Soil pH in Boulder, CO: Lime or Sulfur?

Typical topsoil pH in Boulder, CO

Typical topsoil in Boulder, CO is pH 6.75, already right for vegetable garden (6-7). Test before adding anything.

What Boulder needs, by plant

• Already in range: lawn (cool-season grass), vegetable garden, tomatoes, roses.
• Sulfur: potatoes 3 lb/100 sq ft over 2 applications; strawberries 1.8 lb/100 sq ft; blueberries 4.1 lb/100 sq ft over 3 applications; azaleas and rhododendrons 3.5 lb/100 sq ft over 2 applications; blue hydrangeas 3.5 lb/100 sq ft over 2 applications; pink hydrangeas 1.2 lb/100 sq ft.

What to do next

If your test lands near 6.75, you can skip lime and sulfur for lawn (cool-season grass), vegetable garden, tomatoes, roses.

How mixed the soil is

Of the 355,501 surveyed acres in Boulder, CO, 55% are near neutral (6.5-7.3), 26% are moderately acidic (5.5-6.5), 15% are alkaline (above 7.3), 4% are strongly acidic (below 5.5). Mapped soils run from pH 4.7 to 8.5, a spread of 3.8 units across 258 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 17.8% clay and 1.69% 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. 17% of mapped components carry free carbonate (lime already in the ground).

Boulder against the rest of Colorado

pH 6.75 is 0.42 below the Colorado median of 7.17. 12 of 63 Colorado counties have more acidic topsoil. Nationally it is more alkaline than the US county median of 6.05.

Closest in pH within the state: San Miguel (6.54), Gunnison (6.6), Larimer (6.6), Jefferson (6.72), Custer (6.75), Douglas (6.81), El Paso (6.81), Archuleta (6.83).

The range in Colorado runs from San Juan at 5.53 to Alamosa at 8.28; Boulder sits 44% of the way up.