Soil pH

pH quantifies the concentration of hydrogen ions in the soil solution. Because the scale is logarithmic, each whole-number step represents a tenfold change in hydrogen-ion activity, so a soil at pH 5 is far more acidic than one at pH 6, not just marginally more so.

Soils naturally range from strongly acidic — common in high-rainfall regions where basic cations have leached away — to alkaline, common in arid regions or over calcareous parent material.

Most plant nutrients are most chemically available within a moderate pH range, though the ideal range differs by crop and soil type and is not a single universal number. Outside that range, specific nutrients can become locked up in insoluble forms or, conversely, can reach concentrations toxic to roots.

Standard soil tests measure pH in a slurry of soil mixed with water or a dilute salt solution such as calcium chloride, using a calibrated pH meter or, in simpler field kits, a colorimetric indicator. Because pH can vary across a field and with sampling depth, representative composite sampling is important for a reliable result.

Where soil testing indicates pH outside the range suited to a planned crop, growers commonly apply liming materials to raise pH or elemental sulfur and other acidifying products to lower it. Rates depend on the soil’s buffering capacity — closely tied to clay and organic matter content — as well as the target crop and local guidance.

• Confirm pH status with a laboratory soil test before amending
• Match liming or acidifying rates to soil buffering capacity, not just the current reading
• Recognize that nitrogen fertilizers containing ammonium can acidify soil gradually over time
• Retest periodically, since pH changes slowly and amendments take time to react through the profile