Making and Testing PlantNutrients

Students’ Sheet

Introduction

Plants need nutrients for healthy growth. However, sometimesthe soil does not contain enough nutrients. This may be because it lacked nutrients in the first place (not all soils have the same composition) or because it has been used for growing plants and the available nutrients have already been ‘used up’.

At time likes these, gardeners, growers and farmers turn to fertilisers. Added to the soil, they refresh the amounts of nutrients.

Activities

Activity1: Magnesium sulfate

You are going to make a sample of magnesium sulfate from sulfuric acid and magnesium carbonate.

You must wear eye protection. At the concentrations you are using the sulfuric acid is not dangerous, but it is an irritant. Take care when using it.

Instructions

  1. Place a small beaker on a tripod and gauze.
  2. Use a measuring cylinder to measure out and pour 20 cm3 of dilute sulfuric acid into the beaker.
  3. Add about 1 g of magnesium carbonate a little at a time (use a spatula) to the acid. Stir well between additions until there is no more fizzing.
  4. Turn on a Bunsen burner and light it. Adjust the burner so that the flame is non-luminous. Move the Bunsen under the gauze and heat the mixture in the beaker gently for 1-2 minutes.
  5. If the mixture is clear (no cloudiness) add a little more magnesium carbonate and stir. Continue to do this until the mixture is distinctly cloudy.
  6. Filter the warm solution and collect the filtrate in an evaporating basin. The filtrate should be clear. Place the basin on the tripod and gauze.
  7. Turn on a Bunsen burner and light it. Adjust the burner so that the flame is small and non-luminous. Move the Bunsen under the gauze and heat the mixture in the evaporating basin gently. Evaporate the solution to about one-half its original volume (about 10 cm3).

Care: The solution must not boil (this will cause the hot solution to spit out of the basin).

  1. Let the mixture cool. Crystals of magnesium sulfate will begin to form. Filter the mixture when it is cold.
  2. Put the filter paper on a watch glass and open it so that the crystals are on top.
  3. Dab the crystals dry with a clean piece of filter paper, cover the watch glass with piece of dry filter paper and leave it to dry completely at room temperature.
  4. Tip the crystals into a suitably labelled sample tube and stopper.

Questions

1. Describe your product.

2. Name the gas produced when magnesium carbonate is added to sulfuric acid.

3. Write a balanced symbol equation for the reaction between sulfuric acid and magnesium carbonate.

4. Magnesium sulfate consists, like all matter, of particles. Name the particles in magnesium sulfate.

Activity 2: Ammonium sulfate

You are going to make a sample of ammonium sulfate from sulfuric acid and ammonia solution.

You must wear eye protection. At the concentrations you are using both the sulfuric acid and the ammonia solution are irritants. Take care when using them.

Instructions

  1. Place an evaporating basin on a tripod and gauze.
  2. Use a measuring cylinder to measure out and pour 15 cm3 of dilute sulfuric acid into the evaporating basin.
  3. Use another measuring cylinder to measure out 35 cm3 of ammonia solution. Stir the acid and slowly add 25 cm3 of ammonia solution to the basin.

Check the pH, it should be less than 7. Also, there should be no smell of ammonia.

  1. Now add more ammonia solution, 1 cm3 at a time using a dropping pipette. After each addition check the pH of the mixture.

Stop adding ammonia solution once the pH is over 7. There will be a slight but definite smell of ammonia in the basin.

  1. Turn on a Bunsen burner and light it. Adjust the burner so that the flame is small and non-luminous. It must not be roaring. Move the Bunsen under the gauze and heat the mixture in the evaporating basin gently. Evaporate the solution to about one-fifth its original volume (about 10 cm3).

Care: The solution must not boil (this will cause the hot solution to spit out of the basin).

  1. Let the mixture cool. Crystals of ammonium sulfate will begin to form. Filter the mixture when it is cold.
  2. Put the filter paper on a watch glass and open it so that the crystals are on top.
  3. Dab the crystals dry with a clean piece of filter paper, cover the watch glass with piece of dry filter paper and leave it to dry completely at room temperature.
  4. Tip the crystals into a suitably labelled sample tube and stopper.

Questions

  1. Describe your product.
  2. Write a balanced symbol equation for the reaction between ammonia and sulfuric acid.
  3. What is the pH of a neutral solution?
  4. Ammonium sulfate consists, like all matter, of particles. Name the particles in ammonium sulfate.
  5. It is important to dry the ammonium sulfate crystals at room temperature and not to heat them. Suggest a reason for this.

Activity 3: Testing your products

Nutrients essential for healthy plant growth are made from one or more atoms of certain elements. They vary in the quantities needed by plants.

The three elements whose compounds are needed in the largest quantities are:

Nitrogen, N

Phosphorus, P

Potassium, K

The next three needed, but in smaller quantities are

Sulfur, S

Magnesium, Mg

Calcium, Ca

Finally, plants also need much smaller quantities of compounds of several other elements.

Questions

The two compounds you made (magnesium sulfate and ammonium sulfate) can be used to provide plants with some of the nutrients they need.

1. Which ‘essential elements’ are provided by (a) magnesium sulfate, (b) ammonium sulfate?

2. Following your answer to question 1, find out why these elements are important for healthy growth and what happens to plants they do not get sufficient quantities of them.

3. You will be given protocols for investigating the effects of nutrients on the growth of seedlings produced from germinated seeds.

  • Soil culture
  • Water culture
  • Floating culture

Use the protocols to design an experiment to investigate the effectiveness as nutrients of the two compounds you made.

If time allows you may have the opportunity to carry out the investigation.

Science & Plants for Schools:

Making and Testing Nutrients: p. 1