2. Learning objectives
To state the mechanism of CEC
To define pH correctly
To identify the effect of pH on nutrient
availability for specific nutrients
To state TWO mechanisms by which pH may
adversely affect nutrient availability
To describe TWO mechanisms by which
excess of certain nutrients may make others
unavailable
To apply this knowledge to the diagnosis of
plant problems
3. Plant nutrient review -
macronutrients
Nitrogen – used by plants in two forms.
Ammonium (cation) and nitrogen (anion). Vital
for green, leafy growth – deficiency symptom
yellowing of older leaves first.
Phosphorous – vital for root development and
germination. Deficiency symptoms – stunted
growth and bluish/purple tinged leaves.
Potassium – vital for fruiting and flowering and
hardiness. Deficiency symptom – brown, often
curling edges to leaves, poor flowering and
fruiting.
4. Plant nutrient review – key
micronutrients
Iron – vital component of chlorophyll.
Deficiency symptoms – interveinal chlorosis,
younger leaves.
Calcium – vital component of cell walls.
Deficiency symptom – ‘bitter pit’ in apples,
‘blossom end rot’ in tomatoes, black spots on
leaves.
5. Review Question answers
1. Because nutrient presence in the soil may not
mean it is available to the plant or that the
plant can take it up. A leaf test may also be
required.
2. Nitrogen – older leaves first because the plant
can move nitrogen from plant tissues in the
event of deficiency.
3. To the amounts needed by the plant not to the
importance of the nutrients.
6. CEC - Terms used
Ion – an atom (single elemental particle) or
molecule (combination of more than one
elemental particles) in which the number of
protons and electrons are not equal –
therefore has an electrical charge.
Cation – an ion with a positive charge.
Anion – an ion with a negative charge.
Remember – opposite charges attract. Like
charges repel.
7. Cation Exchange Capacity
Clay and humus particles have a negative
surface charge.
Certain nutrient (and other) ions found in soil
water have a positive charge (cations).
Negative charges attract positive charges, so
holding these ions in the soil.
Which cations are held depends on their
relative concentrations in the soil water.
8. What is cation exchange?
The ability of one cation to displace
another from the negative charge site
on the clay or humus particle.
This means that the nutrient cations
can become available to the plant as
they are displaced.
Exchange sites therefore act as a
reserve for cations of nutrients in the
soil.
10. Soil pH
A measure of how acid or alkaline a soil is.
Technically a measure of concentration of
hydrogen ions.
Scale runs 1 – 14 where 1 is very acid, 14
very alkaline
Plants grow best (with some exceptions) in
the range 6.5 -7.5
11. Soil pH scale
Logarithmic scale – each point is 10
times greater than the one before. So
pH 7 is 0 (balanced), pH 6 is ten times
more acidic, pH 5 is 100 times more
acidic (10 x 10), pH 4 is 1000 times
more acidic (10 x 100) etc. Alkaline
soils are less acidic so pH 8 is 10
times less acidic than pH7 etc.
12. Soil pH – link to cation
exchange mechanism
Cations fall into two types – acidic and basic (or
alkaline). H+1 (hydrogen) and Al+3 (aluminium) are
acidic; Ca+2 (Calcium), Mg+2 (Magnesium) and K+1
(Potassium) are alkaline or ‘basic’ (meaning that
they have acid neutralising capacity).
So the more acidic the soil the more H+1 ions will
be either free in the soil water or attached to the
negative charge points on clay or humus soil
particles. This displaces the nutrient cations that
can then leach from the soil.
13. Plant nutrients and pH
So high soil acidity can lead to soils being
unable to hold alkaline (basic) nutrient
cations. This means that these are subject
to leaching and may become deficient.
Soil pH can also affect the way that plant
nutrients form chemical compounds with
other elements. If these compounds are less
soluble in soil water then the nutrients may
become unavailable to the plants.
This is called Immobilisation
14. Plant nutrient insolubility
For example: Phosphorous forms an insoluble
compound with Iron at low pH and with Calcium at
high pH.
Excessive application of calcium will lead to an
insoluble compound being formed with potassium.
Another possible cause of deficiency in the plant is
ion antagonism – that is high concentration of one ion
(like calcium) may block the root surface membrane
and prevent it from taking in another (such as
magnesium).
16. pH Worksheet answers
1. The concentration of hydrogen ions in soil
solution
2. More alkaline (the higher the number above 7
the more alkaline it is)
3. Positively charged (cations)
4. 1000 (7=0, 6=10 times, 5= 100 times, 4 = 1000
times)
5. Hydrogen ions will be exchanged for calcium
ions on the surface of clay or humus particles.
The calcium is displaced into the soil water .
17. pH worksheet answers
6. Iron
7. As the level of calcium (lime) in the soil itself is
normal it is unlikely to be low pH. Either
potassium (forms insoluble compound) or
magnesium (ion antagonism) is in excess .
Nutrient tests should diagnose
8. Nitrogen deficiency. Either the soil lacks
nitrogen because no N fertilizer or organic
matter has been applied for some time or the
soil pH is below about 5 or above 8.5. To tell
which it is carry out a soil nutrient test for N and
a pH test.
18. Learning Outcomes
To state the mechanism of CEC
To define pH correctly
To identify the effect of pH on nutrient
availability for specific nutrients
To state TWO mechanisms by which pH may
adversely affect nutrient availability
To describe TWO mechanisms by which
excess of certain nutrients may make others
unavailable
To apply this knowledge to the diagnosis of
plant problems
