The need for nutrients

What You Will Need:

• Two identical plastic cups, or, two identical plant pots about 10 cm high.
  
• Seedlings from the same seed tray. Ideally, use ones that look identical or very similar.
• Vermiculite, purchased from garden stores. This substance is safe to handle and very low in nutrients.
• Nitrophoska fertiliser. It should contain the following nutrients and approximate percentages: 12% nitrogen (N), 5% phosphorous (P), 14% potassium (K), plus sulphur (S) and magnesium (Mg).
• A teaspoon.
• A measuring jug (but a plastic cup will do).
  
• Water.

What to do

Setting up the fair test*

• Three quarters fill two identical plastic plant pots with vermiculite.
• To one of the pots add 1 level teaspoon of nitrophoska fertiliser and mix it in so it is spread evenly throughout.
• Don't add any fertiliser to the other pot.
• Select two identical seedlings and gently wash any existing potting mix from their roots.
• Plant one seedling in each pot. Make sure they are firmed in so they don't flop over.
  
  
  
  
• Using a measuring jug or cup, sprinkle each plant with exactly the same amount of water.
• Place both plants in the same sunny position.
• Water both plants with exactly the same amount of water each day, ensuring they don't wilt.
• After one, two and three weeks, compare and record the growth of the two plants using: 1) height measurements, 2) leaf counts, 3) leaf colour observations, and 4) any other observations you can make.

*A fair test is a simple way of describing a controlled experiment.

Questions

1. How did the growth of the plants compare?
2. What evidence do you have for better growth of one of the plants?
3. What was in the nitrophoska fertiliser that might have helped growth?
4. What was the purpose of washing any existing potting mix off the roots before planting the seedlings?
5. Discuss what a fair test is. Now, state five ways this fair test was made 'fair'.
6. Give examples of things you could do to make this test 'unfair'. Would the results of the experiment still be useful?
7. Why is it important not to apply too much fertiliser to plants in the garden or on the farm?

Applying your knowledge

Design and conduct other fair tests that find out things like:

• how much fertiliser gives the best growth
• what the effects are of too much or too little fertiliser
• the types of nutrient sources, such as commercial fertilisers, compost and potting mix, that give best growth.

Related resources

The value of compost

Photo credits: Vermiculite (Peter E. Smith, NSIL).