Soil pH in Washington, ID: Lime or Sulfur?

Typical topsoil pH in Washington, ID

Typical topsoil in Washington, ID is pH 6.95, already right for vegetable garden (6-7). Test before adding anything.

What Washington needs, by plant

• Already in range: lawn (cool-season grass), vegetable garden, roses.
• Just outside, test first: tomatoes (off by 0.2).
• Sulfur: potatoes 3.5 lb/100 sq ft over 2 applications; strawberries 2.3 lb/100 sq ft over 2 applications; blueberries 4.6 lb/100 sq ft over 3 applications; azaleas and rhododendrons 3.9 lb/100 sq ft over 2 applications; blue hydrangeas 3.9 lb/100 sq ft over 2 applications; pink hydrangeas 1.6 lb/100 sq ft.

What to do next

If your test lands near 6.95, you can skip lime and sulfur for lawn (cool-season grass), vegetable garden, tomatoes, roses.

How mixed the soil is

Of the 680,052 surveyed acres in Washington, ID, 90% are near neutral (6.5-7.3), 7% are alkaline (above 7.3), 3% are moderately acidic (5.5-6.5). Mapped soils run from pH 5.6 to 8.2, a spread of 2.6 units across 201 soil components. 90% 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 23.6% clay and 2.59% 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. 3% of mapped components carry free carbonate (lime already in the ground).

Washington against the rest of Idaho

pH 6.95 is 0.06 below the Idaho median of 7.01. 19 of 44 Idaho counties have more acidic topsoil. Nationally it is more alkaline than the US county median of 6.05.

Closest in pH within the state: Nez Perce (6.7), Teton (6.87), Blaine (6.91), Bannock (6.93), Gem (6.99), Lincoln (7.01), Owyhee (7.02), Gooding (7.04).

The range in Idaho runs from Valley at 6.04 to Jefferson at 8.06; Washington sits 45% of the way up.