NPK Fertilisers (GCSE Chemistry)

NPK Fertilisers

NPK Fertilisers

Formulating NPK Fertilisers

• NPK fertilisers contain three elements. There are three main elements in the formulation of NPK fertilisers. These are nitrogen, phosphorous and potassium.
• NPK fertilisers improve agricultural productivity. We can gain higher yields of crops by using NPK fertilisers. They contain specific nutrients to which are essential for plant growth.
• Fertilisers need to be soluble. A basic requirement of fertilisers is that they need to dissolve in water, so that plants can absorb them through their roots.

Using Ammonia

• Raw materials are used to create NPK fertilisers. In the production of NPK fertilisers, raw materials are used. Through using a series of integrated processes, these can be converted into NPK fertilisers. 
• NPK fertilisers are specifically formulated. Within NPK fertilisers, there are specific proportions of each nutrient. These exact percentages of nitrogen, phosphorous and potassium enable crops to flourish. An NPK fertiliser is an example of a formulation.
• Ammonia contains nitrogen. As we saw earlier, one of the elements in NPK fertilisers is nitrogen. Ammonia contains nitrogen, which means it can form nitrogen containing compounds.

• Ammonia can form nitric acid. Through reacting with oxygen and water, ammonia forms nitric acid. This nitric acid can react with ammonia to produce the soluble salt, ammonium nitrate.
• Ammonia can form ammonium salts. As we have just seen, we can react ammonia with nitric acid to form ammonium nitrate. This contains lots of nitrogen.

NH₃ + HNO₃ → NH₄NO₃

• Ammonium salts are basic. We previously learnt how to test the pH of compounds. A low pH indicates that a substance is acidic, whilst a high pH indicates that it is basic. Some ammonium salts are basic.
• Ammonium nitrate can be made in the lab. We can form ammonium nitrate in the lab. This process involves various steps, using low concentrations of reactants to undergo titration and crystallisation.
• Lab processes form a small amount of product. Despite being safer than the industrial production of ammonia, the lab process only forms a small amount of product. This product is pure ammonium nitrate crystals. 
• Ammonium nitrate can be made in industry. We can also make ammonium nitrate on a much larger scale through industrial processes. This is quite a dangerous reaction as it gives out a lot of energy in the form of heat. The product is extremely concentrated ammonium nitrate.

Using Potassium

• Potassium compounds are found through mining. Potassium compounds, such as potassium chloride, or potassium sulphate can be found through mining. These provide a good source of potassium for NPK fertilisers.
• Potassium salts are soluble. There is no need to further process potassium salts as they are all soluble and can be applied directly to plants.

Using Phosphate

• Phosphate rocks can be found through mining. As with potassium salts, phosphate salts are abundant in rocks, like calcium phosphate,  and can be mined to extract phosphates.
• Phosphate rocks are insoluble. Phosphate rocks found through mining are insoluble. This means that we must modify them into soluble salts, before they can be used as fertilisers.
• Phosphate rocks can produce soluble salts. When treated with nitric acid or sulphuric acid, phosphate rocks can produce soluble salts. These can then be used as NPK fertilisers.

Phosphate Salts

• Phosphate rocks can be treated with nitric acid. When treated with nitric acid, phosphate rocks will form a soluble salt called calcium nitrate and also phosphoric acid.

calcium phosphate + nitric acid →   calcium nitrate + phosphoric acid

• Calcium nitrate is soluble and can be used as a fertiliser. The calcium nitrate produced from this reaction, contains nitrogen. It is a soluble salt which can be used in NPK fertilisers.
• Phosphoric acid produced cannot be added directly to plants. Phosphoric acid is an acid, and therefore cannot be added to plants directly. It is neutralised with ammonia, to make ammonium phosphate. This contains both the nitrogen, “N” and the  phosphorus,“P”, needed in NPK fertilisers.

Ammonia + phosphoric acid →    ammonium phosphate

• Ammonium phosphate is a soluble salt. Ammonium phosphate is soluble and a valuable component of NPK fertilisers.
• Phosphate rocks can be treated with sulphuric acid. When treated with nitric acid, phosphate rocks will form a soluble salt called calcium sulphate and another soluble salt called calcium dihydrogenphosphate. This is known as super phosphate.

calcium phosphate + sulphuric acid →   calcium dihydrogen phosphate + calcium sulphate

Super phosphate is soluble and an important addition to NPK fertilisers

• Phosphate rocks can be treated with phosphoric acid. When treated with nitric acid, phosphate rocks will form a soluble salt called calcium phosphate. This product can be called a triple superphosphate. There are no other by-products.

calcium phosphate + phosphoric acid →  calcium dihydrogen phosphate