Your county's soil pH vs your yard's: how far apart can they be?
The short answer
A county figure tells you which direction your soil probably leans. It does not tell you your number. Across the 3,131 counties on this site, the gap between the most acidic and most alkaline mapped topsoil within a single county has a median of 2.7 pH units, and in 1,171 counties (37%) it is 3 units or more. pH is logarithmic, so 3 units is a thousand-fold difference in acidity.
How often a county is one soil, and how often it is a patchwork
Each county page shows the share of land in four bands: strongly acid (below 5.5), acid (5.5-6.5), neutral (6.5-7.3) and alkaline (above 7.3). When one band covers at least 60% of the county, the typical figure is a fair first guess. That is true in 1,925 counties. In the other 1,206 (39%), the county is a patchwork, and the page says so.
The most mixed counties
These counties have the widest spread of mapped topsoil pH:
- Yukon-Koyukuk Census Area, AK: pH 3.4 to 10, typical 5.9
- Manistee, MI: pH 2 to 8.1, typical 5.06
- Kern, CA: pH 3.9 to 9.8, typical 7.49
- Okanogan, WA: pH 4.4 to 10.1, typical 6.45
- Sublette, WY: pH 4.3 to 10, typical 6.97
Why your yard can differ even from the typical soil
The soil survey maps undisturbed soil. Houses are not built on undisturbed soil: grading strips topsoil and exposes subsoil, which is often more alkaline in dry regions and more acidic in wet ones; fill and bagged topsoil arrive from elsewhere; concrete foundations and paths leach lime into the soil beside them; and decades of lawn fertiliser slowly acidify. Any of these can put a yard a full pH unit or more from its county.
What to do with that
Use the county figure to decide what to test for and what not to panic about, then test. A lab test through your state's cooperative extension service usually costs less than one bag of the wrong amendment, and also reports buffer pH, which is what actually sets the lime rate. If your result and the county figure disagree, believe the test.