Soil pH in Fairfax, VA: Lime or Sulfur?

Typical topsoil pH in Fairfax, VA

Typical topsoil in Fairfax, VA is pH 5.4, too acidic for vegetable garden. Expect roughly 7.7 lb of lime per 100 sq ft, but only after a soil test confirms it.

What Fairfax needs, by plant

• Already in range: potatoes, blueberries, azaleas and rhododendrons, blue hydrangeas.
• Just outside, test first: strawberries (off by 0.1).
• Lime: lawn (cool-season grass) 7.7 lb/100 sq ft over 2 applications; vegetable garden 7.7 lb/100 sq ft over 2 applications; tomatoes 7.7 lb/100 sq ft over 2 applications; roses 7.7 lb/100 sq ft over 2 applications; pink hydrangeas 5.6 lb/100 sq ft over 2 applications.

What to do next

If your test confirms pH near 5.4, spread about 77 lb of ground limestone per 1,000 sq ft of vegetable bed in 2 doses, ideally in fall.

How mixed the soil is

Of the 187,001 surveyed acres in Fairfax, VA, 73% are strongly acidic (below 5.5), 16% are alkaline (above 7.3), 9% are moderately acidic (5.5-6.5), 2% are near neutral (6.5-7.3). Mapped soils run from pH 4.3 to 7.5, a spread of 3.2 units across 330 soil components. 73% of the land falls in one class, so the county figure is a fair first guess for most yards here.

Clay, organic matter and rates

Topsoil here averages 18.1% clay and 1.46% organic matter, which this site treats as loam. Moving it one pH unit takes about 7 lb of lime or 2.3 lb of sulfur per 100 sq ft. None of the mapped components carry free carbonate.

Fairfax against the rest of Virginia

pH 5.4 is 0.16 above the Virginia median of 5.24. 95 of 127 Virginia counties have more acidic topsoil. Nationally it is more acidic than the US county median of 6.05.

Closest in pH within the state: Russell (5.37), Powhatan (5.38), Dickenson (5.39), Rockbridge (5.39), Colonial Heights City (5.4), Washington (5.41), Frederick (5.42), Grayson (5.42).

The range in Virginia runs from Newport News City at 4.76 to Poquoson City at 6.36; Fairfax sits 40% of the way up.