Chemistry

Fertiliser: Making the most of phosphate

This lesson introduces you to the elements, compounds and mixtures that are all around us, particularly those used in agriculture.

In this topic you will investigate the following:

• What are the relationships between atoms, elements, compounds and mixtures?

• How do we use the periodic table to tell us information about different elements?

• How and why are different elements used for different purposes?

So, let’s get digging!

This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmosforschools.com

Introduction: Fertiliser (P1)

Chemical elements are the building blocks that make everything in the universe. And all living things need these elements togrow. Like steel in a car factory, they are the raw materials used by plant and animal cells to help them to them grow, divide andfunction.

Some of these raw materials, like carbon and oxygen, plants get from the air through their leaves. Others have to come from thesoil through the plants’ roots.

The most important of these are nitrogen, phosphorus and potassium.

But some soil does not have enough of these nutrients and so farmers and gardeners have to add them. We call this specially madeplant food fertiliser.

The problem is, though, that while there is plenty of nitrogen and potassium in the world, we have used a lot of the world’sphosphorus and some people are worried we might run out.

Some plants that we use for food aren’t very good at making the most of the phosphorus the farmer gives them. So to help stopthis waste, scientists have been looking for ways to make the plants use less. That way, farmers will save money as they don’t haveto buy as much fertiliser, our supplies will last longer and we can grow more food to feed the growing population.

Read the full Cosmos Magazine article here

Left: Humans have been farming for centuries, this piece of ceramic art depicts farming in Ancient Greece. Right: Modernday farming. Image credits: Stravaganza & iStock.

Question 1

Propose: The Cosmos Magazine article states that "plants just cannot do without" nitrogen, phosphorus and potassium. What do youthink plants need these elements for?

Gather: Fertiliser (P1)

Video credit: Ted Ed / YouTube.

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Question 1

Recall: The density of an atom's nucleus is massive.

True

False

Question 2

Recall: Atoms contain

a nucleus made of electrons.

a nucleus made of clusters of protons and electrons.

a nucleus made of neutrons which is surrounded by a cloud of protons.

a nucleus made of clusters of protons and neutrons which is surrounded by a cloud of electrons.

If you blew up an atom to the size of a stadium, like the Melbourne Cricket Ground, its nucleus, sitting at its centre, wouldbe the size of a marble! Right: Basic diagram of the atom. Image credit: Wikipedia.

The smaller particles found inside an atom are known as subatomic particles. These include protons, neutrons and electrons, andcan be seen in the above diagram of the atom.

Subatomic particles can have a charge. Protons have a positive charge while electrons have a negative charge. Neutrons have neithera positive nor a negative charge and are neutral.

Question 3

Complete: Use the above information to complete the table below.

Subatomic particle Charge Location

negatively charged

proton

found in the nucleus

Atoms are like building blocks. Everything is made of atoms!

An element is a substance that is made up of atoms of only one type. For example, the element oxygen is made up of only oxygenatoms, while the element carbon is made up of only carbon atoms.

Question 4

Complete: What are the missing words in the following three sentences? Add your answers in the table below.

The element potassium is made up of only [blank 1] atoms.

The element [blank 2] is made up of only nitrogen atoms.

The element phosphorus is made up of only [blank 3] atoms.

[blank 1] [blank 2] [blank 3]

The elements that are known to us are arranged in a table known as the periodic table.

Each element in the periodic table has a symbol and an atomic number. An element's symbol helps us identify it without spelling outits name (e.g. 'Al' is the symbol for aluminium). An element's atomic number tells us how many protons there are in each atom ofthat element.

The periodic table of elements.

You may download this high resolution version of the periodic table if you wish to view it in more detail or print it out.

CLL_Periodic_Table.pdf

Question 5

Research: Perform an internet search to find out where the symbols for nitrogen, phosphorus and potassium came from.

Left: N-P-K composition labelling on a bag of fertiliser. Right: Fertilisers provide plants with the essential nutrients theyneed to be healthy. Image credit: iStock.

Plant fertiliser is a mixture composed of various different elements. By law, a fertiliser manufacturer must tell the consumer whatthe total percentage composition of each element is. For the elements nitrogen, phosphorus and potassium they do this by writingthe N-P-K composition on the packaging, like in the image above.

For example, if a bag of fertiliser is labeled 18-24-6 it contains 18% nitrogen, 24% phosphorus and 6% potassium.

Question 6

Calculate: A 5 kg bag of fertiliser contains 6% nitrogen. What is the mass (in kilograms) of nitrogen in the bag?

Process: Fertiliser (P1)

The same elements can sometimes be arranged as a mixture or a compound. Here you can see iron and sulphur as amixture on the left, and as a compound, iron sulphide, on the right.

As we have seen, elements are made up of atoms. Elements can combine in different ways to make mixtures and compounds.

Watch this short video about elements, mixtures and compounds.

Video credit: nuclearrabbit / YouTube.

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In the video, the presenter describes a mixture of iron andsulphur in the watch glass. These are not chemically bonded,just mixed together.

She then explains that, when heated in the test tube, the ironand sulphur react to form a new compound known as iron

sulphide. This means that the elements iron and sulphur arenow chemically bonded. Notice that the properties havechanged?

Question 1

Recall: What test was performed to demonstrate the difference between the mixture and the compound?

The NPK fertiliser described in the Cosmos Magazine article is a mixture. Different elements in that mixture perform different

functions to help promote plant growth. Phosphorus is one of those elements.

Question 2

Research: Find out why phosphorus is so important for plant growth.

In the Cosmos Magazine article, concerns are raised over the predicted limited supply of phosphorus in the future.

A prediction of the annual phosphorus production over the next century alongside expected global population growth.Source: http://www.shenzhenstuff.com.

Question 3

Interpret: What information does the above image convey? What is the message it is trying to get across?

Hint: Think about what this source predicts will happen around the year 2033 and the implications this might have on a growing global

population.

Image credit: iStock

Question 4

Create: Cosmos Magazine has decided to produce it's own brand of NPK fertiliser and has hired you to create a billboardadvertisement for it. Use pencils and paper to create your advertisement. Be sure to show how at least one of the elements foundin an NPK mixture can help to promote plant growth.

Once you have completed your ad, photograph or scan it and upload it below.

Drag and drop file here

Apply: Fertiliser (P2)

Experiment: How much fertiliser is best?

From seed to seedling. Image credit: iStock.

To investigate the effect of fertiliser on plant growth.

We have learned that NPK fertiliser is a mixture of different elements which each provide nutrients to plants. We use fertilisers tosupplement soil when there is a deficiency in a particular nutrient. Nutrients such as nitrogen, phosphorus and potassium areessential to the survival of plants. But just how much fertiliser do plants need?

In this experiment, we will be testing how different amounts of fertiliser affect the growth of mustard seeds.

For this experiment, you will need:

5 Petri dishes

25 mustard seeds (5 for each Petri dish)

Filter paper (to sit at the bottom of each Petri dish)

12 mL each of 0%, 25%, 50%, 75% and 100% fertiliser solutions (these will need to be prepared by your lab technician orteacher for you)

A place to store each Petri dish that has access to sunlight

Our independent variable (what we are changing or testing) is: The amount of fertiliser (%) added to each Petri dish.

Aim

Background information

Materials

Variables

Our dependent variable (what we are measuring or observing) is: The growth of the seeds.

Inquiry questionsFor your experiment, you will need to consider each of the following points:

1. How you will measure the growth of the seeds. For example, you could measure their mass (g) or height (mm).

2. How will you make sure that your experiment is controlled? That is, how you will keep the conditions in each of your Petridishes the same to maintain the integrity of the experiment?

3. How long will you leave your seeds to grow and how often will you measure their growth?

4. What will be your hypothesis for this experiment?

Question 1

Design: Write a step by step experimental method to show how you will use this equipment to test the effect of fertiliser on thegrowth of mustard seeds.

Question 2

Three parts of the experimental design are quite interesting:

We are using filter paper rather than soil in each Petri dish.

Five seeds are placed in each Petri dish rather than one.

We are using 0% fertiliser in one Petri dish.

Judge: Why do you think each of these decisions has been made?

Question 3

Complete: Use the table below to clearly state your controls and variables.

Hint: Make sure that you show how you will measure plant growth, include units and consider that there are lots of other variables that

need to be controlled.

Independent variable Dependent variable Controlled variables

The amount of fertiliser (%) added to each Petridish.

Each petri dish will receive either 0%, 25%, 50%,

75% or 100% fertiliser

Question 4

Judge: What would be an appropriate hypothesis for your experiment?

Results

Question 5

Construct: You will need to construct a table that clearly shows your results. You may also wish to include photos, sketches andgeneral observations to support your data.

Discussion

Question 6

Evaluate: In these experiments, do your results tell you how well NPK fertilisers work compared to other mineral supplements?How could the method be changed to gather this information?

Question 7

Write a conclusion that would respond to your aim and hypothesis.

Hint: You can consider the following questions when writing your conclusion.

What were you able to find out - does NPK fertiliser affect the growth of plants?

What is the best concentration of NPK fertiliser to use on growing mustard seeds?

Conclusion

Career: Fertiliser (P2)

Benjamin Norton has always loved science. When he was a child, he tried to build a laser out of a cardboard tube, a torch, andsome red cellophane tape. Today, as an experimental physicist at Griffith University, he gets to build real lasers.

Surprisingly, Ben didn’t study Physics in high school. He wantedto be a marine biologist, but changed his mind when he foundout he could study lasers at Griffith University’s Photonics andNanoscience course. Now, he couldn’t be happier. Most of hisday is spent working on his lasers, which he builds from scratch.

In 2012, Ben used one of his lasers to help take a photo of asingle atom. Atoms are the building blocks that make up ouruniverse. They are also incredibly tiny - a single strand of yourhair is 1 million atoms wide. Imagine trying to take a photo ofsomething that small! To make things even harder, the laserbeam Ben uses is over a hundred thousand times wider thanthe atoms he photographs. Trying to align the laser beam to theatom is like using a bulldozer to pick up a single grain of rice. Toease this process, he freezes the atom and holds it in place withelectric fields in a vacuum before he photographs it with anextremely powerful microscope. The process can take a longtime, but it’s well worth the trouble. Ben’s photo is the highestresolution ever taken of an atom to date. “The best thing aboutthis job is the feeling you get when you see something no one inthe world has ever seen before,” he says.

When he isn’t in the lab, Ben loves playing American football andsoccer. He also enjoys scuba diving off the coast of Queensland,where he lives.

Question 1

Imagine: Pretend that you are Ben and have been working on this project for many years. You are about to see an atom for thevery first time!

What do you think would be the most exciting part about this moment?

Cosmos Lessons team

Education Editor: Bill CondieArt director: Robyn AdderlyProfile author: Yi-Di NgLesson authors: Deborah Taylor and Daniel Pikler