Soil can become acidic due to a number of causes. Rain water dissolves carbon dioxide from atmosphere forming carbonic acid. Organic decay from plants and animal results n formation of acetic, citric, oxalic and carbonic acids. When excess H+ ion accumulate in the soil colloid, the soil become acidic. To cure acidity of soil, liming is usually carried out. Calsium carbonate (limestone), calsium oxside (quicklime) or calsium hydrokside (slaked lime) may be used. The Ca2+ ions added can replaced the H+ ions and thus raise the pH of soil. However, the exact quantity of lime to be added depends on the type of soil

Science Project Example : Soil pH

Alkaline soils are usually found in dry, arid areas. Water evaporates from surface, leaving behind a layer of basic salts. One way to remedy the situation is to add sulphur. Sulphur is readily oxidized by soil bacteria to sulphuric acid which helps to make the soil less alkaline. There is no optimum pH for plant growth. Certain plants grow well in slightly acidic conditions while others prefer a more alkaline condition. Thus, farmers must always check the pH of the soil before growing any crops


Calsium sulphate (or barium sulphate) powder

Universal indicator solutions

Garden soil

Green beans


Calcium Hydroxide


1. Air dry small sample of garden soil. When it is dry, crumble it to fine powder

2. Pour the soil into a test tube to a depth of 1 cm

3. Add 1 cm depth of calsium sulphate (or barium sulphate) powder

4. Add distilled water until the tube is half full

5. Stopper the tube and shake vigorously for 30 seconds

6. Allow the soil particles to settle (the calcium sulphate or barium sulphate powder will help the soil particles to flocculate and settle

7. Add 10 drops of universal indicator solution and shake for 10 seconds

8. When the soil particles have settled, compare the colour of he solution with the standart colour chart. (Don’t expect perfect match, choose the colour nearest to it

9. Half fill 4 paper cups with same garden soil. Label one as control. To second cup, add 0,5 g of sulphur powder. Mix it into the top layer of soil. Label this cup

10.To the third cup, add 0,5 g of calcium hydroxide. Again, stir it carefully into the top soil. Label the cup

11. To the final cup. add 2 g of calsium hydroxide, stir and labeled the cup

12. Leave the cup for 2 weeks, lightly watering the occasionally. Test the pH of the soil using steps 1 to 8

13. Place 10 green beans into each paper cup and observe their growth for one week

      a. what is the average height of the plants in each cup

      b. Which plants have the biggest leaves

      c. What is the best pH range for growing green beans

Further Investigations

1. Investigate the effect of soil pH on the following  balsam, cactus, ferns, hisbiscus, morning glory, maize, sweet potato, white wheat, carrot, onons, potato, tomato, etc. What pH ranges would you reccommend for growing each of the above

2. Collect soil from various sources and the test their pH

    a.Which plants would grow the best in the soil samples you tested

    b. How much lime or sulphur should be used for each type of soil to raise or lower the pH by 2           units

3. Is the effect the same if calcium carbonate used? how much should be used to raised or lower the pH by 2 units

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