Soil pH in Clay, IA: Lime or Sulfur?

Typical topsoil pH in Clay, IA

Typical topsoil in Clay, IA is pH 6.63, already right for vegetable garden (6-7). Test before adding anything.

What Clay needs, by plant

• Already in range: lawn (cool-season grass), vegetable garden, tomatoes, roses.
• Just outside, test first: strawberries (off by 0.1), pink hydrangeas (off by 0.1).
• Sulfur: potatoes 3.7 lb/100 sq ft over 2 applications; blueberries 5.4 lb/100 sq ft over 3 applications; azaleas and rhododendrons 4.8 lb/100 sq ft over 3 applications; blue hydrangeas 4.8 lb/100 sq ft over 3 applications.

What to do next

If your test lands near 6.63, 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 360,261 surveyed acres in Clay, IA, 63% are near neutral (6.5-7.3), 29% are moderately acidic (5.5-6.5), 8% are alkaline (above 7.3). Mapped soils run from pH 5.7 to 8.2, a spread of 2.5 units across 289 soil components. 63% 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 29.8% clay and 5.13% organic matter, which this site treats as clay. At 5.13% 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. 20% of mapped components carry free carbonate (lime already in the ground).

Clay against the rest of Iowa

pH 6.63 is 0.27 above the Iowa median of 6.36. 82 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: Cerro Gordo (6.59), Story (6.59), Worth (6.6), Dickinson (6.62), Greene (6.63), Calhoun (6.64), Webster (6.65), Palo Alto (6.7).

The range in Iowa runs from Van Buren at 6.02 to Monona at 7.11; Clay sits 56% of the way up.