Lesson 9 – Part A: SOIL pH
Lesson 9 – Part B: TESTING SOIL pH
Lesson 9 – Part C: (FOR SENIOR STUDENTS) THE pH SCALE
SOIL pH
WHAT IS SOIL pH?
Soil pH is a measurement of how acidic (sour) or alkaline (sweet) soil or potting mix may be. Lemon juice is acidic. It contains a lot of citric acid and has a sour taste. Alkalis are things that reverse acidity. In soil, we use crushed chalk to add calcium and reverse acidity. Calcium makes soil more alkaline, or sweeter.
The chart below only shows the soil pH scale from 4 to 10 because plants can't grow in soil where the pH is lower than 4 or higher than 10. Soil or potting mix that has a pH of 7 (in the middle of the scale) is called neutral because it is neither acid nor alkaline.
An easy way to remember the pH scale is – the more calcium in soil, the higher the pH number.
WHY IS SOIL pH VERY IMPORTANT?
Soil pH controls how active earthworms and helpful bacteria and fungi are in your soil. They can all work best in a soil pH close to 7.
The pH of your garden soil also controls the amount of different minerals that plants can soak up and use for food. When soil pH is too high or too low, some minerals become stuck to other minerals or to soil particles and plants can't soak them up.
Plants need lots of major minerals and only tiny amounts of others – these are called trace (or minor) elements. In the chart below, plants need large amounts of the first 6 food minerals, and only tiny amounts of the next 6 minerals. The horizontal white bands in the chart show how much of the different foods plants are able to soak up at each pH number.
• In the dark green area of the chart the pH is between 6 and 7.5. Most fruits and vegetables grow well in this pH range because they can soak up all the food minerals in the correct amounts that they need for good health.
• The lighter green area has a pH of 5.5-6. Potatoes, strawberries and blueberries prefer this pH, and most other plants can manage to grow and produce a crop if soil has this pH.
• In the grey areas of the chart, plants may not get enough of the foods they need in large amounts and will get poisonous amounts of some foods they should only have in tiny amounts.
• Very few plants can survive when soil pH is in the brown areas.
If plants can't get enough of the right foods when they need them, they are more likely to catch diseases and be attacked by pests.
This may look like a disease on the capsicum but it is 'blossom-end rot' – a sign that the plant does not have enough calcium. If there is not enough calcium in your soil (the pH number is low), your plants will grow very slowly, new growth may become black, or leaves may become yellow. Fruits on tomato, capsicum, eggplant, cucumber, pumpkin, watermelon and zucchini plants can't form properly. Celery will go black in the centre and potatoes will be soggy when dug up. The cabbage family will have small twisted leaves, grow very slowly, and have a lot of caterpillars eating its leaves.
However, if there is too much calcium in your soil (the pH number is high), potatoes and beetroot will be rough and scabby, and celery will have cracked stems. Turnips will be hollow and cauliflower heads will go brown or the plants will not form heads at all.
You can see that keeping soil pH in your garden beds in the green areas of the chart can avoid a lot of problems with your food crops.
The best way to keep soil in a good pH range is to add compost to the topsoil in your garden beds.
Well-made compost has a pH of 6.5 – perfect for growing most vegetables and fruits. When you mix compost with soil in garden beds it helps to raise the pH number of acidic soils and lower the pH number of alkaline soils.
The electrically charged places on humus in compost (that you learnt about in Lesson 3) also stop plants from soaking up too much of the minerals they need in very tiny amounts.
Over time, the pH of soil in garden beds will change, and it is a good idea to check the pH of your soil at least once a year to see if the pH number has become too low or too high.
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TESTING SOIL pH
Don't forget your hat and gloves!
The only way to find the pH of your garden soil is to test it with a special pH soil test kit. A good time to test the pH of your garden soil is when you are getting the bed ready to grow some plants – after you have added compost to the bed, or when you are going to grow a green manure.
To test your soil you will need a bucket, a trowel and a pH test kit.
Use a hand trowel to take about 6 samples of soil up to 10 cm deep from different parts of the section of the
garden bed that you want to test. 
Put the soil samples into the bucket. If the soil is not damp, add just enough water to make it damp – not wet enough for mud pies. Use the trowel to mix the soil very well. This will take at least one minute.
Take about a teaspoonful of the well-mixed soil from the bucket and put it on the small white plastic sheet that is included in the test kit. Place the plastic sheet on a level surface. Carefully add
several drops of the liquid to the top of the soil sample to moisten it. Replace the lid on the bottle. Stir the sample with the plastic rod to spread the coloured liquid through the mixed sample.
Dust the sample with the white powder from the puffer bottle included in the kit. Replace the lid on the bottle and wait for one minute.
During that time, the white powder will change colour. After one minute, compare the colour of the powder on the sample with the colour patches on the chart by holding the chart close to the sample. Don't allow the chart to touch the sample or the chart will soak up some of the dye, and it will be difficult to see the correct colour in future tests. Next to each colour on the chart is a number. Read the number that matches the colour of your soil sample. This is the overall pH of your garden bed.
If the pH number is not close to the green or pale green areas in the pH chart above, you may have to put some dolomite or agricultural lime onto your garden bed to add calcium and raise the pH number, or mix some sulphur through the topsoil to lower the number.
Place the soil sample that you used for testing into a garbage bin, and put the rest of the soil from the bucket back onto your garden bed.
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THE pH SCALE (for senior students)
The method of measuring acidity or alkalinity of soil is called soil pH because it measures the amount of electrically charged particles called hydrogen ions in soil. The 'p' stands for potential and 'H' is the chemical symbol for hydrogen.
Hydrogen ions are measured because they can replace calcium ions on the electrically charged places on humus and clay particles in soil. Displaced calcium ions are washed down through soil away from plant roots during rain or watering, and soil becomes more acidic.
However, increasing amounts of hydrogen ions in soil are shown as progressively lower numbers on the pH scale because the scale is calculated using a complicated method called negative logarithms. As you learnt in the lesson above, an easier way to remember what the pH scale means is:
The more calcium in soil, the higher the pH number.
Using the method of negative logarithms for the pH scale also means that a pH of 6 is ten times more acidic than 7, and a pH of 5 is a hundred times more acidic than 7. Consequently, it will take a long time to raise a soil pH of 5 to a suitable pH for growing most vegetables in your garden without the help of humus in your garden soil.
To keep your garden healthy, it is advisable to check the pH of soil in garden beds at least once a year because plant roots and decaying organic matter release hydrogen ions into the soil, and soil pH will become more acidic over time.
In areas of Australia where water contains lots of calcium ions, soil pH can rise above a level that is good for plant growth if there is insufficient humus in soil. Soil in these areas will also need testing and adjustment when necessary.
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