Acidity in the Soil. Unit. Year levels 5 6. About this activity. Demonstrating ph levels in substances. Curriculum Links. Science

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1 Unit Year levels 5 6 Acidity in the Soil growth and survival of living things are The affected by the physical conditions of their environment (ACSSU094). Scientific knowledge is used to inform personal and community decisions (ACSHE217). With guidance, pose questions to clarify practical problems or inform a scientific investigation, and predict what the findings of an investigation might be (ACSIS231). About this activity Soil testing is very popular in Kitchen Garden Schools, and it is enormously useful in the garden not just for planning where plants will go, but also to involve students in an understanding of the needs of plants and the reasons behind our treatment of the soil (with compost and other additives such as lime). It goes a small way towards explaining why some plants love to grow together and others won t tolerate the same conditions. You can even explain to students why we don t put citrus and onions into our worm farms, using a basic understanding of ph levels to show that these are acidic items that make worms uncomfortable. Demonstrating ph levels in substances Many people have heard of this ph demonstration using red cabbage water, but have put it aside as too hard. Do try it! It really is easy particularly if you have bits of red cabbage from the garden and it is lots of fun. As with any experiment, try it out first on your own; it s easy to do at home or in the kitchen, and doesn t need special equipment. See the notes in Lesson 3 about which substances to try testing so that you get a full range from acid to alkaline. Test a variety of substances but don t forget to test the soil from your garden, plus perhaps your compost bins, the worm farm and even under specific plants (label the samples, e.g. Under the rhubarb, Under the banana tree, Chicken coop, Wet compost, middle bin and so on...) and allow time for the sediment to settle. You can use the demonstration to lead to a discussion about acidity, alkalinity and ph levels in the soil, the requirements of plants or the way our taste buds sense acidic foods (vinegar, lemon juice very sour!) as opposed to alkaline foods (chalky substances soft and sweetish). When testing the soil using this method, you could check your findings against the results of one of the readily available commercial soil testing kits on the market. Several Kitchen Garden Schools have told us that they get the best value out of the activity when they introduce it with questioning and discussion with the class first because once the students see the change in colours in the solutions, it s a little hard to bring them back to thinking about plants for a few minutes! Stephanie Alexander Kitchen Garden Foundation 2011 <RX PD\ FRS\ DQG FRPPXQLFDWH WKLV PDWHULDO IRU QRQ FRPPHUFLDO DQG HGXFDWLRQDO SXUSRVHV SURYLGHG \RX UHWDLQ DOO DFNQRZOHGJHPHQWV SURYLGHG ZLWK WKH PDWHULDO

2 Acidity in the Soil Lesson 1 Discussion Ideas Year levels 5 6 The growth and survival of living things are affected by the physical conditions of their environment (ACSSU094). Scientific knowledge is used to inform personal and community decisions (ACSHE217). Location The kitchen or the classroom Duration: 15 minutes Vocabulary acid/acidic alkali/alkaline base/basic hydration neutral ph scale solution substance trace elements Getting started To start, you might discuss how we, as gardeners, try to create the ideal environment for our plants to ensure that they grow well. Revisit the basic needs of plants: sunshine for energy, water for hydration, soil for nutrients. Our own experiences Discuss how smart gardeners recognise these different needs and adjust the water, soil and sunshine (sun/shade) to suit the plants they want to grow. Collect students experiences and examples of things we have done to alter the soil in our garden (such as adding compost and spreading mulch, chicken poo or lime). You could also discuss taste as a way of introducing acids. Ask students to give examples of foods that taste bitter, salty, sweet and sour. Can they name some sour foods (lemon juice, vinegar and dishes with these ingredients such as salad dressings)? These foods are acids. What do we know about acids? Can we name any other acids? Which recipes have we used acids in, and why do you think we used them? (Sometimes acid is used for the sour flavour and sometimes to take advantage of the way it emulsifies with oils, like olive oil and vinegar shaken vigorously together in a dressing. It can also be used in a marinade to break down and help tenderise ingredients such as meat or fish.) The opposite to acidic substances are alkaline substances, sometimes called alkali or base substances. Alkaline substances often taste chalky, mealy and sweet. Do students know of anything alkaline? It s hard to think of ingredients, but bicarbonate of soda is very alkaline (you don t want to taste it as it s too strong to be pleasant). Create a class list of vocabulary and examples as you go. Extensions / Variations Have the students look at gardening reference material, such as books or the Gardening Australia website ( Search for acid soil and also for alkaline soil, and plants that prefer one or the other. Do any of the plants mentioned grow in our garden? If they do, does this tell us anything about how and why we grow particular plants together? What does this make students think about plant families? What do we do to adjust the soil acidity to suit our plants? (Such as adding lime for some plants; compost or manures for others.) Students may extend this concept in part by exploring crop rotation.

3 Acidity in the Soil Teacher Resource The ph scale The ph scale is an international standard that has been tested and agreed on. Having a standard measurement of acidity enables us to talk about acidity or alkalinity of substances in a consistent way. The name ph refers to potential Hydrogen or power of Hydrogen which means that it is a measure of the number of hydrogen ions. The scale runs from very acidic at 0 to very alkaline at 14. Neutral water is at 7 this is a measure of pure water (but most of our tap water has traces in it that shift the ph slightly, nearer to 6). The ph (potential Hydrogen) scale Concentration of hydrogen ions compared to distilled water ph rating Sample substances at this level ph = 0 battery acid, hydrochloric acid ph = 1 sulphuric acid ph = 2 lemon juice, vinegar ph = 3 orange juice, fizzy drinks 1000 ph = 4 tomato juice 100 ph = 5 bananas, healthy lake water, coffee 10 ph = 6 milk, urine, saliva 1 ph = 7 pure water 1/10 ph = 8 sea water, eggs 1/100 ph = 9 bicarbonate of soda 1/1000 ph = 10 milk of magnesia 1/ ph = 11 ammonia 1/ ph = 12 soapy water 1/ ph = 13 bleach, oven cleaner 1/ ph = 14 liquid drain cleaner

4 Acidity in the Soil Lesson 2 Soil Health and ph Levels Year levels 5 6 The growth and survival of living things are affected by the physical conditions of their environment (ACSSU094). Scientific knowledge is used to inform personal and community decisions (ACSHE217). Location The kitchen or the classroom Duration: 15 minutes Why does ph matter? When the soil is too acid, phosphorus binds to other elements in the soil in a way that prevents plant roots from taking it in. Plants need phosphorus, and they can t get it when it is stuck to something else. The same thing happens to other trace elements that plants need, so soil that is too acidic starves the plants. Acidic soil also inhibits beneficial bacteria that ideally grow around the plant s roots and help the plant to fix nitrogen in the soil. These little changes can make a healthy plant yellow and weak, and susceptible to diseases and pests. Getting started Revisit the class list of activities and vocabulary from the previous lesson. If students looked up acid-loving and alkaline (lime-loving) plants, discuss their findings. Soil health and plant health Soil can be acidic or alkaline acidic soil is sometimes called sour and alkaline soil is sometimes called sweet soil. Most plants are very sensitive to acidity and will die if exposed to very acidic or very alkaline soil. We make soil more acidic when we add compost, manure and other organic material. We make it more alkaline when we add lime (ground limestone or dolomite lime). The ph scale Look at students lists of examples of acidic foods. If someone says lemon juice is very acidic and someone else says orange juice is very acidic, does that mean they are exactly the same? (No.) Ask: Okay then, how do we communicate what the acidity is? An international agreement is called a standard, or a scale, and it helps us to talk about a measure like acidity in confidence that we are talking about exactly the same measure. Introduce the ph scale and discuss students experiences of some of the substances on the list. Explain that we are going to test some of the things in our kitchen but also the soil in our garden. Some plants prefer to grow in more acidic soil than others (blueberries, potatoes, radishes, carrots, beans and peanuts are some of the plants that love slightly acidic soil between 6 and 5.5 on the scale). Many Australian indigenous plants prefer slightly acidic soil, as much of the soil here is naturally acidic. Most of our vegetables prefer soil that is very slightly on the acid side of neutral (ph 6 to 7 remember 7 is neutral). (An obvious point to make here is that these vegetables come from other countries, where the soil is slightly different.) Ask: Why would we need to know the acidity of the soil in our garden beds? }} To ensure that we plant the kinds of plants that will grow best and produce beautiful fruit and vegetables for our garden. }} To see if we need to add anything to the soil to help the plants that are already there.

5 Acidity in the Soil Lesson 3 Hands On! Doing the ph Test Year levels 5 6 With guidance, pose questions to clarify practical problems or inform a scientific investigation, and predict what the findings of an investigation might be (ACSIS231). Resources red cabbage (½ or more of one small cabbage) knife chopping board kettle for boiling water a large bowl, jug and strainer that can handle boiling water several very clean clear glass jars (same size is best) measuring spoons ½ tsp each of bicarbonate of soda, lemon juice, vinegar and other materials to test torch (optional) white paper (optional) Location The kitchen, garden or classroom Duration: minutes Before the lesson First, chop ½ a red cabbage into small pieces. Shredded, as for coleslaw, works well. You can use the other half of the cabbage in cooking. Boil a litre of water and pour the boiling water on the cabbage. (Try a large pyrex bowl.) Let it sit until it cools. You will see that the water is a rich purple-red colour. Strain the cabbage, keeping the purple-red liquid in another bowl or a jug. You could do the preparation to this point before class. The pale cabbage can go in the compost now. Make sure you keep all the purple water it s best if it s in a white or seethrough jug so that the students can see what colour you started with (plastic should be okay because it s cool now). You can keep this cold purple water in the fridge for 2 3 days if you need to. Doing the demonstration Set up your clean, clear glass jars or cups in a space where students can see them. You can put white scrap paper under them so that the colour change will be obvious, and you can also write the substance on the paper next to each jar e.g. Bicarbonate of soda. In each jar, measure ½ teaspoon of each of the substances that you want to test: ½ tsp bicarbonate of soda, ½ tsp lemon juice, ½ tsp tartaric acid etc. Add 1 tablespoon of fresh water and stir until the powder has become a paste. (Important: Use a clean spoon for each one, otherwise you will contaminate each substance and ruin the results!) Predicting results Ask students to predict what will happen before you mix each one in. Measure 60 ml of the purple cabbage water and add it to each of the cups or jars in sequence. You will see a sudden change of colour in each one as the purple liquid mixes in. Pink is acid; red slightly acid. Dark blue is neutral. Greenish yellow is alkaline. Get the students to arrange the jars in a row according to how acidic they think they are. Don t show them the full scale just yet. If they need prompts, talk about mixing light or paint colours (blue and yellow make green, therefore a green liquid goes between a yellow and a blue liquid). Below is the scale for the class to check their assumptions. Discuss the process and how they made decisions.

6 Acidity in the Soil Lesson 3 p2 ph scale for Red Cabbage Water ph number Colour Pink Dark Red Violet Blue Blue Green Greenish Yellow Notes The activity works best if you have a variety of substances with varying levels of acidity. Try these as the basic set: bicarbonate of soda lime or lemon juice vinegar tartaric acid (not cream of tartar). Try also: powdered chalk a crushed clove of garlic mashed or puréed apple, banana, potato a dash of milk garden soil, compost from the worm farm, soil from under a native plant or tree. Mix powders with a bit of water by stirring or shaking well just before adding the cabbage water. To test soil, put about ¼ cup soil in the bottom of the jar, add ¼ cup water and shake vigorously. Add red cabbage water, shake again and let it settle until you can see the colour of the liquid. (This settling can take about half an hour so you might prepare and label these first. Hold them up to the light or shine a torch through them on a white background to show the colour clearly.) Try testing more than one spot of the garden and discussing with students why it would vary.