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FERTILISERS
As the population of the world increases, there is need to increase crop yields. We provide the plants
with nutrients and we do this by using fertilisers.
Organic fertilisers Plant remains or animal excretions such as plant compost and animal manure.
Inorganic fertilisers Manufactured in chemical industrial processes and chemical plants.
Inorganic Fertilisers
Plants need nutrients for healthy growth. In order for soil to be able to support and sustain growth,
these important nutrients are required. Mineral nutrients are classified as macronutrients and
micronutrients. Macronutrients also consist of primary and secondary nutrients.
Macronutrients
Macronutrients are divided into primary and secondary nutrients.
Primary nutrients
The three elements that are sourced in minerals in the soil are (N), phosphorus (P) and potassium (K).
These nutrients are mineral nutrients that dissolve in water in the soil and are absorbed by the roots of
the plants.
Nitrogen
The air is made up of about 78% of nitrogen but plants are not able to access the element in this form. It
is unreactive in the form of nitrogen gas ( N
2
). However, in nature, lightning causes oxygen to react with
nitrogen in the atmosphere to form soluble nitrogen oxides, such as NO, NO
2
, and N
2
O
4
. These soluble
oxides will dissolve in atmospheric water such as clouds and become rain. They will be now ready for
absorption by plants. Nitrogen can also be converted to soluble nitrates (NO
3
-
), before plants can absorb
it. Nitrifying bacteria in the soil produces nitrates from decaying plant and animal matter. Nitrogen is
essential for proteins, which make strong stems and healthy leaves. Nitrogen deficiency in a plant is
seen by its stunted growth, weak stem or yellowish leaves.
Phosphorus
Phosphorus is a very reactive non-metal and it is not found as a free element in the soil, but rather in
minerals such as phosphates (PO
4
3-
). Phosphates help in root development, ripening of fruit and
germination of seeds. It assists in converting the sun’s energy and other chemicals such as nitrogen into
usable food for plants. Phosphorus deficiency is evident by purplish colour of leaves from the
accumulated sugars created by photosynthesis that cannot be used in the absence of sufficient
phosphorus, low fruit yields and sickly looking plants.
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Potassium
Potassium is a very reactive metal and is also not found as a free element in the soil. Potassium occurs in
the Earth’s crust as minerals, such as feldspars and clays. Potassium is leached from these by
weathering.
Potassium can also be present in potash. Potash is made up of potassium oxide (K
2
O) and potassium
hydroxide (KOH) and is used in fertilisers. Plants will utilise potassium salts such as potassium chloride
(KCI) and potassium nitrate (KNO
3
) to survive frost and resist diseases, as well as improving quality of
flowers and fruit. Potassium also regulates water balance in plants and controls the rate of
photosynthesis and respiration. It helps in the movement of sugars in plants. Potassium deficiency is
recognized by a plant’s low resistance to disease, burnt-looking leaves and poor fruit yield.
Secondary nutrients
The secondary nutrients are calcium (Ca) magnesium (Mg) and sulphur (S). There are many of these
nutrients found in the soil. Sulphur is found in sufficient amounts from the slow decomposition of
organic matter.
Micronutrients
Micronutrients are elements essential for plant growth and are required in very small amount. These
include iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo) and chlorine
(CI). Recycling organic matter, such as leftover leaves and stems of plants after the crop has been
harvested, is a sustainable way of providing micronutrients.
Manufacturing of fertilisers
As the demand for food increases as the population increases, crops need to be grown more quickly and
often in unfavourable conditions. Production of fertilisers help in the growth of plants and vegetables.
Nitrogen fertilisers
Nitrogen is a vital ingredient for fertilisers. The air is made up of approximately 78% of nitrogen.
However, the nitrogen needs to be separated from all other gases in the air and this is done by a process
called fractional distillation. Fractional distillation separates mixtures of liquids by making use of their
differing boiling points.
Nitrogen based fertilisers are produced by the use of three process namely The Haber process, The
Ostwald process and The Contact Process.
The Haber Process
The Haber process is used to produce ammonia (NH
3
).
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The equation for the reaction is:
3NH
2
+ N
2
2NH
3
Condition for reaction: Catalyst of iron or iron oxide
Gases are subjected temperature of approximately 450⁰C
Pressure of approximately 300 atmospheres
Ammonia is also used in the manufacturing of fertilisers and in the making of nitric acid (HNO
3
).
The Ostwald process
The Ostwald process is used to produce nitric acid (HNO
3
).
The process is made up of three steps:
Step 1: 4NH
3
+ 5O
2
⇋ 4NO 6H
2
O
This step is known as the ‘catalytic oxidation of ammonia’. In this step a platinum
gauze is used to speed up the reaction. It takes place at a temperature of
approximately 800⁰C.
Step 2: 2NO + O
2
→ 2NO
2
Step 3: 3NO
2
+ H
2
O → 2HNO
3
+ NO
The NO produced in step 3 is collected and re-used in Step 2.
Condition for reaction: Pressure of 4 10 atmospheres
Temperature of about 900⁰C as the reaction is highly exothermic
Catalyst of platinum
The Contact Process
The Contact process is used to produce sulphuric acid (H
2
SO
4
)’
The process is made up of four steps:
Step 1: S + O
2
→ SO
2
Step 2: 2SO
2
+ O
2
2SO
3
In this step vanadium pentoxide (V
2
O
5
) is used as a catalyst. It also takes place at a
temperature of approximately 450⁰C.
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Step 3: SO
3
+
H
2
SO
4
→ H
2
S
2
O
7
(H
2
S
7
O
7
is called fuming sulphuric acid or oleum)
Step 4: H
2
S
2
O
7
+ H
2
O → 2H
2
SO
4
(fuming sulphuric acid id diluted to form H
2
SO
4
)
Condition for reaction: Catalyst of vanadium pentoxide in step 2
Temperature of 450⁰C
Pressure is kept at 1-2 atmospheres
Uses: Sulphuric acid is used in the production of fertilisers, battery acid, detergent, synthetic fibres
and paint.
Phosphate fertilisers
The production of phosphoric acid (H
3
PO
4
)
The sulphuric acid produced in the Contact process is used to produce phosphoric acid. The sulphuric
acid reacts with phosphate rock to produce phosphoric acid. An example of such is the reaction of
sulphuric acid with the rock fluorapatite (Ca
5
F(PO
4
)
3
)
Ca
5
F(PO
4
)
3
+ 5H
2
SO
4
+ 10H
2
O 5CaSO
4
.2H
2
O + HF 3H
3
PO
4
Uses: This phosphoric acid is used to produce superphosphate fertilisers.
Phosphoric acid is used to remove rust and is major in nausea medications and production of
teeth whitening toothpastes.
Potassium fertilisers
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Potash is mined as the mineral potassium chloride (also called muriate of potash, MOP), potassium-
magnesium sulphate (K
2
SO
4
.MgSO
4
also called sulphate of potash magnesia, SOPM) or mixed sodium-
potassium nitrate (also called Chile saltpetre).
Advantages of inorganic fertilisers:
High nutrient concentration
Nutrient immediately available to the crop
Pure and consistent
Disadvantages of inorganic fertilisers:
Nitrogen-based fertilisers can cause soil acidification when added which can lead to nutrient
decrease in the soil.
Many inorganic fertilisers contain heavy metals such as cadmium and it contaminates the soil.
Eutrophication occurs and the washed away fertilisers stimulate excessive plant growth evident
in the formation of algae on the surface of the water.
Nitrogen-based fertilisers can also lead to pest problems by increasing the birth rate
Inorganic fertilisers can run into our drains when it rains and contaminate the water thus killing
fish.
Organic fertilisers
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Another method that has been used traditionally is to let livestock such as cattle graze in the fields,
once the crop has been harvested. The manure is then left on the lands and provides nutrients to the
new crop when it is planted.
Advantages of organic fertilisers:
Most organic fertilisers are slow release fertilisers which reduce the risk of leaching and
eutrophication.
Soil chemistry is improved by the use of organic fertilisers.
Costs are lower
Disadvantages of organic fertilisers:
Nutrient levels are less concentrated than in organic fertilisers
The composition is variable
Improperly used organic fertilisers may contain pathogens from animal and plant matter that
are harmful to humans or plants.
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