One number from 0 to 14 tells you which side a liquid is on.
Below 7 is acidic, 7 is neutral and above 7 is basic, or alkaline. The scale is logarithmic, so each whole step means a tenfold change. That small detail explains rain, garden soil and the slowly changing sea.
Pick the range your home test kit shows. The bar shows how much the RHS suggests you may need to change the soil for most garden plants.
Based on Royal Horticultural Society advice on soil pH. Test results within three months of adding lime, fertiliser or organic matter can mislead.
TWO NUMBERS WORTH KNOWING
Small numbers, big changes.
One figure for ordinary rain, one for how far the ocean has moved.
≈ 5.6 pH
pH of ordinary, unpolluted rain
Rain is slightly acidic because carbon dioxide dissolves into it to form weak carbonic acid. The EPA says acid rain usually has a pH between 4.2 and 4.4.
NORMAL RAIN · US EPA
≈ 30%
Rise in surface ocean acidity since the industrial revolution
NOAA says surface ocean pH has fallen by about 0.1 units. Because the scale is logarithmic, that small drop is roughly a 30 percent increase in acidity.
THE OCEAN · NOAA
NOAA's Pacific Marine Environmental Laboratory gives the fall as 0.11 pH units, an exact change in hydrogen ion concentration of 28.8 percent. Seawater is still basic; acidification describes the direction of change.
FOUR THINGS TO TRY
Put the scale to work.
You do not need a lab. A garden test kit, a splash of vinegar and a careful look at cleaning labels cover most everyday pH questions.
Test the soil before you plant
The RHS suggests checking pH before planting a new bed, fruit or vegetables, or when leaves are yellowing.
Try the vinegar fizz
Add vinegar to a soil sample. If it fizzes, the RHS says free chalk or limestone is present.
Match plants to the pH
Rhododendrons, camellias, heathers and blueberries want acid soil. On chalk, choose plants that like alkaline ground.
Never mix bleach with other cleaners
UKHSA warns that bleach mixed with acids such as vinegar gives off chlorine gas, and with ammonia gives off irritating chloramines.
AI-generated image, not a photograph of a real person or place.
WHY RAIN IS ACIDIC
Acid from the air, not just from factories.
AI-generated image, not a photograph of a real person or place.
Even clean rain is slightly acidic, at about pH 5.6, because carbon dioxide in the air dissolves into raindrops as weak carbonic acid.
Acid rain is something more: the EPA says sulphur dioxide and nitrogen oxides, mostly from burning fossil fuels, react in the air to form sulphuric and nitric acids. Two thirds of the sulphur dioxide in the US atmosphere comes from electric power generators.
Questions
Questions people ask
What does pH actually measure?
The US EPA describes pH as an expression of hydrogen ion concentration in water, on a scale of 0 to 14. As hydrogen ions increase, the solution becomes more acidic and the pH number falls.
Is basic the same as alkaline?
Nearly, in everyday speech. The EPA notes that basicity is tied directly to pH, while alkalinity is a separate measure of how much acid a solution can neutralise.
Why is a small change in pH a big deal?
The scale is logarithmic, so one whole unit is a tenfold change in hydrogen ions. NOAA says a fall of about 0.1 in ocean pH means roughly 30 percent more acidity.
What pH should garden soil be?
The RHS says a slightly acid pH of 6.5 suits the widest range of plants, except lime-hating ones such as rhododendrons and heathers, which prefer pH 5.1 to 6.0.
Why should bleach not be mixed with vinegar?
UKHSA says mixing sodium hypochlorite, the active ingredient in household bleach, with acidic products produces chlorine gas, which irritates the eyes and throat. Use one product at a time.
