AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    Potassium chloride, potassium sulfate and phosphate rock are obtained by…

    Full specification wording

    Production and uses of NPK fertilisers: Potassium chloride, potassium sulfate and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser.

    NPK fertilisers supply nitrogen, phosphorus and potassium.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 6 of 8
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    NPK fertilisers supply nitrogen, phosphorus and potassium. Potassium chloride and potassium sulfate are mined as soluble potassium salts, so they dissolve in soil water and roots absorb the potassium ions. Phosphate rock is also mined, but it consists mainly of calcium phosphate, which has very low solubility. Insoluble compounds cannot release phosphate ions quickly enough into soil solution for crops to absorb, so spreading phosphate rock directly gives little available phosphorus. To make it useful, the rock must be treated with an acid to convert the insoluble calcium phosphate into soluble phosphate salts.

    For example: Treating phosphate rock with sulfuric acid produces a mixture containing calcium sulfate and soluble calcium phosphate, which plants can take up.

    What earns the marks

    • Potassium chloride and potassium sulfate are obtained by mining and are soluble potassium salts used to supply the K in NPK fertilisers.
    • Phosphate rock is obtained by mining and is mainly insoluble calcium phosphate.
    • A fertiliser must release its nutrient ions into soil solution so plant roots can absorb them.
    • Phosphate rock is too insoluble to release phosphate ions at a useful rate, so it is not applied directly as a fertiliser.
    • Treating phosphate rock with an acid converts insoluble calcium phosphate into soluble phosphate salts that plants can absorb.

    Common mistakes and how to put them right

    • Thinking phosphate rock contains no phosphorus.

      Put it right: It does contain phosphorus, but in an insoluble form that plants cannot readily absorb.

    • Assuming all mined minerals are directly usable as fertilisers.

      Put it right: Solubility in soil water is the key requirement, and phosphate rock fails this test.

    • Confusing the roles of potassium and phosphorus.

      Put it right: Potassium salts supply K, while phosphate rock is a source of P after chemical treatment.

    Exam tips

    • Link each named substance to its nutrient: potassium chloride and potassium sulfate supply potassium, while phosphate rock supplies phosphorus after treatment.
    • Use the word soluble when explaining why phosphate rock must be treated before use.
    • If asked why a substance is not a fertiliser, refer to the availability of nutrient ions to roots rather than to the element being absent.

    Try the lesson

    The start of the Production and uses of NPK fertilisers: Potassium chloride, potassium sulfate and phosphate rock are obtained by mining, but phosphate rock cannot be used directly as a fertiliser. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Potassium chloride and potassium sulfate are both obtained by mining. Which element do these two compounds supply to plants?

    Listen

    NPK fertilisers supply nitrogen, phosphorus and potassium. Potassium chloride and potassium sulfate are mined as soluble potassium salts, so they dissolve in soil water and roots absorb the potassium ions. Phosphate rock is also mined, but it consists mainly of calcium phosphate, which has very low solubility. Insoluble compounds cannot release phosphate ions quickly enough into soil solution for crops to absorb, so spreading phosphate rock directly gives little available phosphorus. To make it useful, the rock must be treated with an acid to convert the insoluble calcium phosphate into soluble phosphate salts. For example, treating phosphate rock with sulfuric acid produces a mixture containing calcium sulfate and soluble calcium phosphate, which plants can take up. A strong answer earns marks for these points. Potassium chloride and potassium sulfate are obtained by mining and are soluble potassium salts used to supply the K in NPK fertilisers. Phosphate rock is obtained by mining and is mainly insoluble calcium phosphate. A fertiliser must release its nutrient ions into soil solution so plant roots can absorb them. Phosphate rock is too insoluble to release phosphate ions at a useful rate, so it is not applied directly as a fertiliser. Exam tip: Link each named substance to its nutrient: potassium chloride and potassium sulfate supply potassium, while phosphate rock supplies phosphorus after treatment.

    See it

    A top-down flowchart. A single node labelled Mining branches into three mined materials: Potassium chloride, Potassium sulfate and Phosphate rock. Potassium chloride and Potassium sulfate both lead into one node reading Soluble potassium salts: dissolve in soil water, roots absorb potassium ions. Phosphate rock leads to a node reading Mainly insoluble calcium phosphate: too insoluble to release phosphate ions at a useful rate. From that node, one arrow goes to Not applied directly as a fertiliser and another goes to Treated with an acid, which leads to Soluble phosphate salts that plants can absorb.
    Shows that potassium chloride, potassium sulfate and phosphate rock are all obtained by mining, but only the soluble potassium salts can be used directly, while insoluble phosphate rock must be treated with acid to form soluble phosphate salts.

    Key words

    Say what each word means before you turn the card.

    NPK fertilisers

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    Solubility and nutrient availability

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    Mining of raw materials

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    Industrial processing of minerals

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    See every definition
    NPK fertilisers
    Fertilisers that supply nitrogen, phosphorus and potassium, with the potassium coming from mined soluble potassium salts.
    Solubility and nutrient availability
    A fertiliser must release its nutrient ions into soil solution so plant roots can absorb them, and insoluble compounds cannot do this quickly enough.
    Mining of raw materials
    Potassium chloride, potassium sulfate and phosphate rock are obtained by mining.
    Industrial processing of minerals
    Phosphate rock must be treated with an acid to convert the insoluble calcium phosphate into soluble phosphate salts.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. A single node labelled Mining branches into three mined materials: Potassium chloride, Potassium sulfate and Phosphate rock. Potassium chloride and Potassium sulfate both lead into one node reading Soluble potassium salts: dissolve in soil water, roots absorb potassium ions. Phosphate rock leads to a node reading Mainly insoluble calcium phosphate: too insoluble to release phosphate ions at a useful rate. From that node, one arrow goes to Not applied directly as a fertiliser and another goes to Treated with an acid, which leads to Soluble phosphate salts that plants can absorb.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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