2. Learning objectives
To identify the effect that soil texture has on soil pores
and soil water
To state and define the different types of soil water
To correctly define Field Capacity, Permanent Wilting
Point and Available Water Content
To state the mechanism of CEC
To define pH correctly
To identify the effect of pH on nutrient availability for
specific nutrients
To state that excess of certain nutrients may make
others unavailable
To apply this knowledge to the diagnosis of plant
problems
3. Soil water and soil pores
• Soil pores are found mainly between
soil aggregates.
5. Definitions of soil water
states
Saturation Point: all the pores are
filled with water, there is no air.
Field Capacity: the water remaining in
the intermediate and micro-pores
once gravitational water has drained.
Permanent Wilting Point: All capillary
water is used; only unavailable
hygroscopic water remains.
6. Available Water Content
The amount of water that a soil holds
between the point at which Field
Capacity is reached and Permanent
Wilting Point.
The total of the capillary water
remaining in the soil at any time.
The water that plants can use.
7. Soil Water Worksheet
answers
1. Soil texture can be defined as the
relative proportions of sand, silt and
clay particles in a soil. Soil mineral
particles form aggregates when they
are bound together by clay and
humus in the soil. Soil pores are the
spaces between soil aggregates.
2. Sand – largest; silt – next largest;
clay- smallest
8. Soil Water Worksheet
answers
3. 1m3
of clay – smaller particles means less
volume per unit of surface area.
4. (a) Saturation Point; (b) Field Capacity; (c)
Permanent Wilting Point; (d) Available Water
Capacity.
5. (a) the water that drains from the soil; (b) the
water held in soil pores that plants can use;
(c) water held tightly to the surface of soil
particles that plants cannot use.
6. The water lost from the surface of plant
leaves.
9. 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.
10. 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.
12. 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
13. 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.
14. 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.
Al+3
increases in concentration in significantly acidic soils
as it is soluble in acidic conditions. Aluminium is toxic to
plants.
15. 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
16. Plant nutrient immobilisation
For example: Phosphorous forms an
insoluble compound with Aluminium 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 (here,
magnesium).
18. 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.
19. 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.
20. Learning Outcomes
To identify the effect that soil texture has on soil pores
and soil water
To state and define the different types of soil water
To correctly define Field Capacity, Permanent Wilting
Point and Available Water Content
To state the mechanism of CEC
To define pH correctly
To identify the effect of pH on nutrient availability for
specific nutrients
To state that excess of certain nutrients may make
others unavailable
To apply this knowledge to the diagnosis of plant
problems
