How to Modify Soil pH

Sometimes you’ll find yourself in a situation where slightly more acidic or basic soil conditions would be ideal. This can be done quite easily by changing the pH.

To increase the pH reading of light sandy loams one full point (e.g., from 5.5 to 6.5) apply 35 lbs. of ground limestone (agricultural limestone) to 1,000 sq. ft.; on medium loam, 50 lbs.; and on heavy clay loam, 70 lbs.

To reduce the pH of a light sandy loam one full point (e.g., from 6.0 to 5.0) apply 10 lbs. of dusting sulfur to 1,000 sq. ft.; on medium loam, 15 lbs. will be needed, and on heavy clay loam, 20 lbs.

A word of caution is needed on a common recommendation – the use of aluminum sulfate for acidifying soils. This chemical is widely used in greenhouses for acidity for azaleas. However, as a pot plant, an azalea is only intended to live through the flowering period for which it was forced and then can be discarded. The permanent effect of aluminum on its roots means nothing. In garden soils, however, aluminum is a bad actor if a pH of 5.5 or lower is to be maintained, damaging roots and stunting plants. Ammonium sulfate can be used instead. Although not nearly as effective pound for pound as dusting sulfur, it has the added advantage of supplying nitrogen in ammonium form which acid-loving plants can use. Use double the amount recommended for sulfur, but be sure to dissolve ammonium sulfate in water for applying.