Soil pH in San Juan, WA: Lime or Sulfur?

Typical topsoil pH in San Juan, WA

Typical topsoil in San Juan, WA is pH 5.56, too acidic for vegetable garden. Expect roughly 6.3 lb of lime per 100 sq ft, but only after a soil test confirms it.

What San Juan needs, by plant

• Already in range: potatoes, strawberries, azaleas and rhododendrons.
• Just outside, test first: blueberries (off by 0.1), blue hydrangeas (off by 0.1).
• Lime: lawn (cool-season grass) 6.3 lb/100 sq ft over 2 applications; vegetable garden 6.3 lb/100 sq ft over 2 applications; tomatoes 6.3 lb/100 sq ft over 2 applications; roses 6.3 lb/100 sq ft over 2 applications; pink hydrangeas 4.9 lb/100 sq ft.

What to do next

If your test confirms pH near 5.56, spread about 63 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 0 surveyed acres in San Juan, WA, 65% are moderately acidic (5.5-6.5), 34% are strongly acidic (below 5.5), 1% are near neutral (6.5-7.3). Mapped soils run from pH 4.7 to 7.3, a spread of 2.6 units across 126 soil components. 65% 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 9.9% clay and 9.09% organic matter, which this site treats as loam. At 9.09% organic matter the soil buffers like a heavier one, which pushes it into the loam bracket. 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.

San Juan against the rest of Washington

pH 5.56 is 0.85 below the Washington median of 6.41. 11 of 39 Washington counties have more acidic topsoil. Nationally it is more acidic than the US county median of 6.05.

Closest in pH within the state: Whatcom (5.41), King (5.45), Pierce (5.48), Kitsap (5.49), Skagit (5.58), Clark (5.62), Island (5.67), Snohomish (5.68).

The range in Washington runs from Grays Harbor at 4.93 to Adams at 7.45; San Juan sits 25% of the way up.