• Nutrients required by plants 
• Functions and sources of the primary mineral nutrients
• The industrial production of fertilisers
• Flowchart: The industrial production of fertilisers
• N:P:K fertilisers 
• Excessive use of fertiliser and the environment
• Alternatives to inorganic fertilisers

The fertiliser industry

The need for fertilisers is increasing due to:

• the growth in the world population – more people means more food is necessary and
• the decrease in available agricultural land – less land means the available land must produce a lot of high quality food at a high rate (fast).

Nutrients in the soil are used by the plants that grow and these nutrients must be replaced.
Inorganic fertilisers are produced industrially in the Haber, Contact & Ostwald processes at chemical plants like SASOL.

7.1 Nutrients required by plants

NB: DEFINITIONS

• Promote: to help or encourage
• Essential: absolutely necessary
• Inorganic: not produced by a living organism; not containing carbon
• Mineral: an inorganic solid substance that occurs naturally (not man-made)
• Inert: unreactive
• Sufficient: enough

Did you know?

• Bone meal is produced when the bones of dead animals are ground into powder.
• Kelp meal is dried seaweed.
• Granite meal is finely ground granite rock.

Activity 1

1. Which of the following is a primary mineral nutrient that is needed by plants?
   1. N
   2. C
   3. Mg
   4. Na (2)  [2]

Solution
1. A  [2]

7.2 Functions and sources of the primary mineral nutrients

Atmospheric nitrogen is fixed into compounds containing nitrates (NO³⁻) or ammonium ions (NH4+) so that the nitrogen can be taken in by plants and used as nutrient.

• Atmospheric fixation: fixation by lightning;
• Biological fixation: fixation by bacteria in the ground and by the roots of legumes;
• Industrial fixation: fixation by industrial processes like the Haber process.

hint

• “Fixed” means “attached”.

1. In the Haber process:
   Steam reforming of methane (natural gas) in the presence of a platinum catalyst to form a mixture of carbon monoxide (CO) and hydrogen (H₂) gases (synthesis gas).
   
2. In the Contact process:
   2.1 If SO₂(g) is not dried before step 2, it reacts with water to form sulphurous acid, H₂SO₃(aq) which forms acid rain, a type of rain which is produced as a side-effect of human industrial activities, which is corrosive to structures and harmful to living things.
   SO₂(g) + H₂O(ℓ) → H₂SO₃(aq)
   2.2 SO3(g) reacts with water to form gaseous sulphuric acid, H₂SO₄(g) which escapes into the atmosphere and forms acid rain.
   SO₃(g) + H₂O(ℓ) → H₂SO₄(g)
3. In the Contact process:
   3.1 The 1st step is known as the catalytic oxidation of ammonia.
                               _Pt
   4NH₃(g) + 5O₂(ℓ) → 4NO(g) + 6H₂O(g)
   3.2 NO₂ (nitrogen dioxide) is a brown gas which reacts with water to form HNO₃(g) which escapes into the atmosphere and forms acid rain.

Fluorapatite, Ca5(PO4)3F

1. is mined at Phalaborwa;
2. is insoluble in water and can’t be absorbed by plant roots;
3. reacts with sulphuric acid, H₂SO₄(ℓ) to produce phosphoric acid, H₃PO₄(aq).

Superphosphate

1. is produced when fluorapatite reacts with sulphuric acid H₂SO₄ and is a mixture of Ca(H₂PO₄)₂·H₂O and CaSO₄ (gypsum)

Triple Superphosphate

1. is produced when fluorapatite reacts with phosphoric acid H₃PO₄ to form Ca(H₂PO₄)₂·H2O (no CaSO₄ is formed).

7.3 The industrial production of fertilisers

7.4  Flowchart: The industrial production of fertilisers

e.g. Worked example 1
Study the diagram below that illustrates the industrial preparation of nitric acid and answer the questions that follow.

1. 1.1 Write down a balanced chemical equation for the formation of product A.
   1.2 Name the catalyst used during the reaction in Question 2.1.
   1.3 What name is given to the reaction in Question 2.1? Give a reason for this name.
   1.4 Write down a balanced chemical equation for the formation of product B.
   1.5 Name the products A and B.
   1.6 Write down a balanced chemical equation for the formation of product C.
   1.7 Name product C.
2. Ammonium nitrate is important for its use in fertilisers and explosives. It can be prepared by the reaction of nitric acid and ammonia.
   2.1 Write down a balanced equation for the preparation of ammonium nitrate.
   2.2 What kind of fertiliser is ammonium nitrate?
   2.3 Which characteristic of ammonium nitrate makes it suitable for use as a fertiliser?
   2.4 Which characteristic of ammonium nitrate makes it suitable to be used in explosives?
3. Ammonium sulphate is often used as a fertiliser to supplement nitrogen and sulphur shortages in plants.
   3.1 Compile a flow chart that indicates all the industrial steps for the preparation of ammonium sulphate from air, natural gas and sulphur.
   3.2 Give balanced equations for all the reactions that take place during the preparation of ammonium sulphate from air, methane and sulphur.

Solutions

1. 1.1                         _Pt
   4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)
   1.2 Platinum or Rhodium
   1.3 Catalytic oxidation of ammonia
   1.4 2NO(g) + O₂(g) → 2NO₂(g)
   1.5 A: Nitrogen monoxide B: Nitrogen dioxide
   1.6 3NO₂(g) + H₂O(ℓ) → 2HNO₃(aq) + NO(g)
   1.7 Nitric Acid
2. 2.1 HNO₃ + NH₃ → NH₄NO₃
   2.2 Primary mineral nutrient
   2.3 High nitrogen content per mass
   2.4 Nitrogen bonds are easily broken and therefore it decomposes rapidly and easily (explodes).
3. Answers for 3.1 and 3.2
   • O₂ + N₂ (air)
   • CH₄ (methane)
   • S
   • To produce: (NH₄)₂SO₄ (ammonium sulphate)

7.5 N:P:K fertilisers

As plants need a large amount of the primary mineral nutrients (nitrogen, phosphorus and potassium) and these need to be replenished (replaced) in the soil to make sure that crops grow well to provide enough food, NPK fertilisers that contain a mix of these nutrients, are usually used. They are produced by mixing a nitrogen fertiliser, a phosphorus fertiliser and potassium chloride (KCℓ).

Step by step
Step 1. Find the mass of the bag.
Step 2. Find the percentage (%) of fertiliser in the bag (the number in brackets).

• 26% fertiliser in bag
  ∴ 74% inert compounds

Step 3. Find the mass of fertiliser in the bag.

• mass of fertiliser = percentage fertiliser  × mass of bag
                                        100
  = 26 × 10Kg = 2,6 Kg
    100

Step 4. Find the total number of parts of nutrients in the bag. (Add the numbers representing the ratios of each of the elements, N, P and K.)

Step 5. For each element (N, P and K) find the percentage of the element in the fertiliser:

• % element = ratio element × % fertiliser   
• %N=    ratio N      ×% fertiliser = 3 × 26 = 8,67
            total ratio                          9
• %P=     ratio P      ×% fertiliser = 1 × 26 = 2,89
            total ratio                           9
• %P=     ratio K     ×% fertiliser = 5 × 26 = 14,44
            total ratio                          9

Step 6. For each element (N, P and K) find the mass of that element in the bag.

• mass element = ratio element × % fertiliser × mass of bag
                                total ratio              100   
  • 3  × 26   × 10 kg = 0,87 kg
    9    100
  • 1 ×  26   × 10 kg = 0,29 kg
    9    100
  • 5 × 26   × 10 kg = 1,44 kg
    9    100

The per component totals (0,87; 0,29; 1,44) add up to 2.6kg.

e.g. Worked example 2
A bag of fertiliser has the following information on it: 3 : 2 : 3 (26).

1. What information can you deduce from these numbers?
2. Calculate the percentage composition of the fertiliser in the bag.

Solutions

1. It shows the ratio N : P : K.
   3 + 2 + 3 = 8therefore the fertiliser mixture consists of:
   ³/₈ parts N; ²/₈ parts P and ³/₈ parts K
   These nutrients make up 26% of the mass of the bag content. The other 74% is made up of gypsum, lime and sand.
2.  %N=     ratio N     ×% fertiliser
              total ratio
   • =3 × 26 = 9,75
       8
   • = 2 × 26 = 6,5
        8
   • = 3 × 26 = 9,75
        8

7.6 Excessive use of fertiliser and the environment

The correct application of fertiliser to crops is essential for high quality, fast growing crops but using too much or unnecessary fertiliser has a negative effect on the environment.

• Groundwater is contaminated by fertiliser leaching (spreading by water) into the ground.
• Soil becomes acidic (pH decreases). Many plants do not grow in acidic soil.
• Invasive plants grow excessively while indigenous plants die. Invasive plants are undesirable non-indigenous (foreign) plants that grow too fast and out-compete local plants.
• Fertiliser in dams and rivers leads to “eutrophication”, defined below.
• High nitrate concentrations in drinking water decreases the ability of haemoglobin in the blood to carry oxygen and leads to ‘blue baby syndrome’.

7.6.1 Eutrophication

NB: REMEMBER
An overabundance (excess, too much) of nutrients (N and P) in water leads to:

• the fast and excessive growth of algae and other water plants (this is called ‘algal bloom’) and bacteria
• causing a drop in the oxygen concentration in the water and
• the death of fish and other living organisms in the water.

hint: VOCABULARY

• Overabundance / excess: more than is needed
• Shortage: not enough
• Invasive plants: plants that do not naturally grow in a certain area
• Indigenous plants: plants that grow naturally in (are native to) a certain area
• Swamps: ground that is uncultivated but is usually covered with water
• Erosion: soil and rock is removed from the Earth’s surface by wind, rain or water
• Stunted growth: reduced growth leading to smaller, underdeveloped plants

Activity 2

1. Which ONE of the following is NOT associated with eutrophication in water?
   1. Dead zones
   2. Algal bloom
   3. Depletion of oxygen
   4. Increased aquatic life (2)  [2]

Solution

1. D [2]

7.6.2 The effect of a shortage or excess of the primary nutrients (N, P and K) on plants

7.7 Alternatives to inorganic fertilisers

Although fertilisers are essential for the fast growth of high quality crops, the negative effects of inorganic compounds on the environment must be taken into account. Alternative sources of organic nutrients that can be used to ensure good crops are:

• Bone meal;
• Animal manure;
• Natural plant compost;
• Bat guano (faeces);
• Fish emulsions;
• Kelp meal

Advantages of organic fertilisers:

• Break down and release nutrients more slowly than inorganic fertilisers, so there is less chance of the fertiliser leaching into the soil and causing contamination of groundwater;
• Usually cost less and
• Are often available for free.

Disadvantages of organic fertilisers:

• Not enough is available for large scale usage;
• Provide less nutrients – more has to be used;
• Slow release of nutrients sometimes harms plants;
• Slow release may cause nutrients to be available too late in the plant’s growth cycle. 

Activity 3

Fertilisers allow farmers to grow crops in the same soil year after year. However, environmental problems, such as eutrophication, are associated with the application of fertilisers.

1. State ONE PRECAUTION that a maize farmer can take to prevent eutrophication. (1)
   Nitric acid is an important reactant in the production of ammonium nitrate, a nitrogen-based fertiliser.
2. Write down the name of the industrial process for the production of nitric acid. (1)
3. Write down a balanced equation for the preparation of ammonium nitrate from nitric acid. (3)  [5]

Solutions

1. Use fertilisers sparingly  / Do not over-fertilise
   Make use of precision (computerised) application of fertilisers
   Ensure that water from fields does not run into rivers/dams
   Redirect water from fields into reservoirs/away from rivers/dams  (any one) (1)
2. Ostwald process  (1)
3. HNO₃  + NH₃ → NH₄NO₃  (3)  [5]

Activity 4

The rapidly increasing human population is resulting in an ever-increasing demand for food. To meet this demand, farmers apply fertiliser to the same cultivated land EACH YEAR.

1. Explain why farmers have to apply fertilisers to their land each year. (1)
2. Write down one negative impact that over-fertilisation can have on humans. (1)
3. Sulphuric acid is an important substance used in the manufacture of fertilisers.
   The equation below represents one of the steps in the industrial preparation of sulphuric acid.
   2SO₂(g) + O₂(g) ⇋ 2SO₃(g) ∆H<0
   3.1 Write down the name of the process used to prepare sulphuric acid in industry (1)
   3.2 Write down the name or the formula if the catalyst used in 2.3.1 (1)
   3.3 Is the forward reaction endothermic or exothermic? Give a reason for your answer. (1)
   3.4 Write down the name or formula of the fertiliser formed when sulphuric acid reacts with ammonia. (2)  [7]

Solutions

1. Fertilisers replenish nutrients depleted by the growing of crops  (1)
2. Damage to crops/soil  resulting in small or no harvest/ less income. Excessive fertiliser seeps into groundwater and contaminates drinking water  or runs into rivers and/or dams and causes eutrophication  which may result in less income /famine/starvation /poor quality drinking water /fewer recreation areas/ environmental damage/ death of wild animals  (any one) (1)
3. 3.1 Contact process  (1)
   3.2 V₂O₅  vanadium pentoxide  (any one) (1)
   3.3 Exothermic as∆H < 0  (1)
   3.4 (NH₄)₂SO₄  OR / Ammonium  sulphate  (2) [7]

Activity 5

Ammonia, ammonium nitrate and ammonium sulphate are three important nitrogen-containing fertilisers. The flow diagram below shows how these fertilisers are produced in industry.

1. Use the information in the flow diagram above and write down the following:
   1.1 Name Process 1 (1)
   1.2 Balanced equation for Process 2 (3)
   1.3 Name or Formula for compound X (1)
   1.4 Balanced equation for the preparation of ammonium sulphate using sulphuric acid and compound Y (3)
   1.5 Name or Symbol of the primary nutrient in ammonium sulphate (1)
2. Write down one positive impact of fertilisers on humans (1)
3. Write down two negative impacts of the use of ammonium nitrate as fertiliser, on humans. (2)  [12]

Solutions

1. 1.1 Fractional distillation of liquid air  (1)
   1.2 N₂ + 3H₂ → 2NH₃ (reactants  products  balance  (3)
   1.3 Nitric acid / HNO₃  (1)
   1.4 H₂SO₄ + 2NH₃ → (NH₄)₂SO₄ (reactants  products  balance (3)
   1.5 Nitrogen / N  (1)
2. Enhance growth of crops/plants  to produce more food for humans  food security for humans  production / application of fertiliser  results in job creation selling of fertilisers  stimulates the economy  (any one) (1)
3. (Excessive) nitrates in water (eutrophication)  can result in blue baby syndrome or cancer  (Excessive) nitrates/ammonium ions in water can result in poor quality drinking water  or death of fish  or less food  or fewer recreational facilities  or famine due to killing plants / crops from the excess  or excessively changing the pH of the soil and thereby reducing the food production (any two) (2) [12]