4.10.4.2 - Producing NPK fertilisers

NPK formulations and agricultural productivity

Fertilisers supply nutrient elements that plants need, helping improve agricultural productivity when those nutrients limit growth.

NPK fertilisers contain compounds of nitrogen, phosphorus and potassium. N, P and K are the chemical symbols for these elements. The fertiliser provides them in compounds, not as a mixture of pure nitrogen gas, phosphorus and potassium metal. Plants need suitable available nutrient forms, so the solubility and chemical form matter as well as the total element quantity.

An NPK fertiliser is a formulation: a mixture designed to contain appropriate proportions of ingredients for its intended use. Various salts are combined to give suitable nutrient percentages. Different crops or soil conditions can require different formulations. Adding a salt supplies the elements present in that compound; it does not make its entire mass equal to the mass of one nutrient.

Calculating nutrient masses

Illustrative formulation with elemental mass percentages: a $20\,\mathrm{kg}$ fertiliser contains $10\%$ nitrogen, $5\%$ phosphorus and $8\%$ potassium. Convert each percentage to a fraction of the total mass.

The remaining mass includes other elements in the salts and other formulation ingredients. Commercial labels may use a different convention; read what the percentages represent.

Finding the fertiliser mass needed

A second formulation contains $15\%$ nitrogen by elemental mass. To supply $3.6\,\mathrm{kg}$ of nitrogen, divide the nutrient target by its mass fraction.

The fertiliser mass must exceed the nitrogen mass because nitrogen is only part of the mixture.

More fertiliser does not necessarily give more useful growth indefinitely. Supply must match the crop's needs and the soil; excess nutrients may be lost rather than taken up and can cause environmental harm. Industrial manufacture combines chemistry with controlled mixing so the formulation delivers the intended composition consistently.

Ammonia, ammonium salts and nitric acid

Ammonia provides a route from atmospheric nitrogen to nitrogen-containing fertiliser salts.

Ammonia is manufactured by the Haber process. It acts as a base and reacts with acids to form ammonium salts. The acid determines the negative ion and therefore the salt's name. With nitric acid it forms ammonium nitrate:

With sulfuric acid it forms ammonium sulfate:

These reactions combine ammonia with the acid rather than producing water by the usual acid–metal hydroxide route. Ammonium nitrate contains nitrogen in both the ammonium and nitrate ions; ammonium sulfate also supplies nitrogen, but not phosphorus or potassium. One ammonium salt alone is therefore not a complete NPK fertiliser.

Ammonia is also used to manufacture nitric acid by reactions involving oxygen and subsequent processing. Nitric acid can then react with more ammonia to make ammonium nitrate or treat phosphate rock. The same ammonia feedstock therefore supports the connected route ammonia → nitric acid → useful salts and phosphate processing.

Industrial NPK production is integrated: products from one process become raw materials for another, then salts are combined in controlled proportions. This is more than mixing three pure elements in a container. The chemistry determines which nutrients each salt can provide.

At Porsgrunn, Norway, on 10 June 2024, Yara inaugurated a 24 MW renewable-hydrogen plant built by Linde Engineering. It uses water electrolysis and renewable electricity to supply hydrogen for ammonia and fertiliser production. Changing the hydrogen source changes the upstream resource route; ammonia still supplies nitrogen to the later reactions.

Mined potassium salts and phosphate rock

Some fertiliser ingredients are obtained from mineral resources rather than manufactured from gases.

Potassium chloride and potassium sulfate are obtained by mining and can provide potassium in suitable fertilisers. Their formulae are $\ce{KCl}$ and $\ce{K2SO4}$. The choice depends on the desired formulation and the other ions appropriate for the crop and soil. Both contain potassium, but their composition is different, so equal masses do not automatically supply equal masses of potassium.

Phosphate rock is also obtained by mining. It contains phosphorus in phosphate compounds, but cannot be used directly as an effective fertiliser in the specified production route. The phosphate compounds are poorly soluble, so they do not readily supply available phosphate to plants. Having the correct element somewhere in a solid is insufficient if it cannot enter a form roots can use.

Chemical treatment with suitable acids converts phosphate rock into more soluble products. This increases the availability of its phosphorus. Compare the situation with potassium salts that are already suitable soluble inputs: both resources are mined, but they do not need identical processing before incorporation into fertilisers.

These raw materials are finite mineral resources. Extraction, processing and transport have energy and environmental impacts, so efficient formulation and application can reduce unnecessary resource consumption. Recovery or reuse of nutrients can also matter in a wider resource plan, but a question about the specified mined inputs should name the actual compounds and their roles.