Investigating Fertilisers: the effects of minerals on plant growth

This practical was developed for the Scottish Highers and is in the appropriate format, but the protocol can equally well be used for other UK specifications.

Fertilisers play a key role in increasing the productivity of food crops, whether that’s tomatoes in a school greenhouse or vast fields of maize. Ensuring the correct balance of minerals is crucial. In this SAPS Scotland Higher practical, students investigate the effects of different levels of minerals (nitrates, phosphates and potassium) on the growth of radishes.

This experiment is inexpensive, easy to set up, reliable and generates plenty of quantitative results, suitable for analysis. However, it does require a light bank and the radish seed take 2-3 weeks to grow to a suitable size followed by 2-3 days to dry to a constant dry mass.

When they photosynthesise, plants make carbohydrates which are composed of the elements carbon (C), hydrogen (H) and oxygen (O). To convert these to proteins, the element nitrogen (N) must be added. Most plants obtain their nitrogen from the soil in the form of nitrates. Similarly, to convert carbohydrates to ATP or nucleic acids, the element phosphorous (P) must be added. This is obtained from the soil as phosphates. Potassium (K) and iron (Fe) solutions are also absorbed by plants from the soil for growth.

Such substances (e.g. nitrates, phosphates, potassium salts and iron salts) are inorganic (i.e. not derived form living things) and are collectively known as mineral salts. The ones that plants need in the greatest quantities are known as the “macro-elements”.

Summary of method

In this practical, students sow radish seed in ‘soil’ containing different levels of 3 minerals – nitrates, phosphates and potassium. These minerals are present inside N:P:K fertiliser pellets which slowly release their contents to the surrounding soil. The ‘soil’ in which the seed is sown is a 50:50 mixture of peat and vermiculite. This has a very low mineral content and so the level of minerals can simply be controlled by varying the number of fertiliser pellets added to the ‘soil’.

After sowing, the radish seedlings are kept well watered, in a warm temperature under constant illumination. After 18-21 days they are harvested and dried. Measuring the dry mass of the plants at the different mineral levels is a reliable indicator of their overall growth (if the mass of water is included it can give misleading results as the percentage of water in living things is usually very large and often very variable).

Download the full protocol from the link on the right.