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Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ SSooiill TTyyppeess && SSooiill FFoorrmmaattiioonn ~~
Rock Types
There are three principle rock groups on the
surface of the earth.
These are:
1. Igneous Rock
2. Sedimentary Rocks
3. Metamorphic Rocks
Igneous Rock
Igneous rocks were the first ever rocks formed
on the earth.
4.5 billion years ago the surface of the earth
was covered in molten lava and volcanoes.
Due to changes in heat patterns of the Sun or
variations in the distance between the earth
and the sun, the molten lava began to cool.
When the lava cooled it solidified and formed
rocks.
1
Leaving Certificate Ag. Science – Soil Science
2
Rocks formed by cooling of molten lava are
called Igneous Rocks.
Examples of Igneous rocks include Granite and
Basalt (below).
Igneous rocks are made of the following
elements:
Oxygen
Silicon
Aluminium
Iron
Calcium
Sodium
Other metals and
non-metals.
Granite is made up of three main components:
1. Feldspar
2. Mica
3. Quartz
Leaving Certificate Ag. Science – Soil Science
3
Feldspar is pink in colour and has K, Al Si.
Mica is black and contains the same as Feldspar
as well as Fe and Mg.
Quartz contains both Feldspar and Mica.
Basalt and Granite are formed from the same
material and differ only due to how they cooled
down.
Granite cooled more slowly than Basalt and
therefore gave very large crystals.
Sedimentary Rocks
As the name suggests, the fusing of various types
of sediment forms these rocks.
The material can either be another type of rock
(Igneous) or shells on the bottom of the ocean.
Sandstone is formed by sediments of quartz,
(broken up by water, heat and frost as well as
Glacial forces).
Limestone is formed by the fusing of pieces of
shells and other materials containing Calcium
carbonate (CaCO3)
Leaving Certificate Ag. Science – Soil Science
Chalk, Iron stone and shale are other example
of sedimentary rocks.
Metamorphic Rocks
These rock arise form sedimentary rocks when
they are exposed to pressure and heat.
Examples are Slate (changed from shale) and
marble (from Limestone)
Soils and types of Rock
The condition of the soil is highly dependant on
the type of bedrock.
Example: Soils formed from granite would be
acidic (pH < 7).
Soils from Limestone would be basic. (Think of
the use of lime)
Also drainage depends on the underlying rock.
Soil Formation
Soils are formed by the physical and chemical
breakdown of rocks 4
Leaving Certificate Ag. Science – Soil Science
Physical breakdown is caused by the Freeze –
Thaw phenomenon.
Chemical breakdown occurs when soluble
components in the rock are dissolved by water
(A very strong solvent)
This is further emphasised by various types of
dissolved acids in rainwater.
Carbonic acid, Nitric acid and Sulphuric Acid
ate common in rainwater.
Soil formation is also greatly affected by the
presence of plant life on the surface of rocks.
5
Leaving Certificate Ag. Science – Soil Science
Early plant life like Lichens (See them on trees
and rocks) can breakdown rocks.
Lichens get minerals from rocks by secreting an
acidic liquid into the rocks. This further breaks
down the rocks decays slowly.
When lichens die they also form the first layer of
humus (decaying material) that will eventually
become the soil.
When the humus becomes a few inches thick,
other plants (like grasses and mosses) can begin
to grow on it. The roots of these plants then can
break the rock further.
The more plants that grow in the humus the
more soil material is available.
When the soil gets thicker still, tree and shrubs
get a foothold and further break the soil with
their roots.
Formation of the Peat Bogs
15000 years ago, after the Ice Age, glaciers left
a vast amount of small, shallow lakes around IRL. 6
Leaving Certificate Ag. Science – Soil Science
Eventually, these shallow lakes were taken over
by vegetation growing from the shore inwards.
This vegetation included plants like reeds and
sedges (type of grass with a typically triangular
stem).
Areas with this type of vegetation are commonly
called fens.
Decaying organic matter filled the lake to
above the water level. It is no longer a lake.
Trees and shrubs then moved in once all the
water had gone. The typical tree would be Birch
or Alder.
The areas are then referred to as CARR or FEN
WOODS.
These trees would eventually die as well and
become part of the decaying mass. They would
not however decay completely because of the
lack of O2.
These conditions are known as ANAEROBIC
conditions.
7
Leaving Certificate Ag. Science – Soil Science
The dominant species at this stage are no longer
reeds and sedges but now are bog mosses
(From the family sphagnum)
These mosses take over because mineral
supplies are low, which does not suit other types
of plants.
These bog mosses can soak up five times their
weight in water and after thousands of years of
Irish weather, these mosses were able to build
up to heights of 10 metres above the once lake
floor.
Why would you add moss peat to your garden?
8
Leaving Certificate Ag. Science – Soil Science
These bogs are called Raised bogs and are
typical of the midlands.
The other type of bog are called blanket bogs
and are found in mountainous areas where high
rainfall is typical (Wicklow, Clare and Kerry to
some extent)
Blanket bogs are so called because they form a
blanket over a wide area. They are quite shallow
(1 - 2 metres deep at most)
Not as much potential for cultivation by large-
scale machinery or for farming.
Bord na Mona and the raised bogs
Blanket bogs are unsuitable for the machinery
mainly because where they are found.
The raised bogs of the midlands are being
currently exploited for fuel and moss peat.
But these natural resources will not last for ever
and research is on going to find out what should
be done with the land after all the turf has been
harvested. 9
Leaving Certificate Ag. Science – Soil Science
It has been notices that Cut Over Bog has great
potential for grassland and large-scale
vegetable growing.
What must be decided is now much of the turf
do they keep.
Experts recommend 1.5 metres but what is the
problem?
*****************************
10
Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ TThhee SSooiill PPrrooffiillee ~~
If one were to cut a pit in the ground your would
notice that the soil is made of distinct layers ot
HORIZONS.
In very simple terms there are three main
horizons, A, B & C.
The A Horizon is commonly called the Top Soil.
It contains a large proportion of Humus (Organic
Matter), roots, bacteria and living organisms
(Nematodes and Earthworms).
The humus gives the top soil its characteristic
dark colour.
In humid temperate climates like our own, the A-
horizon or topsoil is subject to leaching (Losing
minerals by washing out).
Clay and humus can also be leached out of the
A Horizon.
Below the A-horizon is the B Horizon.
11
Leaving Certificate Ag. Science – Soil Science
It has intermediate characteristics and it often
referred to as the sub soil.
The B horizon owes it origin to the activity of
earthworms.
The earthworms swallow the humus from the
topsoil (with it minerals etc) and transfer them all
around the B Horizon.
The B Horizon is also strongly influenced by the
leached materials from the A Horizon.
The B Horizon is often a strong colour (Orange in
iron rich soils) when leaching is a factor.
The B-horizon may also have a high percentage
of clay than the A and C horizons.
The C Horizon is the Parent Material or the
original rocks from which the soil was formed.
The main ingredient of the C Horizon is broken
pieces of under lying rock.
Certain soils, called Derived Soils contain
material other than the original rock but material
that were transported to that location by Winds,
Rivers or Glaciers. 12
Leaving Certificate Ag. Science – Soil Science
To further complicate things the A, B and C
Horizons can be subdivided further.
When the A Horizon has been subjected to
leaching, the lower part of the horizon is pale
and light coloured and almost looks like it has
been bleached.
The upper half of the A horizon is still dark as
humus is continuously being formed from the
decaying organic matter.
The upper layer is then called the A1 horizon
and the leached area the A2.
An A3 horizon may be present if you notice
another layer between the leached area and
the B Horizon.
The B Horizon is commonly recognised by the
accumulation of leached materials from the A
horizon.
The area where most of the accumulation
occurs is the B2 Horizon.
The B2 Horizon is then referred to by what is
accumulated. 13
Leaving Certificate Ag. Science – Soil Science
I.E. B2h stands for Humus
B2ir stands for Iron
Where the B2ir has accumulated to such a
serious extent that a layer of Iron Oxide has
formed, which results in water logging, we refer
the B2 Layer as the IRON PAN.
There is also an O Horizon.
This is the layer of organic matter on the surface
of the soil.
This can be further divided into the O1 and O2
Horizons. (Totally decayed and not decayed)
14
Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ SSooiill CCoommppoossiittiioonn ~~
Soils are made up with the following
components.
Mineral particle (Sand, Silt and clay)
Dead organic matter (Humus)
Living organic matter (Plant roots and bacteria)
Water
Air
Mineral salts
15
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25%
45%25%
5%
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�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������
Composition of Soils
����Air Mineral Matter
����Water
����Other Material
Leaving Certificate Ag. Science – Soil Science
Above illustrates the ideal composition of soil,
25% Air, 25 % H2O, 45% Mineral Matter & 5% O.M.
1. Soil Texture - Particles
The texture of a soil depends on the relative
mixture of sand, silt and clay particles.
The most common method of classifying soils is
based on the percentage clay in the soil. E.g.
Soils that contain 0 – 5 % clay are known as
sandy soils. (More on this later)
The particles in the soil are classed on their size.
Anything over 2mm in diameter are referred to
as pebbles or stones.
Particles from 2 mm to 0.5 mm are called sane
particles.
From 0.5 mm to 0.002 mm are called silt
particles.
Any particle under 0.002 mm are referred to as
clay.
Sand and Silt are similar in composition and are
formed by physical breakdown of rocks. 16
Leaving Certificate Ag. Science – Soil Science
Clay particles are formed by both physical and
chemical breakdown of rocks.
As mentioned before soils are classified by the
amount of clay in the soil.
0 – 5 % Clay Sandy Soil
5 – 10 % Clay Sandy Loam
10 – 20 % Loam
20 – 30 % Clay Loam
30 – 40 % Clay Soil
40 % Up Heavy Clay Soil
A more common and accurate way at looking
at the type of soil is by using a soil triangle.
Generalised Soil Types
Sandy Soils
o Have large air holes.
o Free Draining soils
o Is easy to work with (light)
o Dries out quickly
o Minerals are easily leached.
17
o Poor soil with little or no nutrients.
Leaving Certificate Ag. Science – Soil Science
o Is a warm soil.
Clay soils
o Holds water easily
o This protects from leaching of minerals.
o Is naturally fertile soil.
o Very poor drainage, which can lead to
water logging.
o Is a cold soil.
Loam soils
o Intermediate characteristics of both clay
and sandy soils.
o More advantages and fewer
disadvantages than sandy or clay soils.
o A good mixture is 40 % Sand, 40 % Silt and
20 % clay.
While the nature of soil depends on the particle
composition, the amount of humus in the soil is
also a major factor
18
Leaving Certificate Ag. Science – Soil Science
2. Humus
Humus is the product of the breakdown of
organic matter.
It is the remains of dead animals and plants.
Fully decayed material forms CO2, water and
mineral salts. But the incomplete product is a
dark sticky material called HUMUS.
Normal sandy soils as mentioned before are free
draining and soil water could leach out
(including the minerals dissolved in it).
Humus in the soil absorbs the water and can
greatly improve the quality of otherwise poor
(sandy) soils.
Both clay and humus can hold minerals in the
soil. The minerals are in the form of cations.
This is known as cation exchange capacity.
Humus is useful to the soil because:
i. It contains minerals (the type and
amount depend on the source of the
humus).
19
Leaving Certificate Ag. Science – Soil Science
ii. It holds minerals in the soil due to its high
cation exchange capacity.
iii. It improves and strengthens the crumb
structure of heavy soils.
iv. It forms clay humus complexes, which
improves soil stability.
v. Its dark colour improves the warming
capabilities of the soil (i.e. it is able to
absorb more heat from the sun)
vi. Can make the soil more acidic (an
advantage and disadvantage
depending on the use of the soil).
There are two categories of humus: Fast
decaying and slow decaying.
Fast Decaying Humus:
Usually formed from soft parts of plants
(cellulose based material)
Important as food source for earthworms
and bacteria.
The material decays in a matter of months.
Slow decaying Humus: 20
Leaving Certificate Ag. Science – Soil Science
Usually formed from hard parts of plants
(lignin based materials)
It is important for soil improvement (See the
points above)
Decays over a period of several years and
forms stable complexes with clay particles.
Soils are grouped also by the amount of humus
in the upper horizons.
< 10 % O.M. Mineral soil.
10 – 17 % Humus soil
17 – 35 % Slightly peaty soil
35 – 50 % Peaty soil
> 50 % Peat (Turf)
3. Soil Micro organisms / bacteria
The micro-organisms in the soil include bacteria,
fungi and algae but bacteria are the most
important group.
The most important function of bacteria is the
breakdown of organic matter into humus.
21
Leaving Certificate Ag. Science – Soil Science
Also play a major role in the Nitrogen cycle
(More later)
Also important in the creation and destruction of
carbon in the soil.
4. The Earthworm and its role in the soil.
The earthworm plays a major role in the structure
of the soil.
The number of earthworms is an indication of
the fertility of the soil.
In good (fertile) soil there may be as many as
150-200 worms per metre squared.
That is the same as 1.5 – 2 million worms per
hectare.
Earthworms improve the soil in the following
ways:
a. They eat their way through the soil and mix
the ingested material with mucus in their
guts. This helps to improve soil crumb
structure.
22
Leaving Certificate Ag. Science – Soil Science
b. Depositing soil in different places and
mixing horizons.
c. Improve drainage of heavy clay soils
d. Introduces more air into the soil.
e. When they die the further increase the
amount of organic matter.
5. Soil Water
Water is important in the soil for some obvious
reasons:
a. Required for the growth of plants and
animals (like earthworms and even bacteria)
b. All chemical reactions that take place in
the soil require water for them to proceed.
c. But too little or too much water can be
harmful to the soil and its inhabitants.
There are four extreme conditions to consider
when discussing soil water
i. Flooding – When all available space in the
soil is taken up be water and there is excess
on the surface of the soil. 23
Leaving Certificate Ag. Science – Soil Science
ii. Water logging – When all available spaces
are taken up by water in the soil but there is
no excess on the surface of the soil.
a. The presence of the water is not the
problem here but the resultant
absence of air.
b. The area where all the air spaces is
taken up by water is called the
Water Table
iii. Field capacity – This is when a soil is drained
to the extent that only the gravitational
water will be lost. This means the water that
would normally move down under the force
of gravity.
a. Some water will remain attached to the
surface of soil particles like clay or silt.
The water covers the particles in a fine
film.
b. Large spaces are filled with air while the
smaller spaces are filled with water. The
24
Leaving Certificate Ag. Science – Soil Science
small spaces are called capillaries and
the water inside them capillary water.
c. This is the ideal condition of soil.
d. If the water level in the soil is less than
the field capacity, then the amount of
water required is the soil water deficit.
e. Measured in mm of water
f. Clay soils can hold more water and are
said to have a greater field capacity.
iv. Wilting Point – Is the condition of a particular
plant when it has extracted all the available
water from the soil. It varies for each plant as
each plant has different requirements.
6. Soil Air
Air is required in the soil for the growth of roots,
bacteria and earthworms.
It is essentially the same in composition as
atmospheric air
a. 78 % Nitrogen
b. 21 % Oxygen 25
Leaving Certificate Ag. Science – Soil Science
c. 0.03 % Carbon Dioxide
d. 0.97 % Other gases
Plants use O2 for the uptake of minerals from the
soil.
Most the useful bacteria in the soil are aerobic
(i.e. they need Oxygen to live) Anaerobic
bacteria do not need oxygen.
Certain types of bacteria can use the Nitrogen
in the soil and change it into nitrates and other
compounds which are useful to the plant.
These bacteria are called Nitrogen fixing
bacteria and include Nitrobactor and
Azotobacter.
Sandy soils contain more air than clay soils as a
general rule.
7. Mineral Nutrients
These are not to confused with mineral particles
(sand, silt and clay) but are elements or
compounds that are either bound chemically to
clay particles or are dissolved in water in the soil. 26
Leaving Certificate Ag. Science – Soil Science
Plants require them as nutrients for growth and
repair of their cells and also for certain chemical
reactions to take place.
They are classified by their amount of use.
The minerals required in large amounts are
called Macro – Nutrients. Examples are N, P, K,
Ca and Mg.
Minerals required in small amounts are called
Micro – Nutrients. Examples are Fe, Mn, Cu, and
B.
They are also known as minor elements or trace
elements.
8. Soil Temperature
Water is very slow to attract heat
This means that soil with a lot of water in it harder
to heat up than dry soil.
In fact 1kg of water needs more than ten times
the heat to raise its temperature by 1 degree, as
does 1 kg of dry soil.
27
Leaving Certificate Ag. Science – Soil Science
Clay soils, as mentioned earlier can attract more
heat than sandy soils, and therefore need more
heat to raise its temperature.
This is why clay soils are referred to as cold soils
and sandy soils are called warm soils.
On average, clay soils need 50 % more heat
than sandy soils to raise their temperatures by 1
degree.
Aspect, colour and altitude also affect the
heating capabilities of a soil.
9. Soil pH
pH is a measure of the acidity of a soil.
It is important to consider because plants require
certain pH’s to grow well.
If pH changes the plant may not be able to
produce as much, or may in fact die as a result.
Earthworms and soil bacteria thrive in neutral or
near neutral conditions.
In soils, pH ranges mainly from 4 to 9.
Soils can be classified by their pH values 28
Leaving Certificate Ag. Science – Soil Science
< 4.5 = Very Acidic
4.6 – 5.2 = Strongly acidic
5.3 – 5.9 = Moderately acidic
6.0 – 6.5 = Slightly acidic
6.6 – 6.9 = Near Neutral
7.0 = Neutral
7.1 – 7.5 = Slightly Alkaline
7.6 –8.3 = Moderately alkaline.
In Ireland, soils tend to become acidic with time,
and most soils need to be limes (increase the
pH) every four years or so.
29
Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ The National Soils Survey - The Soil Groups ~
1. Introduction
In the 1960’s An Forás Taluntas began the
National Soil Survey, and it is still continuing to
this day.
The research team are based in Johnstown
castle in Co. Wexford & it was started to identify
soils with similarities in their horizons throughout
the country.
The main aim was to then eventually classify all
the soils in Ireland into the Great Soil groups and
Series.
This would then be used to determine the best
possible use for the types of soil in a particular
area.
2. The General Soil Map of Ireland
This was first published in 1969, with a second
edition in 1980. 30
Leaving Certificate Ag. Science – Soil Science
These show the distribution of the major soil
groups throughout Ireland (and each county
also) as well as a discussion on their land use
potential.
What are the major soil groups:
a. The Podzols
b. Brown Podzolics
c. Grey – brown Podzolics
d. Brown Earths
e. Gleys
f. Rendzinas
g. Lithosols
h. Blanket peats
i. Basin Peats
3. The Brown Earths (In more detail)
These soils are mature, well drained mineral soils.
They have not suffered from serious cases of
leaching (loss of minerals)
They have a uniform profile (i.e. No distinct
horizons or layers) 31
Leaving Certificate Ag. Science – Soil Science
The Brown Earths in Ireland are mainly found in
areas where the underlying rock is acidic, and
therefore the soil is acidic.
With regular liming and fertilising the soils can be
quite a productive soil.
4. The Podzols
These soils are typical of areas where the parent
rock is acidic (like granite) and the annual
rainfall is quite high (> 1000 mm) and a
moderate, cool climate.
These soils show serious cases of leaching of
both mineral and humus.
These materials are translocated into the B
Horizon.
This process is caused PODZOLISATION.
When serious cases occur and large amounts of
iron oxide (rust) have been leached from the A
horizon into the B horizon, an iron pan may form
when these cement together.
32
Leaving Certificate Ag. Science – Soil Science
This in turn causes major problems for drainage
and root penetration.
If water logging or flooding occur, then little or
no oxygen will be available and organic matter
will eventually form an O Horizon.
This is the first stage of a formation of a peat and
the soil is now known as a Peaty Podzol.
When the O Horizon becomes deeper than 30
cm, then the soil is no longer podzol, but is now
a blanket peat.
Podzols are not very useful as tillage soils, or for
grazing. This is due to their poor drainage and
poor root penetration.
In cases where Iron Pans have not formed,
Podzols may be useful for Forestry.
Podzols must be managed carefully or they may
very quickly become unsuitable again.
33
Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ SSooiill FFeerrttiilliittyy ~~
1. Introduction
All plants require certain elements and minerals
to carry out functions in the plant.
These minerals allow for proper growth of the
plant.
A lack of the minerals results in a deficiency
disease or stunted growth.
An essential element is defined as “a chemical
element required for the normal growth of the
plant”.
2. Nature of Plant Nutrients
There are 17 essential elements, with Carbon,
Hydrogen and Oxygen making up most of plant
tissue (95%)
These elements however are taken from the air
and are rarely responsible for limiting crop
growth.
34
Leaving Certificate Ag. Science – Soil Science
The other 14 are taken up from the soil and are
more likely to restrict growth if lacking.
Six of these elements are required in large
amounts and they are called macro elements.
The other eight are called micro elements and
are only required in small amounts.
Macro Elements (Soil) Macro Elements (Air) Micro Elements (Soil)
Nitrogen Carbon Iron Phosphorous Hydrogen Manganese
Potassium Oxygen Boron Calcium Molybdenum
Magnesium Copper Sulphur Zinc
Chlorine Cobalt
Table 1: Macro and Micro Elements
3. The Major Elements
The major elements occur in the soil in two major
forms: Complex, insoluble forms and Simple,
soluble forms.
The conversion of each element from its
complex form into the simple form is very
important. 35
Leaving Certificate Ag. Science – Soil Science
The complex and simple forms of each elements
are below: Major Element Complex Form Simpler Forms
Nitrogen (N) Organic: (Proteins, amino acids etc)
Ammonium Salts (NH4+) Nitrite Forms (NO2-) Nitrate Forms (NO3-)
Phosphorous (P) Inorganic: Rock minerals,
and phosphates Organic: Nucleic Acids
Phosphates of Calcium, magnesium and
potassium (HPO42- or H2PO4-)
Potassium (K) Inorganic: Rock minerals
e.g. felspar, micas, silicates
Potassium ions on soil colloids (K+)
Calcium (Ca) Inorganic: Rock minerals e.g. felspar, calcites etc
Calcium ions on soil colloids (Ca2+)
Magnesium (Mg) Inorganic: Rock Minerals as above and clays
Magnesium ions (Mg2+)
Sulphur (S) Inorganic: Rock minerals
Organic: Proteins and amino acids
Sulphites and sulphates of calcium and
magnesium (SO32- and SO4-)
4. Nitrogen
Nitrogen is an important component of plant
proteins, chlorophyll and nucleic acids.
36
Leaving Certificate Ag. Science – Soil Science
Therefore it is vital for growth, photosynthesis and
cell reproduction
It is the most important and widely used fertiliser
element.
A deficiency in N causes sharp reductions in crop
yields.
When N is abundant in soil, the crop grows
quickly, is dark green in colour and fruits and
seeds have high protein levels.
When N is short, growth is restricted and the plant
goes yellow.
The ways in which N is supplied, used and reused
is outlined in the Nitrogen Cycle:
Figure 1: The Nitrogen Cycle 37
Leaving Certificate Ag. Science – Soil Science
The significant parts of the Nitrogen Cycle are as
follows:
i. Fertiliser Application
Organic and Inorganic fertilisers contain
N in Urea, Ammonium and Nitrate forms.
The Urea are generally converted into
Nitrates in good soils.
Some of the fertiliser materials are
converted in proteins and are
immobilised, not available to plants.
ii. Nitrogen Fixation
Nitrogen fixation is the process of
changing atmospheric nitrogen into
other, simpler forms (e.g. NH4+)
The principle bacteria are called
Rhizobium.
These bacteria are also found on root
nodules on legumes (e.g. Clover)
The bacteria have a symbiotic
relationship with the plants, meaning
each get what they want. 38
Leaving Certificate Ag. Science – Soil Science
The Rhizobium provide N for the plant,
while the plant provide food.
This is why clover is of huge importance in
pasture - land.
iii. Organic Matter Breakdown
When organic matter or manure is
applied to soil, the complex forms of N in
the material are converted in NH4+.
iv. Nitrification
Nitrification is the conversion of NH4+
(Ammonium) into Nitrates using O2.
There are two stages involved.
Stage 1
2NH4+ + 3O2 2NO2- + 2H2O + 4H+ + Energy
Stage 2
2NO2- + O2 2NO3- + Energy
The result of Nitrification is that NH4+ ions
(which cannot be absorbed by plants) is
made available as NO3-, which can be. 39
Leaving Certificate Ag. Science – Soil Science
v. Plant Uptake
The uptake of NO3- by plants completes
the cycle – N begins at plants and end at
plants.
vi. De-nitrification and Leaching
All parts of the Nitrogen cycle so far are
favoured by good, moist, high pH, warm
soils with the presence of O2.
But De – Nitrification occurs in wet,
anaerobic or waterlogged soils.
It results in a loss of N from the soil and its
conversion to N2O or Nitrous Oxide and
N2 gas.
Leaching also results in the loss of NO3-, so
fertiliser application should be done in
spring or summer.
5. Phosphorous
Phosphorous is involved in plant growth and in
cell division.
It is the second most critical plant nutrient.
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Leaving Certificate Ag. Science – Soil Science
A lack of P results in stunted growth and a bluish
colour on the leaves.
Phosphorous in soil exists in a range of complex
and simply forms.
Phosphorous ions are released by chemical and
biological degradation processes.
If the soil has a pH of 5.5 or less, the P reacts with
Iron, Manganese and Aluminium compounds
and are made almost unavailable to plants.
Above pH 7.5, a similar problem occurs with
Calcium, making P unavailable.
This is called phosphorous fixation or
immobilisation.
Therefore, to improve P availability, the soil
should always be kept between a ph of 6 and 7.
6. Potassium
Potassium performs a number of function in
plants, including disease resistance and
carbohydrate translocation.
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Leaving Certificate Ag. Science – Soil Science
A lack of K results in withering of the vegetation
and a loss of yield.
The availability of K+ is affected by clay mineral
composition.
Some clays can trap K+ ions into the inside of the
clay colloid, making the ions unavailable for the
plant.
This is known as potassium fixation or
immobilisation.
The areas where these soils are found are
recorded, and the farmers on this land must
apply more P fertiliser on the land.
7. Calcium, Magnesium and Sulphur
These minerals, although major elements, are not
applied in fertilisers for two reasons:
The are relatively abundant in soils
They are not subject to immobilisation like N,
P and K.
Lime and inorganic fertilisers contain these
elements also as part of the active parts. 42
Leaving Certificate Ag. Science – Soil Science
8. Trace Elements
Trace elements like Fe, Mn, B, Cl etc are involved
in enzyme systems of the plant and are therefore
important for normal plant function.
Deficiency of the major elements is rare but
often occurs with trace elements.
This occurs for two reasons:
i. Lack of the element in the parent rock
ii. Over liming or high pH
These deficiencies can be rectified by soil
applications or foliar sprays.
9. Soil pH and Liming
The pH of the soil influences the availability of
elements (both major and minor) in the soil.
Most elements are at maximum availability at pH
6 – 7 except for some minor elements.
This is the ideal pH and therefore should be the
aim for good soil management.
Liming is an essential farming operation due to
acid leaching. 43
Leaving Certificate Ag. Science – Soil Science
Figure 2: Nutrient Availability and pH
Liming is carried out every 5 – 10 years,
depending on the soil and rainfall and
evaporation amounts.
Liming raises the pH by replacing H+ ions (acid
ions) with Ca++ ions.
It takes up to two years for the liming to take full
effect.
Ground limestone is the main source of lime in IRL 44
Leaving Certificate Ag. Science – Soil Science
Ground limestone must meet the following
guidelines:
i. Should have a TNV (Total Neutralising
Value) of not less than 90%
ii. All particles must be less than 3.35mm
iii. At least 35% must pass through a 0.15mm
sieve.
iv. Must have more than 3.0% water.
10. Soil Analysis
Soil must be analysed to measure its fertility and
the fertiliser requirements
It involves three steps:
i. Soil Sampling
Soil must be sampled in various areas of the
field considering different coloured areas &
sloped areas.
Consider the past history of the field
Take a minimum of 25 samples, usually in a W
shape throughout the field are taken at root
level. 45
Leaving Certificate Ag. Science – Soil Science
A composite sample is made by mixing all of
these samples.
ii. Soil Analysis
Soil analysis generally means determining
lime requirements, available P and available
K.
Available N is hard to measure so is not done
generally
Most testing is carried out by Teagasc, in their
soil testing lab at Johnstown Castle.
The samples are dried and sieved and
treated with Morgan’s extracting solution
and another solution for P & K.
The amount of P and K is measured in ppm
(parts per million)
Lime requirements are expressed in tonnes
per hectare.
Morgan’s extracting solution mimics a
growing plant and takes out P & K like a
plant would. 46
Leaving Certificate Ag. Science – Soil Science
iii. Interpretation of soil analysis
The results of the soil analysis are compared
to previous reference tests carried out by
Teagasc.
This allows the technician to give guidelines
for the farmer on P and K application and
liming requirements.
11. Finding N, P and K in soils
Certain chemicals are used to analyse soils and
determine the presence of N, P and K.
Examples of chemicals are given below:
Test for Reagent Positive result
Nitrates (NO3-) Diphenylamine Blue colour
Phosphates (HPO42-) Ammonium Molybdate Yellow precipitate
Sulphates (SO42-) Barium Chloride White cloudiness
Chlorides (Cl-) Silver Nitrate White precipitate
47
Leaving Certificate Ag. Science – Soil Science
SSooiill SScciieennccee ~~ FFeerrttiilliizzeerrss ~~
1. Introduction
A fertilizer or manure are materials that contain
one or more of the essential elements and are
applied to soils to encourage crop growth.
The amount added is decided on after soil
analysis.
Fertilizers are mainly inorganic and are
manufactured materials.
Manures are organic (made from plant and
animal wastes).
2. Fertilizers
Most of the fertilizers sold in Ireland contain N, P
and K either singly or in a combination.
Fertilizers are manufactured from the following:
Atmospheric N Ammonia or Nitric Acid
Mineral rock phosphates (North Africa)
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Leaving Certificate Ag. Science – Soil Science
Natural potassium salts (potash) imported
from Eastern Europe.
Fertilizers containing one nutrient element are
called straight fertilizers and those with a
combination are compound fertilizers.
Straight Fertilizers
The most important straight fertilizers in Ireland
are Calcium Ammonium Nitrate (CAN), Urea
and Ground Rock Phosphate.
Calcium ammonium nitrate is the largest selling
fertilizer in Ireland.
Nitrogen occurs in a nitrate form and
ammonium form.
The acidifying property of NH4+ is buffered by
the Ca in CAN and is therefore very useful as a
source of N.
CAN must be spread as soon as it is exposed to
air, as wastage and caking will occur
otherwise.
49
Leaving Certificate Ag. Science – Soil Science
The use of Urea is increasing due to the high
levels of N in its compound (46%)
It doesn’t give a higher crop response than
CAN, however, because it takes longer to
change to Nitrate form.
On sunny dry days, urea can decompose and
can be wasted to the atmosphere.
It is therefore recommended that urea is only
spread before May 1st, after August 15th or
when rain is forecast.
It should also only be used as a top dressing
(over established crops) when wet.
Ground rock phosphate is used as a straight
fertilizer mainly for the forestry industry
These soils are nearly all acidic and the P is
released slowly to the trees.
In normal soils, GRP is of little use.
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Leaving Certificate Ag. Science – Soil Science
Compound Fertilizers
Compound fertilizers are made from mixing
straight fertilizers or by chemical synthesis of
other raw materials.
In chemically synthesised fertilizers, P is more
available than in GRP.
Compound fertilizers are named by listing their
percentage content of N, P and K.
For example 10:10:20 contain 10% N, 10% P and
20% K.
The remaining percentage is made up of
chemicals and impurities of various kinds.
The most common fertilizers are shown below:
Fertilizer
18 : 6 : 12
27 : 2.5 : 5
0 : 7 : 30
0 : 10 : 20
10 : 10 : 20
24 : 2.5 : 10
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Leaving Certificate Ag. Science – Soil Science
Each of the fertilizers is designed for a particular
purpose. The nutrients are balanced as to reflect the
uptake of the particular crop. 18 : 6 : 12 is designed for silage and hay crops. 27 : 2.5 : 5 is used on heavily stocked grassland. 0 : 7 : 30 is used for autumn spreading on land
set aside for silage. The farmer must decide which compound is
most suitable for the crop chosen.
3. Fertilizer Application
Fertilizers can be applied in three ways:
A. Drilled into the soil along with seeds using a
seed drill
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Leaving Certificate Ag. Science – Soil Science
B. They can be broadcast with a fertilizer
spreader (with reciprocating arm) and mixed
during cultivation processes.
C. They can be broadcast onto a growing crop.
This is called top dressing.
Straight and compound fertilisers are all sold in
granular form.
This improves fertilizing in two ways:
Reduces caking
More accurate spreading
4. Manures
Manures are waste materials of plants and
animals.
There are five main sources of manure:
i. Farmyard Manure
ii. Animal slurry
iii. Straw
iv. Seaweed
v. Sewage sludge
53
Leaving Certificate Ag. Science – Soil Science
Farmyard manure is a mixture of faeces, urine
and bedding, and is a by product of winter
housing of animals.
It is high in organic matter and is beneficial to the
soil.
Animal slurry is a mixture of mainly urine and
faeces, collected from under the floor of slatted
units.
Both animal slurry and farmyard manure have
similar low concentrations of minerals.
The average composition is:
0.5 % Nitrogen
0.15 % Phosphorous
0.60 % Potassium
Both materials are therefore quite bulky materials
when used a fertiliser materials.
5. Application of Organic Manure
Farmyard manures are spread using a muck
spreader.
54
Leaving Certificate Ag. Science – Soil Science
The spreader is filled using a tractor and a front
loader.
It is then torn up and spread from the side of the
spreader.
Slurry spreaders suck slurry into the tank using a
tractor driven pump.
When being applied the pump is reversed and
the material is pumped out.
It hits a deflector plate, which spreads the slurry
in a band of about 6m wide.
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