Soil pH is measured on a scale from 0 to 14. A value of 7 is neutral, lower numbers are acidic, and higher numbers are alkaline. Each whole-number step represents a tenfold change, so pH 5 is much more acidic than pH 6.
pH affects nutrient availability and the activity of soil organisms, but plants do not all prefer one “neutral” range. Blueberries favour strongly acidic soil; many vegetables grow well around pH 6 to 7; other plants tolerate alkaline conditions.
Simple colour-strip kits can be an observation activity, but ordinary red-and-blue litmus paper does not give a useful garden reading. A laboratory soil test provides a calibrated pH result and amendment advice based on the soil and crop.
Do not add lime to “sweeten” soil or sulfur to acidify it until a test shows a need. These materials react slowly, the amount depends on texture, and too much is difficult to undo. An adult should apply any recommended amendment exactly as directed.