Promoting quick and healthy plant growth is important for farmers all around the world. NPK fertilisers are key in facilitating this. They contain the elements nitrogen, phosphorous and potassium. There are several types of reactions needed to produce these fertilisers, many of which have very specific conditions as detailed below.

• What are NPK fertilisers?
  • NPK fertiliser composition
• The Haber process and the use of NPK fertilisers
  • Ammonia reaction with phosphoric acid
• Obtaining potassium and phosphorus for NPK fertilisers
• NPK fertiliser uses

What are NPK fertilisers?

Compounds of nitrogen, phosphorus, and potassium can be used as fertilisers. This increases the crop yield by allowing them to grow bigger and improving the productivity of agriculture.

NPK fertilisers are easy to use, widely available, don't have a bad smell, and contain enough nutrients in order for more crops to be grown.

NPK Fertiliser Composition

Nitrogen, phosphorus and potassium play a vital role in plant growth. For example, potassium helps plants, flowers, and healthy fruits to grow. Healthier leaves are grown due to nitrogen and finally, phosphorus plays a role in allowing for healthy roots to form. Plants' life processes can be affected if these three elements are lacking.

The Haber Process and the Use of NPK Fertilisers

The Haber process is a chemical process that is used in the production of ammonia (NH₃) from nitrogen gas (N₂) and hydrogen gas (H₂). Ammonia is a key ingredient in the production of NPK fertilizers, as it is used as the source of nitrogen.

The NPK fertiliser production begins with the synthesis of ammonia through the Haber process. Nitrogen gas from the air is combined with hydrogen gas, and the mixture is fed into a high-pressure reactor containing a catalyst. The catalyst facilitates the reaction between nitrogen and hydrogen to produce ammonia. The ammonia is then purified and stored for use in the production of NPK fertilizers.

The ammonia produced through the Haber process is combined with phosphoric acid (H₃PO₄) to produce ammonium phosphate ((NH₄)₃PO₄), which is high in nitrogen and phosphorus. Potassium can be added to create a complete NPK fertilizer.

Ammonia Reaction With Phosphoric Acid

When ammonia reacts with phosphoric acid, a neutralisation reaction takes place.

The word equation for this reaction is:

\( \text{ammonia + phosphoric acid} \longrightarrow \text{ammonium phosphate}\)

The symbol equation for this reaction is:

\(3NH_3 \space (aq) + H_3PO_4 \space (aq) —› (NH_4)_3PO_4 \space (aq)\)

Obtaining Potassium and Phosphorus for NPK Fertilisers

Mining can take place to obtain potassium sulfate, potassium chloride, and phosphate rock.

Potassium sulfate and potassium chloride are used as a source of potassium. Phosphate rock is not appropriate to use as a fertiliser directly and this is because of the insolubility of the phosphate salts in the rock. This leads to plants not being able to use them as nutrients.

For soluble salts to be produced, phosphate rock can be reacted with a number of acids. Phosphate rock can be treated with nitric acid, sulphuric acid, and phosphoric acid to create these soluble salts that can be directly used as fertilisers.

Phosphate rock reacts with nitric acid to produce phosphoric acid, H₃PO₄, and calcium nitrate Ca(NO₃)₂. After this, ammonium phosphate, (NH₄)₃PO₄, can be produced when the phosphoric acid is neutralised with ammonia.

Phosphate rock reacts with sulphuric acid and produces a mixture of calcium phosphate, Ca₃(PO₄)₂, and calcium sulfate, CaSO_4. This mixture can also be known as single superphosphate.

Phosphate rock reacts with phosphoric acid and produces just calcium phosphate, Ca₃(PO₄)₂. This product can also be called triple superphosphate.

NPK Fertiliser Uses

NPK fertilizers are fertilizers that contain the three main macronutrients required by plants: nitrogen (N), phosphorus (P), and potassium (K). These nutrients are essential for plant growth and are typically found in the soil in varying concentrations, however, when cultivating crops in the fields or at home, soil can become deficient in these crucial nutrients.

Here are some of the key uses of NPK fertilizers:

• Increase crop yields: Plants require nitrogen, phosphorus, and potassium in relatively large amounts, and deficiencies in these nutrients can limit growth and reduce yields. By providing these nutrients in the form of fertilizer, farmers can help to ensure that plants have the nutrients they need to grow and produce healthy crops.
• Improve plant health: NPK fertilizers can also help to improve the overall health and vigour of plants. This can make them more resistant to pests and diseases, and better able to withstand environmental stresses such as drought or extreme temperatures.
• Correct nutrient deficiencies: Soil tests can help to identify which nutrients are lacking, and NPK fertilizers can be applied to address these deficiencies.
• Boost plant growth: NPK fertilizers can be used to promote rapid growth in plants, which can be useful in situations where fast-growing plants are desirable, such as in the production of turf grass or landscaping.
• Enhance flower and fruit production: NPK fertilizers can be used to enhance the production of flowers and fruits in plants.

While NPK fertilizers can be highly effective in increasing crop yields and improving plant health, they must be used carefully to avoid overapplication and environmental damage. Excess fertilizer can leach into groundwater, leading to pollution and other environmental problems.