Leaving Certificate Ag. Science â€“ Soil Science Soil Science

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


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


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)


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


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


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


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


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


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


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


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


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


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


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


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

��

�

25%

45%25%

5%

��

��

��

Composition of Soils

��Air Mineral Matter

��Water

��Other Material


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


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.


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


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


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


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


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


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


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


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


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


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


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


< 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


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


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


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


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


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


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


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


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


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


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


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.

40


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.

41


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


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


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


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


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


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


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)

48


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


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.

50


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

51


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

52


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


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


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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Leaving Certificate Ag. Science â€“ Soil Science Soil Science

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