About SoilPhCheck
Why this exists
Most advice about soil pH starts with "it depends on your soil", which is true and not much help to someone standing in a garden centre deciding between a bag of lime and a bag of sulfur. SoilPhCheck answers the first half of that question with real data: the typical pH of the topsoil in your county, as mapped by the USDA's own soil scientists, so you know which direction you are likely to need to go before you buy anything.
Where the numbers come from
Every county figure comes from the USDA Natural Resources Conservation Service soil survey (SSURGO), queried through its public Soil Data Access service. For each county, SoilPhCheck takes the pH (1:1 water) of the top mineral horizon of every mapped soil component, leaves out forest litter layers that would make wooded counties read falsely acidic, and weights each component by how many acres of the county it covers. Clay, organic matter and carbonate figures come from the same records. The rates for lime and sulfur, and the reasoning behind each verdict, are set out in the rates guide.
A county is not a yard
The single most important limit: these are county figures. In 1,206 of the 3,131 counties (39%), no single pH band covers even 60% of the land, and the county vs yard guide shows how far apart soils within one county can be. New-build yards are often topsoil trucked in from elsewhere. That is why every page ends with the same instruction: test your own soil before you add anything.
Corrections
If a county's figure looks wrong, please say so through the contact page. Confirmed errors are fixed in the underlying data, which corrects every page that depends on it at once.
Who runs this
SoilPhCheck is independently run. It is not affiliated with the USDA, any extension service, or any fertiliser or garden-supply brand, and nobody pays to influence a verdict.