of a soil to exchange cations
between the soil surfaces and the soil solution
Factors affecting cation exchange capacity
Humus content
Clay content
Soil texture
Highest CEC value as colloidal
humus has high number of –ve
charges
Higher CEC with a higher clay
content in soil
Fine textured soils have a higher
CEC
pH
Low pH level (below 5) leads to fewer cations in the soil. pH can be increased by liming
(increases Ca2+). Increasing pH leads to increase of –ve charges on soil colloids (greater CEC)
Soil
2.2.1 Chemical properties of soil
pH
A measure of the concentration of the
hydrogen ions in a solution . It can also be
expressed as the negative log of the hydrogen
ion concentration:
pH = -log10[H+]
pH scale
0
1
2
3
4
5
6
7
Neutral
Acidic
8
9
10
11
12
13
14
alkaline
pH range of most soils
pH for ideal agricultural soils
5.5 - 8.5
6.5 - 7.5
Acidic ions
Alkaline ions
Hydrogen and Aluminium
Calcium and magnesium
Hydrogen is derived from carbonic acids
Aluminium is derived from granite or sandstone
Derived from limestone
and the application of lime
Available soil nutrients
Availability of certain nutrients is reduced if soils become increasingly acidic or alkaline
Most nutrients are available between pH 6 – 7
Raising the pH by liming can improve the availability of nutrients in acidic soils
Liming:
Spreading ground limestone on soil
Contains calcium and magnesium
pH levels increase and acidic levels of the soil decreases
Reduces acid leaching
Medium-term activity (takes approx. 2 years for full effects)
Ca 2+ and Mg2+ replace H+ and Al3+ ions
Ground limestone that is used for agriculture must consist of the following:
•
•
•
•
Crushed natural limestone that contains calcium carbonate and
magnesium carbonate
Moisture no greater that 2.5%
Total neutralizing value no less than 90%
Pass through a 3.35 mm sieve and not less than 35% through a
0.15 mm sieve
Soil
2.2.1 Chemical properties of soil
Questions
What are the chemically
active particles in soil?
Do humus and clay particles
have a negative charge?
List the chemical properties
that are important in
determining the
characteristics of a soil
What is flocculation?
What are floccules also
known as?
Name two soil colloids
What are soil colloids?
How does flocculation occur?
List three cations that
promote flocculation
What are cations?
What are anions?
What is cation exchange?
What does C.E.C. Stand for?
What is cation exchange
capacity (C.E.C.)?
How are cations released into
soil?
When does cation exchange
take place?
What factors affect cation
exchange capacity?
Define the term leachate
What is pH?
What is the pH value scale?
What is the value given to a
neutral pH?
What do you describe a
solution with a pH less than
7?
What do you describe a
solution with a pH more than
7?
List two acidic ions
List two alkaline ions
Where are hydrogen ions
derived from?
Where are aluminium ions
derived from?
Where are calcium and
magnesium ions derived
from?
Name the liquid used to
determine the pH of a
substance (soil)
What is the optimum pH level
for crop growth?
What pH can reduce the
availability of nutrients to
plants?
How can you increase the
availability of essential
nutrients in an acidic soil?
What is liming?
What does ground limestone
contain?
How does liming work?
What are the advantages of
liming?
What must the moisture
content of ground limestone
for agriculture be?
What must the total
neutralizing value of ground
limestone for agriculture be?
What size sieve should
ground limestone for
agriculture pass through?