Production and uses of NPK fertilisers — AQA GCSE Chemistry

Fertilisers supply the three essential elements nitrogen, phosphorus and potassium (NPK). Nitrogen promotes leafy growth, phosphorus supports root development, and potassium helps disease resistance. Because crops remove these elements from soil, farmers add NPK fertilisers to replace them and improve agricultural productivity. Common examples include ammonium nitrate for nitrogen and potassium chloride for potassium. Plants absorb soluble mineral ions, not intact compounds. While some compounds are naturally soluble, phosphate rock is insoluble and cannot be used directly as a fertiliser. It must be treated with nitric, sulfuric or phosphoric acid to produce soluble salts (like single or triple superphosphate) before plants can absorb the phosphorus ions.

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.

Industry makes NPK fertilisers by combining several feedstocks and process streams rather than by a single reaction. Nitrogen compounds come from ammonia, itself made from nitrogen and hydrogen in the Haber process. Ammonia is neutralised by acids such as nitric acid or sulfuric acid to form ammonium salts. Phosphorus compounds come from phosphate rock, which is treated with nitric acid, phosphoric acid or sulfuric acid to produce soluble phosphates. Potassium compounds come from mined potassium chloride or potassium sulfate. The resulting intermediate solutions are blended, granulated and dried so that each batch contains the required percentages of N, P and K. Because the routes are integrated, a plant can adjust raw materials and process conditions to make different formulations.

A formulation is a mixture designed to give the exact proportions needed for its purpose. NPK fertilisers are formulations because they blend different salts so that the percentages of nitrogen, phosphorus and potassium suit a particular crop, soil or growing season. The nutrient content is usually shown as an N:P:K ratio, for example 10:10:10 or 15:5:10, where each number represents the percentage by mass of that element in the fertiliser. The salts must be soluble so that the ions can dissolve in soil water and be taken up by roots. Different formulations may also include fillers or other nutrients, but the defining feature is a controlled mixture of salts giving appropriate percentages of N, P and K.

Ammonia is a versatile starting material because it is a base and a source of nitrogen. Reacting ammonia with an acid produces an ammonium salt in a neutralisation reaction: ammonia plus nitric acid gives ammonium nitrate, ammonia plus sulfuric acid gives ammonium sulfate, and ammonia plus phosphoric acid gives ammonium phosphate. These salts are water-soluble and supply nitrogen, and in some cases phosphorus, to plants. Ammonia is also oxidised industrially, typically over a heated catalyst, to nitrogen monoxide, which is further oxidised to nitrogen dioxide and dissolved in water to make nitric acid. That nitric acid can then be reacted with more ammonia to make ammonium nitrate, so the two products are linked within an integrated fertiliser process.

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.

Phosphate rock contains insoluble calcium phosphate, meaning plants cannot absorb it. To produce soluble NPK fertilisers, the rock is treated with acids. Reacting phosphate rock with nitric acid produces a mixture of phosphoric acid and calcium nitrate. The phosphoric acid produced can then be reacted with further phosphate rock to manufacture triple superphosphate (calcium phosphate). Alternatively, treating the rock with sulfuric acid produces single superphosphate, which is a mixture of calcium phosphate and calcium sulfate. These acid treatments convert the insoluble rock into soluble salts, allowing phosphate ions to dissolve in soil water and be absorbed by plant roots for healthy growth.

Treating phosphate rock with different acids gives different soluble salts. Nitric acid produces calcium nitrate and phosphoric acid; sulfuric acid produces calcium sulfate and calcium phosphate; phosphoric acid produces calcium phosphate. These salts supply phosphate and sometimes nitrate or sulfate ions to crops. Industrial fertiliser production differs from laboratory preparation: industry uses large-scale continuous plants, locally mined phosphate rock, concentrated acids, controlled temperatures and recycling of waste heat, and it monitors product purity and nutrient content. Laboratory preparation typically uses small quantities, bench apparatus such as beakers and filter funnels, dilute acids and batch methods. Both routes rely on the same reaction between an acid and the basic phosphate rock, but scale, equipment, safety controls and product specification differ.