Educational Checkpoint #2 Soil pH

Welcome everyone to Educational Checkpoint #2 of the 2021 Soil your Undies challenge.

This time we are testing our soil pH, but what is pH?

This week Discovery’s resident soil expert Imogen is taking a look at the pH of our soil. pH stands for the Power of Hydrogen. In simple terms, acids are molecules that contain a lot of hydrogen atoms that can detach to react with other molecules. The opposite of acids are alkalines, also called bases, which are molecules able to take on extra hydrogen. pH is measured on the pH scale from 1 to 14; 1 is strongly acidic, 7 is neutral, and 14 is strongly alkaline.

Some examples of common acids and alkalines might be:

  • Citric acid – found in citrus fruits
  • Acetic acid – the acid found in vinegar
  • Battery Acids – Batteries often use a strong acid to create the chemical reaction that produces electricity
  • Sodium Bicarbonate – Baking soda is weakly alkaline
  • Sodium Hydroxide – the strong Alkaline in drain cleaner capable of dissolving organic matter such as food scraps, hair, and mould

What does this have to do with soil?

Soil pH affects our soil health in several different ways. In just the same way that you don’t want to get strong acid or alkalines on your skin, neither do soil microbes and plant roots. Highly acidic and highly alkaline environments can be very hostile to microorganisms.

The nutrient availability is also affected by pH. Certain chemical elements are more readily dissolved in environments of a particular pH, and are bound up and inaccessible in other pH ranges. The most nutrients are available in the neutral range, around 7. Some major nutrients such as nitrogen, phosphorus, and sulphur are less accessible to plants in acidic soils. Iron, manganese, copper, and zinc are considered trace elements which are still essential for plant growth, but these are inaccessible in highly alkaline soils.


Doing the pH test

The pH kit contains a universal indicator dye which will change colour to reflect the pH of the soil. Because the colour of your soil is likely to be quite dark, the colour of the dye will be difficult to see. Barium sulphate is a white powder with a neutral pH, and by adding a small amount of this to the test, it can soak up the indicator and reveal the colour. Please be cautious when using these chemicals. Universal indicator is not toxic but is likely to stain skin and clothing. Barium Sulphate is also non toxic, but can be a mild irritant if inhaled. Appropriate precautions should be taken when using these chemicals. For detailed step by step instructions, see the attached information sheets.


My soil pH is really low/high! What can I do?

Hopefully your soil pH is somewhere between 6-8, however if you discover it is very low or high there are several ways we can help to improve things. If you have an Alkaline soil with a high pH, you can lower the pH using compost rich in organic matter, or with fertilisers such as ammonium nitrate. If your soil pH is very low and acidic, the pH can be increased using fertilisers such as lime (calcium carbonate), dolomite (magnesium carbonate), or potash (potassium chloride or oxide). If you need to add these kinds of fertilisers and supplements to your soil, be sure to check the pH regularly to make sure that enough and not too much was added.


What if my class is locked down?

As we see in our video, the soil pH test does not need to be done outside in the garden. If you are running your classes virtually, you may wish to demonstrate the pH test for your students on camera. You might have them predict what the pH value will be or have them suggest which colour they can see in the completed test. You will have been given enough materials to complete a few separate tests, so you may wish to save some and redo the test when the students are able to be back in the classroom.

For the participation prize photos, this time we are only asking for a photo of the completed pH test. Please remember to save your photos and submit them together at the end of the challenge.


See you again soon with educational checkpoint #3

Download - Parent/Teacher Information Sheet
Download - Student Activity Sheet