Acidifying soil with elemental sulfur and/or acidifying fertilizers

Alkaline Soils

Fine-textured clay and clay loam soils, and high lime soils with a pH greater than about 7.3, require high rates of acidifying amendments. These soils are not recommended for growing acid-loving plants, because the amendment rates required to lower soil pH initially result in excessive levels of soluble salts and the soil pH will eventually increase. The best strategy on these soils is to select plants that are adapted to high pH conditions.

Neutral to Slightly Acid Soils

For most plants soil acidification is unnecessary, but lowering soil pH is frequently required to grow blueberries, boxwood, pin oak, heaths, heathers, azaleas, and rhododendrons successfully. These plants require an acid soil with a pH between 4.5 and 5.2. If soil pH is above this range, it is usually desirable to lower soil pH to 4.5 prior to planting. Over time pH will tend to increase, especially if the water supply used for irrigation is alkaline as it is in many municipal water supplies.

Elemental sulfur is one amendment that can be used to lower soil pH. The soil type, present pH, and the desired pH are used to determine the amount of elemental sulfur needed. Table 2 provides guidelines to lower the pH of a soil with elemental sulfur by one unit on an area basis and volume basis. Because it reacts slowly with the soil, elemental sulfur should be applied and incorporated to a depth of 6 inches the year before planting for best results. Test soil pH again 3 to 4 months after the initial application. If soil pH is not in the desired range, reapply according to the soil test recommendations.

Iron sulfate can also be used to acidify soils. This material reacts much faster than elemental sulfur, usually within 3 to 4 weeks following application. Do not apply more than 9 lb./100 sq. ft. of iron sulfate in a single application. If higher rates are required, split applications in 1 to 2 month intervals and water frequently to avoid excessive levels of soluble salts.

Aluminum sulfate will also lower pH, but except for blue hydrangea, it is not recommended as a soil acidifying amendment because of the potential for aluminum toxicity to plant roots.

Acid sphagnum peat incorporated into the soil prior to planting will provide a favorable root environment for the establishment of acid-loving plants in high pH soils. Incorporate peat (1 part peat to 1 part soil) at the rate of 1 to 2 cubic feet per plant. The positive effects of acid peat can last 6 to 10 years, but unless other measures are used the pH of the soil will eventually increase.

Ammonium sulfate or urea used as nitrogen fertilizer sources will also help maintain low soil pH. However, do not use them at rates greater than those required to meet the nitrogen needs of the plants. Fertilizers that contain nitrogen only in the nitrate form, such as sodium nitrate, will increase soil pH and should not be used for acid loving plants.

Alkaline Soils