Soil pH in Washington, IA: Lime or Sulfur?

Typical topsoil pH in Washington, IA

Typical topsoil in Washington, IA is pH 6.31, 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, tomatoes, strawberries, roses, pink hydrangeas.
• Sulfur: potatoes 1.8 lb/100 sq ft; blueberries 3 lb/100 sq ft over 2 applications; azaleas and rhododendrons 2.5 lb/100 sq ft over 2 applications; blue hydrangeas 2.5 lb/100 sq ft over 2 applications.

What to do next

If your test lands near 6.31, you can skip lime and sulfur for lawn (cool-season grass), vegetable garden, tomatoes, strawberries, roses, pink hydrangeas.

How mixed the soil is

Of the 361,853 surveyed acres in Washington, IA, 55% are moderately acidic (5.5-6.5), 45% are near neutral (6.5-7.3). Mapped soils run from pH 4.2 to 7, a spread of 2.8 units across 336 soil components. No single class covers 60% of the county, so two gardens a few miles apart here can need opposite treatments. Test your own bed.

Clay, organic matter and rates

Topsoil here averages 26.7% clay and 2.97% 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 Iowa

pH 6.31 is 0.05 below the Iowa median of 6.36. 33 of 99 Iowa counties have more acidic topsoil. Nationally it is more alkaline than the US county median of 6.05.

Closest in pH within the state: Clarke (6.3), Cedar (6.31), Johnson (6.31), Mahaska (6.31), Muscatine (6.33), Scott (6.33), Tama (6.33), Carroll (6.34).

The range in Iowa runs from Van Buren at 6.02 to Monona at 7.11; Washington sits 27% of the way up.