Download Fertilisers

Why do plants need fertilisers?
1. Which 3 elements did mum tell Davy are needed by plants?
Phosphorus is needed to help build a large
healthy root system.
Carrots, potatoes, turnips etc need
fertilisers rich in phosphorus
Lack of phosphorus causes small seeds
and fruit and slow growth
Nitrogen is needed to build
proteins. Proteins are used to
build large leaves and tall
plants.
Potassium helps produce protein and
sugars, and is needed for healthy
flowers e.g. orange blossom and large
tasty fruit.
Lettuces, cabbages, wheat
and spinach need fertilisers
high in nitrogen
Tomatoes, apples, strawberries,
bananas and pear trees need fertilisers
high in potassium
Lack of nitrogen causes
smaller crops with yellowish
leaves and smaller roots
Lack of potassium causes smaller
plants with less resistance to disease,
droughts and frost
2. Copy and complete the table
Element
Why needed by the plant Symptom of lack of
element
Soil contains many elements.
Main soil nutrients.
Name
silicon
Aluminium
iron
calcium
sodium
magnesium
%
25
10
5
4
3
2
3. Plot a bar graph showing soil elements
a. Which 3 elements are the most common in soil ?
b. Which element is the least common?
c. Which 3 elements do plants need?
d. Suggest why farmers need to add fertilisers to the soil.
COPY
The essential elements in fertilisers must be taken up by the plant roots
so the fertiliser must be soluble
Use your data booklet to help you to copy and complete the table
Chemical
calcium
phosphate
potassium nitrate
magnesium
nitrate
potassium
phosphate
essential element
soluble/
insoluble
suitable as
fertiliser?
Natural replacement of soil nutrients
There are 4 main ways nutrients get replaced naturally.
1. Decay and excretion
In nature, nutrients are returned to the soil when dead .........................
and plants decay or when .......................... excrete waste. Harvesting crops
and eating animals stops …………….getting back into the soil naturally.
Composting allows waste plant materials to be used as fertiliser. Slurry pits
store …………..to be sprayed on fields.
2. Root nodules
Some plants such as peas, beans and clover have root nodules. These
plants are known as ‘legumes’
The root nodules contain bacteria which can change nitrogen from the
air into soluble nitrogen (nitrate)
Crop rotation systems use these plants to replace nitrogen in soil.
Different crops are planted in the fields each year. One year a field may
grow wheat, the next year it may be used for growing peas and the
following year it may be used for grazing.
Chemistry counts p 250 shows root nodules
a. What are ‘legumes? Name 3
legumes.
b. How do legumes help to improve the
soil?
c. Explain how crop rotation would help maintain a high level of nitrate in
the soil.
d. What are the disadvantages of crop rotation?
3. Lightning storms
Watch the Demo of sparking Air
80% of the air is nitrogen, which is a very unreactive element because it
has a triple covalent bond.
During a lightening storm, the enormous amount of electrical energy is
able to split the triple bond, enabling the nitrogen to react with oxygen
to form nitrogen dioxide.
This gas dissolves in the water vapour of the air to form nitric acid
which falls to earth in the rain to provide nitrate in the soil.
a. Why is nitrogen so unreactive?
b. Write a word equation to show the reaction of oxygen and nitrogen.
c. What colour was the nitrogen dioxide gas that formed?
d. What colour did the indicator go when the water was added to the flask?
e. What have you learned about the pH of the solution made when nitrogen
dioxide dissolves in water?
f. Write the formulae of nitrogen dioxide, nitric acid and potassium nitrate.
g. Nitrogen dioxide is also produced by car engines because of the high
temperature spark that ignites the fuel. What device is used to remove
nitrogen dioxide from car exhaust systems?
h. Why is nitrogen dioxide removed from exhaust emissions?
ARTIFICIAL FERTILSERS
It is inefficient to use the sparking air method to manufacture nitrate
fertilisers on a large scale because of the enormous energy costs.
Towards the end of the nineteenth century, the demand for food grew
faster than the supply. A new, artificial source of nitrogen was needed.
The challenge was to find a way to get unreactive nitrogen to make
useful compounds. The solution was discovered by Fritz Haber who
developed the 'Haber Process' to make ammonia NH3
Since then, the amount of fertiliser used has increased rapidly.
USE OF FETILISERS
Year
1875
1940
1950
1980
World population
1 billion
2 billion
2.8 billion
5 billion
Potassium fertiliser ( tonnes)
2, 500
63,000
198, 000
320,000
1. Plot two line graphs showing how the use of fertilisers and the population of
the world have changed over the last 125 years.
2. Why has the use of Fertiliser increased over the last 125 years?
PROPERTIES OF AMMONIA
1. The Fountain experiment.
a. Draw a diagram of the apparatus.
b. What two properties of ammonia does the experiment show?
c. A solution of ammonia in water is called aqueous ammonia or ammonium
hydroxide. Copy and complete the equations
ammonia + _________
→
___________ ____________
________(g) + H2O (l)
→
NH4 Cl
(aq)
2. Laboratory Preparation of ammonia
Ammonia is produced by heating an ammonium salt with soda lime which is a
mixture of calcium hydroxide and sodium hydroxide.
a. Copy the equation below and write a word equation for the reaction.
NH4 Cl
+
NaOH
→
NaCl
+ H2O + NH3
b. Draw a labelled diagram of the apparatus
c. What was the effect of ammonia on pH paper?
d. What colour is ammonia?
e. Chemists say that ammonia has a ‘pungent’ smell. How would you describe
it?
AMMOMNIA AND FERTILISERS
Many fertilisers are made from ammonia. Ammonia will neutralise acids
to make ammonium salts which are used as fertilisers.
e..g ammonia + nitric acid →
NH3 +HNO3
→
ammonium nitrate + water
NH4 (NO3 )
+ H2O
1. Write word equations to show how ammonia could be used to make
a. ammonium sulphate
b. ammonium phosphate ( you need phosphoric acid)
2. CREDIT. Write balanced chemical equations for the reactions in question 1
Your teacher may show you how to make ammonium sulphate fertiliser.
THE HABER PROCESS
Copy and complete. The DVD AND CD-ROM will help you
The Haber Process uses hydrogen from ____________ and ___________
from the air to make ammonia.
The reaction uses an ________ catalyst to it to go faster at lower
________________. A ___________speeds up a reaction but is left
unchanged at the end.
The Haber Process uses a temperature of ________ and a pressure of
__________ atmospheres
More _________ would be produced at a lower temperature but the reaction
would happen too slowly to be economic.
The reaction is reversible. This is shown in the equation by the use of the
__________ symbol
Hydrogen + __________
3H2
+
N2
ammonia
2 NH3
Use the CD ROM or other resources to find out about the many uses of
ammonia
THE PRODUCTION OF NITRIC ACID – THE OSTWALD PROCESS
Nitric acid can be made from nitrogen dioxide but, as we have seen it is
expensive to make nitrogen dioxide by direct combination of nitrogen
and oxygen because of the high temperatures needed.
An alternative way to make nitrogen dioxide is by the catalytic oxidation
of ammonia using a platinum catalyst in the Ostwald Process. Ammonia
is oxidised to nitrogen monoxide which reacts with the air to give
nitrogen dioxide.
The nitrogen dioxide dissolves in water in the presence of air to make
nitric acid. Nitric acid is used to make nitrate fertilisers and explosives.
Watch the demonstration
a. What is the main product of the Ostwald Process and how is it produced?
b Which catalyst is used?
c. Why does the catalyst stay at a high enough temperature once the reaction
has started?
d. What evidence was there that nitrogen dioxide had formed?
e. Complete the word equations to show the stages in the Ostwald Process
___________ + oxygen
→ nitrogen monoxide
nitrogen monoxide + ___________ → nitrogen dioxide
nitrogen dioxide + _______ + oxygen
→
nitric acid
PROBLEM SOLVING
Key :
MAKING AMMONIUM NITRATE FERTISLISER
chemical
Process
Use the chemicals and processes shown below to create a flowchart showing how to
produce ammonium nitrate fertiliser
oxygen
ammonia
nitric acid
hydrogen
water
nitrogen
dioxide
Haber Process
(iron catalyst)
nitrogen
Neutralisation
ammonium
nitrate
Ostwald Process
(platinum catalyst)
CREDIT: WHICH FERTILISER TO USE?
Different crops need different amounts of nitrogen, phosphorus and potassium.
Grass, which farmers use for animal feed, needs a lot of nitrogen, so ammonium
nitrate ( NH4 NO3 ), a nitrogen only fertiliser would be suitable.
Wheat requires all three essential elements so a NPK compound fertiliser is used.
NPK fertilisers are usually a mixture of ammonium nitrate (NH4 NO3 ) ammonium
phosphate(( NH4)3 PO4 ) and potassium chloride (KCl).
Some examples of brand name fertilisers are listed in the table along with their NPK
percentages
FERTILISER
NITRAPRIL
USED FOR
GRASS
ICI NUMBER 1
POTATOES,
WHEAT
POTATOES
WHEAT
ICI NUMBER 7
PERCENTAGE
N
P
35
0
K
0
15
15
20
17
17
17
CALCULATING THE PERCENTAGE OF N, P AND K
1. Work out the gram formula mass of the compound
e.g. ( NH4)3 PO4
( 14 + 1+1+1+1) x 3
+ 31 + (16 x 4 )
= 149
So 1 mole is 149g
2. To work out the percentage of nitrogen
Mass of nitrogen in 1 mole is 14 x 3 = 42g
% N = 42 x 100 = 28.2%
149
3. To work out the percentage of phosphorus
Mass of phosphorus in 1 mole is 31g
% P = 31 x 100 = 20.8%
149
How do artificial fertilisers compare to manure?
Manure is 0.5% N
0.2 % P
and 0.4% K
1. Calculate the percentage of nitrogen in
a.
b.
c.
d.
ammonia
ammonium nitrate ( nitram)
Ammonium hydrogen phosphate . ( NH4)2 HPO4
urea CO ( NH2)2
How do their percentages of nitrogen compare to manure?
2. Calculate the percentage of phosphorus in
a. calcium phosphate Ca 3 (PO4) 2
b. calcium superphosphate Ca (H 2PO4) 2
How do their percentages of P compare to manure?
Why is the name ‘superphosphate’ used?
PROBLEMS WITH FERTILISERS
Because nitrate compounds used in fertilisers are so soluble
they easily wash off into rivers and lochs.
High levels of nitrates in lochs and slow moving rivers cause
rapid growth in algae which can harm fish stocks and water
plants.
No –one is sure if the higher levels of nitrate in drinking water could cause
harm to our health. There may be some harm to bottle fed babies although
there is no direct evidence of this. Nitrates may break down to cancer causing
compounds in the stomach.
Possible solutions to the problem

Only use fertiliser when it is most needed e.g. when the crops are
growing fastest.

Use slow dissolving fertiliser pellets

Use less fertiliser.
QUESTIONS
1. Why do nitrate fertilisers wash off easily into rivers and lochs?
2. What happens to lochs and rivers with high levels of nitrate?
3. What problems may be caused by increased levels of nitrate in drinking
water?
4.
Give 3 solutions to the nitrate problem.
LEARNING OUTCOMES FOR TOPIC 14, FERTILISERS
Traffic light the learning outcomes at the end of the Topic
GENERAL
At the end of the Topic you should know
o
Why there has been an increase in demand for fertilisers
o
Which three elements are essential to healthy plant growth
o
Why legumes are important to soil fertility
o
Three other ways in which nitrogen is replaced in the soil naturally
o
Why nitrogen can react during thunderstorms or in a car engine
o
Why fertilisers are important
o
Why fertilisers have to be soluble compounds
o
Why nitrogen is unreactive
o
The conditions that are used in the Haber Process and the name and formula
of the compound that is formed in the Haber Process
o
Why ammonia is an important industrial product
o
The properties of ammonia
o
The main product of the Ostwald Process
o
Which fertiliser is made from nitric acid and ammonia
o
How nitrogen is recycled in the ‘nitrogen cycle’
CREDIT
o
Why different crops need different types of fertiliser
o
How to calculate the % of N, P or K in different fertilisers
o
Why chemical fertilisers are more expensive than organic fertilisers
o
Why the Haber Process is carried out at a moderately high temperature
o
Why not all the nitrogen and hydrogen are converted to ammonia in the
Haber Process
o
How to prepare ammonia in the laboratory
o
Why there is no need to heat the catalyst in the Ostwald Process
NITROGEN CYCLE
The nitrogen cycle is the term used to describe how nitrogen is recycled.
Plants use nitrogen from the soil to make protein. Animals use the
nitrogen from plants to make animal proteins. Animals excrete
ammonium compounds in urine and feaces.
When plants and animals die, the nitrogen in returned to the soil as their
bodies decay. Some bacteria in the soil break down nitrates into
atmospheric nitrogen.
Atmospheric nitrogen
Haber
Process
Lightning
storms
Denitrifying
bacteria in the soil
root nodules
in legumes
nitrates in
the soil
death and decay
proteins in
plants
excretion, death and
decay
protein in
animals