The pH scale measures how acidic or alkaline a substance is. It appears in chemistry classes, on cleaning labels, in cooking, and in discussions of soil quality — yet is frequently misunderstood.
What Is pH?
pH measures hydrogen ion (H+) concentration in a solution. Acids release hydrogen ions; bases accept them. More hydrogen ions means more acidic.
Scale: 0-14, where 7 is neutral (pure water)
Logarithmic: each unit = tenfold change in hydrogen ion concentration
Everyday Examples on the Scale
| Substance | Approx. pH |
|---|---|
| Battery acid | ~0 |
| Stomach acid | 1-2 |
| Lemon juice / vinegar | 2-3 |
| Coffee | ~5 |
| Pure water | 7 |
| Human blood | 7.35-7.45 |
| Seawater | ~8.1 |
| Hand soap | 9-10 |
| Bleach | 12-13 |
pH in Cooking
Adding acid (lemon juice, vinegar, buttermilk) to baked goods reacts with baking soda to produce carbon dioxide, causing rise. Red cabbage's anthocyanin pigments change colour with pH — staying red/pink in acidic conditions, turning blue/green in alkaline conditions.
pH in the Body
Blood pH is maintained within an extremely narrow range of 7.35-7.45 by the lungs and kidneys — even small deviations are medical emergencies. The stomach, by contrast, is highly acidic (pH 1-2) to denature proteins and activate digestive enzymes.
pH in Gardening
Soil pH determines which nutrients are actually available to plant roots and which microorganisms thrive in the surrounding soil. Most garden plants and vegetables grow best in soil that's slightly acidic to neutral, somewhere between pH 6 and 7, which is why this range is such a common reference point in gardening advice.
Acid-loving plants such as blueberries, rhododendrons, and azaleas prefer a notably more acidic range of pH 4.5 to 5.5, and will struggle or show signs of nutrient deficiency if planted in neutral or alkaline soil regardless of how well they're otherwise cared for. Highly acidic soils can be corrected by adding ground limestone, which gradually raises pH over a season or two, while overly alkaline soils can be acidified using sulfur or specific acidifying fertilisers. Testing soil pH with a simple kit before adding any amendments helps avoid the common mistake of overcorrecting in the opposite direction.
Most garden plants prefer slightly acidic soil between pH 6-7. Acid-loving plants like blueberries and rhododendrons prefer pH 4.5-5.5. Ground limestone raises soil pH; sulfur or acidifying fertilisers lower it.
Measuring pH at Home
Litmus paper provides the simplest home pH test, changing colour to indicate roughly whether a substance is acidic or alkaline, though it offers only limited precision compared to other methods. Universal indicator paper improves on this by providing a colour gradient that can be matched against a reference chart to estimate pH to within roughly half a point, sufficient for most household and gardening purposes without requiring specialist equipment.
For more precise readings — useful in brewing, aquarium keeping, or serious gardening — digital pH meters offer accuracy to within 0.01 pH units, though they require periodic calibration using buffer solutions of known pH to remain accurate over time. For most everyday purposes, such as checking garden soil or pool water, the simpler paper-based tests provide more than sufficient accuracy for the decisions you're actually likely to make based on the result.