Soil pH in Clackamas, OR: Lime or Sulfur?

Typical topsoil pH in Clackamas, OR

Typical topsoil in Clackamas, OR is pH 5.7, too acidic for vegetable garden. Expect roughly 9.2 lb of lime per 100 sq ft, but only after a soil test confirms it.

What Clackamas 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) 9.2 lb/100 sq ft over 2 applications; vegetable garden 9.2 lb/100 sq ft over 2 applications; tomatoes 9.2 lb/100 sq ft over 2 applications; roses 9.2 lb/100 sq ft over 2 applications; pink hydrangeas 5.8 lb/100 sq ft over 2 applications.

What to do next

If your test confirms pH near 5.7, spread about 92 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 546,486 surveyed acres in Clackamas, OR, 91% are moderately acidic (5.5-6.5), 9% are strongly acidic (below 5.5). Mapped soils run from pH 4.6 to 6.5, a spread of 1.9 units across 291 soil components. 91% 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 5.22% organic matter, which this site treats as clay. At 5.22% organic matter the soil buffers like a heavier one, which pushes it into the clay bracket. Moving it one pH unit takes about 11.5 lb of lime or 3.4 lb of sulfur per 100 sq ft. None of the mapped components carry free carbonate.

Clackamas against the rest of Oregon

pH 5.7 is 0.6 below the Oregon median of 6.3. 11 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: Lane (5.53), Yamhill (5.55), Columbia (5.57), Washington (5.66), Linn (5.7), Multnomah (5.72), Douglas (5.73), Marion (5.75).

The range in Oregon runs from Clatsop at 4.89 to Malheur at 7.9; Clackamas sits 27% of the way up.