Soil pH in Kauai, HI: Lime or Sulfur?

Typical topsoil pH in Kauai, HI

Typical topsoil in Kauai, HI is pH 5.26, too acidic for vegetable garden. Expect roughly 13.8 lb of lime per 100 sq ft, but only after a soil test confirms it.

What Kauai needs, by plant

• Already in range: potatoes, blueberries, azaleas and rhododendrons, blue hydrangeas.
• Just outside, test first: strawberries (off by 0.2).
• Lime: lawn (cool-season grass) 13.8 lb/100 sq ft over 3 applications; vegetable garden 13.8 lb/100 sq ft over 3 applications; tomatoes 13.8 lb/100 sq ft over 3 applications; roses 13.8 lb/100 sq ft over 3 applications; pink hydrangeas 11.5 lb/100 sq ft over 3 applications.

What to do next

If your test confirms pH near 5.26, spread about 138 lb of ground limestone per 1,000 sq ft of vegetable bed in 3 doses, ideally in fall.

How mixed the soil is

Of the 294,142 surveyed acres in Kauai, HI, 55% are strongly acidic (below 5.5), 31% are moderately acidic (5.5-6.5), 11% are near neutral (6.5-7.3), 3% are alkaline (above 7.3). Mapped soils run from pH 3.2 to 7.9, a spread of 4.7 units across 137 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 39.5% clay and 10.63% organic matter, which this site treats as clay. At 10.63% 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. 3% of mapped components carry free carbonate (lime already in the ground).

Kauai against the rest of Hawaii

pH 5.26 is 0.57 below the Hawaii median of 5.83. 0 of 5 Hawaii counties have more acidic topsoil. Nationally it is more acidic than the US county median of 6.05.

Closest in pH within the state: Honolulu (5.81), Maui (5.83), Hawaii (6.01), Kalawao (6.6).