The measure of acidity or alkalinity (on a 0-14 scale) affecting nutrient availability; most plants prefer slightly acidic soil (pH 6.0-7.0).
In plain terms
pH is acidity (low pH, 0-7) or alkalinity (high pH, 7-14), with 7.0 being neutral. Most nutrients are most available at pH 6.0-7.0 (slightly acidic). Too acidic (below 6.0) or too alkaline (above 7.5) locks up nutrients. Limestone raises pH; sulfur lowers pH. Soil pH determines which plants grow well; acidic soils suit blueberries, azaleas; neutral soils suit most crops.
Why this matters
pH affects nutrient availability. A nutrient-rich soil at wrong pH won't feed plants because nutrients are locked up. Understanding and testing pH prevents frustration.