Calcareous soil: why sulfur will not lower your pH
What calcareous means
Calcareous soil contains free calcium carbonate, essentially crushed limestone, mixed through it. Sulfur lowers pH by turning into acid, and that acid reacts first with any carbonate in the soil. Until the carbonate is gone, pH barely moves. Neutralising 1% free carbonate in the top 6 inches takes tens of pounds of sulfur per 100 square feet, far more than any label rate, and many of these soils carry several percent.
How common it is
SoilPhCheck flags a county as calcareous when at least 30% of its mapped topsoil contains carbonate, or its typical pH is 7.8 or higher, which almost always means carbonate is present. That is 527 of 3,131 counties (17%). On those pages, the advice for acid-loving plants is "use beds or pots", not a sulfur rate.
Where it is concentrated
States with the highest share of calcareous counties:
- Arizona: 13 of 14 counties
- Utah: 25 of 29 counties
- New Mexico: 27 of 33 counties
- Wyoming: 18 of 23 counties
- Nevada: 12 of 17 counties
- Texas: 159 of 254 counties
- North Dakota: 31 of 53 counties
- Montana: 31 of 56 counties
The counties with the most carbonate in their topsoil:
- Reagan, TX: 100% of topsoil carbonate-bearing, typical pH 8.17
- Concho, TX: 100% of topsoil carbonate-bearing, typical pH 8.11
- Terrell, TX: 100% of topsoil carbonate-bearing, typical pH 8.09
- Piute, UT: 100% of topsoil carbonate-bearing, typical pH 8.08
- Schleicher, TX: 100% of topsoil carbonate-bearing, typical pH 7.86
How to check your own soil
Put a spoonful of dry soil in a dish and add a few drops of white vinegar. Fizzing means free carbonate. A lab test will report it more reliably, often as "free lime" or "calcium carbonate equivalent".
What works instead
For blueberries, azaleas and blue hydrangeas on calcareous ground, grow them in raised beds or large containers filled with an acidic mix (peat or pine bark based), and water with rainwater where you can, since hard tap water in these regions carries more carbonate. For everything else, the good news is that most vegetables, lawns and roses tolerate pH up to about 7.5, so the rates guide often lands on "leave it".