Answer: Elemental sulfur is best. The effect is delayed because soil-dwelling bacteria need time to break down the added sulfur and lower the pH. The bacteria oxidize the sulfur; it then combines with water to form sulfuric acid, which acidifies the soil. Because this process relies upon active soil bacteria, sulfur is best applied in spring or summer, when the bacteria are most active. But you shouldn't have to delay planting -- mix lots of acidic peat moss in with the soil in the planting bed.
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