NPK fertilisers contain compounds of all three elements.

The idea

NPK fertilisers are formulations containing compounds of nitrogen (N), phosphorus (P), and potassium (K). These nutrients are supplied as soluble ionic compounds, allowing plants to absorb dissolved ions from soil water. For example, ammonium nitrate provides nitrogen as NH₄⁺ and NO₃⁻ ions, while potassium chloride supplies potassium as K⁺ ions. Crucially, phosphate rock cannot be used directly as it is insoluble; it must be treated with acids (like nitric or sulfuric acid) to produce soluble salts such as ammonium phosphate or calcium phosphate. A fertiliser is classed as NPK only when compounds of all three elements are present, though their relative percentages can vary depending on the specific crop requirements.

What earns the marks

• States that N, P, and K represent the essential elements nitrogen, phosphorus, and potassium.
• Explains that nutrients must be present as soluble compounds rather than free elements to allow for root uptake from soil water.
• Identifies typical soluble compounds used: ammonium nitrate or ammonium sulfate for nitrogen, and potassium chloride or potassium sulfate for potassium.
• Clarifies that phosphate rock is insoluble and must be treated with acid to produce soluble phosphorus compounds (e.g., ammonium phosphate) for use in NPK fertilisers.
• Recognises that an NPK fertiliser is a formulation containing compounds of all three elements, often in varying proportions.

Common mistakes and how to put them right

• Stating that phosphate rock is used directly in NPK fertilisers.

  Put it right by explaining it is insoluble and must first be treated with acid to form soluble phosphates.

• Treating N, P, and K as the elements themselves rather than as nutrients supplied in compounds.

  Put it right by naming a soluble compound that provides each nutrient (e.g., potassium chloride for K).

• Assuming the three nutrients must be present in equal amounts.

  Put it right by explaining that formulations are mixed to contain specific, often different, percentages tailored to crop needs.

Exam tips

• When asked to identify an NPK fertiliser from a list, check the chemical formulas to ensure compounds of nitrogen, phosphorus, and potassium are all present in the same product.
• Remember that phosphate rock must be reacted with an acid (such as nitric, sulfuric, or phosphoric acid) to make the phosphorus soluble and available to plants.