What is soil pH?
The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is alkaline (basic). Each step is ten times: soil at pH 5 has ten times more acid strength than soil at pH 6.
Soil becomes acidic when rain washes away calcium, when some fertilisers break down, and when plant matter rots. It becomes alkaline when it has a lot of limestone or salts, often in dry areas.
How to measure soil pH
You test the pH of the water in the soil.
- Sample: take soil from several spots, 5 to 10 cm deep, and mix it. Remove stones and roots.
- Dry it in shade and crush it.
- Mix 1 part soil with about 5 parts distilled (or clean rain) water. Stir for a minute and leave it for 30 minutes.
- Test the clear water with universal indicator paper or solution (match to the colour chart), or with a pH meter.
- Record the place, date and pH. Repeat three times and take the average.
Use distilled water because tap water can have its own pH and change the result.
How pH changes plant growth
Roots take in nutrients dissolved in soil water. The pH decides which nutrients dissolve and which stay locked.
- Very acidic soil (below 5): calcium, magnesium and phosphorus are short, and aluminium dissolves and harms roots. Leaves turn yellow and plants stay small.
- Neutral soil (6 to 7.5): most nutrients are available. Most crops, such as wheat, rice (slightly acidic), pulses and vegetables, do best here.
- Alkaline soil (above 8): iron, zinc and manganese get locked, so young leaves turn pale yellow with green veins.
Some plants are exceptions: tea and blueberry like acid soil (about 4.5 to 5.5), lucerne and cabbage like it near neutral to slightly alkaline.
Treating the soil
Acidic soil: add lime (crushed limestone, calcium carbonate, or slaked lime, calcium hydroxide). These bases neutralise the acid: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. Add a little at a time and retest after a few weeks.
Alkaline soil: add sulfur (soil bacteria change it to sulfuric acid slowly), compost or other organic matter, and acid-forming fertiliser such as ammonium sulfate.
Do not overdo it. Changing pH is slow, and too much lime can make the soil too alkaline.
Plan a fair test and share results
A fair test changes only one thing, the soil pH. Keep the plant type, seed number, water, light, pot size and temperature the same. Use at least three pots for each pH and measure height every two days for two weeks.
Present your result as a table and a bar graph of pH against average height. State your claim (for example: bean seedlings grew best near pH 6.5), give your data as evidence, and say what could be wrong (few seeds, uneven light). End with advice, for example how a farmer could treat soil at pH 4.5.
Key formulas and definitions
- pH < 7 acidic, pH = 7 neutral, pH > 7 alkaline
- One pH step = 10 times change in acid strength
- Acidic soil + lime (a base) → pH rises
- CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
- Alkaline soil + sulfur, compost or ammonium sulfate → pH falls
- Fair test: change one thing, keep the others the same
Worked examples
1. A soil test gives an orange-red colour that matches pH 4. Is the soil acidic, neutral or alkaline? What would you add?
pH 4 is below 7, so the soil is acidic. Add lime, a base, to neutralise the acid. Add small amounts and retest.
2. Soil A has pH 5 and soil B has pH 7. How many times more acidic is soil A?
Each pH step is a 10-fold change. Two steps give 10 × 10 = 100. Soil A has 100 times more acid strength than soil B.
3. Why do we use distilled water to test soil pH?
Tap water may be slightly acidic or alkaline itself, which would change the reading. Distilled water is neutral, so the colour change comes only from the soil.
4. Bean seedlings grew to these average heights: pH 4: 5 cm, pH 5: 9 cm, pH 6: 14 cm, pH 7: 13 cm, pH 8: 8 cm. Which pH is best? Suggest a pH range for a farmer.
The tallest average is 14 cm at pH 6, with pH 7 close behind at 13 cm. So the best pH is about 6 to 7. A farmer should keep the bean field in this range.
5. Write a word equation for lime (calcium carbonate) neutralising soil acid (hydrochloric acid used as the model).
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. Bubbles of carbon dioxide are seen.
6. A farmer's tea plants have yellow leaves in soil of pH 7.8. The farmer thinks lime will help. Is the farmer right? What would you suggest?
No. Tea likes acid soil (about 4.5 to 5.5) and pH 7.8 is alkaline. Lime would raise the pH even more. Add sulfur or compost to lower the pH slowly, and retest.
Common mistakes
- Testing dry soil with indicator paper. The paper must touch the clear water from the soil mix.
- Using tap water for the soil mix. Use distilled or clean rain water.
- Adding lime to every soil. Only acidic soil needs it; alkaline soil would get worse.
- Changing more than one thing in the experiment (different seeds, different light) and then blaming pH.