NPK fertilisers are formulations of various salts containing appropriate…

The idea

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.

What earns the marks

• Defines a formulation as a mixture designed to give the required proportions of ingredients.
• States that NPK fertilisers blend various salts rather than containing a single compound.
• Explains that the percentages of N, P and K are chosen to suit a crop, soil or season.
• Interprets an N:P:K ratio as percentage by mass of each element.
• Links solubility of the salts to uptake of nutrient ions by plant roots.

Common mistakes and how to put them right

• Reading an N:P:K ratio as a ratio of compound masses rather than percentages of the elements.

  Put it right by treating each number as the percentage by mass of that element.

• Assuming all fertilisers have the same formulation.

  Put it right by explaining that percentages are adjusted for different needs.

• Thinking a formulation is a pure compound.

  Put it right by describing it as a designed mixture of salts.

Exam tips

• When given an N:P:K ratio, state clearly that the numbers are percentages by mass of the elements.
• Use the word formulation to show understanding that the mixture is deliberately designed.
• If asked why salts are used, link solubility to ion uptake by roots.