Soil pH in Washington, OR: Lime or Sulfur?

Typical topsoil pH in Washington, OR

Typical topsoil in Washington, OR is pH 5.66, too acidic for vegetable garden. Expect roughly 5.6 lb of lime per 100 sq ft, but only after a soil test confirms it.

What Washington needs, by plant

• Already in range: potatoes, strawberries, azaleas and rhododendrons.
• Just outside, test first: blueberries (off by 0.2), blue hydrangeas (off by 0.2).
• Lime: lawn (cool-season grass) 5.6 lb/100 sq ft over 2 applications; vegetable garden 5.6 lb/100 sq ft over 2 applications; tomatoes 5.6 lb/100 sq ft over 2 applications; roses 5.6 lb/100 sq ft over 2 applications; pink hydrangeas 4.2 lb/100 sq ft.

What to do next

If your test confirms pH near 5.66, spread about 56 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 395,724 surveyed acres in Washington, OR, 75% are moderately acidic (5.5-6.5), 24% are strongly acidic (below 5.5). Mapped soils run from pH 4.6 to 6.7, a spread of 2.1 units across 285 soil components. 75% 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 21.5% clay and 4.96% 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.

Washington against the rest of Oregon

pH 5.66 is 0.64 below the Oregon median of 6.3. 10 of 36 Oregon counties have more acidic topsoil. Nationally it is more acidic than the US county median of 6.05.

Closest in pH within the state: Polk (5.52), Lane (5.53), Yamhill (5.55), Columbia (5.57), Clackamas (5.7), Linn (5.7), Multnomah (5.72), Douglas (5.73).

The range in Oregon runs from Clatsop at 4.89 to Malheur at 7.9; Washington sits 26% of the way up.