Lowering pH with Sulfur

Soil pH is made lower—more acid, or less alkaline—by adding sulfur. The sulfur may be added in different forms. Farmers use sulfuric acid, which has a quick reaction. Home gardeners usually use soil sulfur (powdered elemental sulfur), ferrous sulfate, or aluminum sulfate. All three are transformed into sulfuric acid by soil bacteria. Soil sulfur is less expensive and used in smaller quantities than the other two.

Ferrous sulfate adds iron as well as sulfur to the soil, and aluminum sulfate adds aluminum, but neither of these have appreciable effect on the soil or plants growing in it. Most soils already contain large amounts of both iron and aluminum. To much aluminum is toxic to plants, but aluminum is only toxic in very acid soils.

Soil bacteria take 6 to 8 weeks to change the sulfur to sulfuric acid. Wait that long before re-testing the soil. As with all bacterial activity, the reaction goes faster in warm, moist soil and slower in dry or cold soils.

Soil can also be made more acid by adding large amounts of peat moss, which is very acid (pH of 3.0 to 4.5). Peat moss is quite expensive, so this isn’t practical for large-scale use, but is suitable for making acid potting mixes and to backfill around acid-loving plants like azaleas and rhododendrons. Mix the peat moss with an equal amount of soil for planting azaleas and rhododendrons.

If your soil contains free lime, it will not be possible to lower the pH. Even large amounts of sulfur will have only a local, temporary effect. Soil with free lime is usually at a pH of 8.2, the pH of limestone. A soil laboratory can tell you if your soil contains free lime and advise you what to do.

Areas with alkaline soil often have alkaline irrigation water, too. If so, the soil will become more alkaline over time. You will probably need to repeat the sulfur application every few years. Use acid-reaction fertilizers to slow down the alkalizing effect of the water. Acid-reaction fertilizers are those whose nitrogen source is ammonium or urea.

Soil Sulfur per 100 sq. ft. to Lower pH to 6.5

Starting pH Sandy Soil Loam Clay Soil
8.5 4.6 5.7 6.9
8.0 2.8 3.4 4.6
7.5 1.1 1.8 2.3
7.0 0.2 0.4 0.7