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Agriculture for Secondary Schools
Colour cards method is advantageous as it is quick and could be used to estimate the
pH range in the field or laboratory and is relatively easy. It is however challenging
as it only provides soil pH by estimation and may be a limitation if the exact pH
value is needed. Moreover, the universal solution can be difficult to find and it is
expensive. Also, there can be many factors that interfere with the pH estimation
such as high soil organic matter content and flooded soils.
Materials needed
Test tube, measuring scoop (or a teaspoon), universal pH indicator solution, pH
colour card, distilled water, air-dried soil, and barium sulphate
Procedures
(i) Place a soil sample in a test tube to about 1 cm level.
(ii) Add an equal amount of barium sulphate.
(iii) Fill the test tube with distilled water about 4 cm from the top and then shake it.
(iv) Allow the content to settle.
(v) Pour 2 - 3 drops of universal indicator solution into the soil.
(vi) Shake the mixture and leave it to settle for 30 minutes. When the indicator is
fully mixed with the soil, it will give off a certain colour.
(vii) Match the colour obtained with the pH colour card.
(viii) The resultant match is an estimation of the soil’s pH.
Alternatively, litmus paper can be used instead of colour cards where the following
materials and procedures will be used.
Materials
Air-dried soil sample, litmus paper, distilled water, 50 ml bottle, and pH colour chart
Procedures
(i) Place a half-full teaspoon of soil into the 50 ml bottle.
(ii) Add about 20 ml of water to the soil (make sure the soil does not become too
wet).
(iii) Bring into contact the litmus paper with the soil solution. The litmus paper will
give off a colour.
(iv) Match the colour from the litmus paper with the pH colour chart.
(v) The resultant match is the estimation of that soil’s pH.
Student’
Student’s Book Form Twos Book Form Three
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