Natural Ways to Make Soil Acidic for Blue Hydrangeas

Sulfur

• The fastest way to change the soil pH is to mix sulfur with it. Over two to three years, bacteria in the soil oxidize the sulfur, creating sulfuric acid. Mix the sulfur with the soil when planting and sprinkle it on the soil at the base of the plant each spring. Using aluminum sulfate is not recommended; it can harm plant roots and has been found in waterways.

Sphagnum Peat Moss

• Another amendment that can be mixed with the soil at planting time is sphagnum peat moss. Soak the peat moss in a bucket to thoroughly wet it before mixing it half and half with the soil in the planting hole. Sphagnum peat is harvested from renewable bogs in Canada and the northern United States.

Fertilizer

• Fertilizers with low phosphorus (the middle number in NPK listings on fertilizer packages) and high potassium (the last number) can help, too. Urea is considered an organic material and fertilizers containing it also will acidify soil. Wood ashes, kelp meal and greensand are other sources of potassium.

Mulch

• Mulching around the base of the plant with sawdust, wood chips, pine needles or compost will help acidify the soil over time as the mulch decomposes. As soil microbes consume the material, they release its nutrients for plant roots, causing flower color to change. Reapply fresh mulch each year.