Page 51 - Agriculture_Form_Three

Agriculture for Secondary Schools

(iv) Put the soil into the cup or container.
(v) Add some distilled water to the soil to turn it into a soil paste (not too wet nor

too dry; it should have some consistency).
(vi) To test acidity; add about 120 ml of baking soda to the already prepared soil
paste and stir lightly (about 1 - 2 minutes). The ratio of soil to baking soda
should be 1:2. If, after stirring, the soil gives off fizzing sounds and forms

bubbles or foams, then that soil is acidic.

(vii) To test alkalinity; add about 120 ml of vinegar to the already prepared soil
paste and stir slightly (about 1 - 2 minutes). The ratio of soil to vinegar should
be 1:2. If, after stirring, the soil gives off a fizzing sound and forms bubbles
and or foams, then that soil is alkaline.

Note: If the soil does not give off fizzing sound, bubbles or foams with vinegar or
baking soda, then that soil is neutral.

Colour cards method: This method uses universal pH indicators. The universal pH
indicators are chemical compounds that change in colour when mixed with either an
acidic or an alkaline solution. In estimating soil pH, these indicators are added to a
soil solution and, depending on the concentration of hydrogen or hydroxide ions, the
colour of the soil solution will change accordingly. The resultant colour change is
then compared with the already known and established pH colour cards. These cards
have a range of colours that represent the pH scale (refer to Table 2.2).

Table 2.2: pH values and their corresponding colours
pH value Colour

< 3 Red
3 - 4 Reddish orange

5 Orange
6 Yellow

7 Yellow green
8 Green
9 Blue green

≥ 10 Blue

Matching the colour obtained from the soil solution to that of the colour cards
provides an estimation of the pH of that soil.

Student’s Book Form Three
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