Lawn and Garden

Printer friendly PDF Modifying the Acidity of Soil

How do I modify the acidity of my soil before planting?

Soil test recommendations for adjusting pH levels are based on the amount of sulfur needed to achieve the desired change in pH. Conversion factors for other acidifying materials can be seen in Table 2.

Soil-TABLE-2

For a small change in pH, any fertilizer containing ammonium (NH4) has the potential to lower pH. Microbes in the soil transform this nitrogen source into nitrate (NO3-). During this process, acid is generated and pH is lowered. This technique is best suited for long-term pH management and for making changes in the range of 0.2 – 0.3 units.

Larger differences between the current and target pH levels require more chemicals, more time, and are more prone to unwanted rapid drops in pH. For example, changing from a pH of 7.8 to 4.8 can be very difficult and time consuming. Small quantities of aluminum sulfate can be used to make moderate changes in pH, but they are ineffective for making large changes.

Additionally, large applications of aluminum sulfate can produce a toxic salt effect. A preferred choice when making large changes in pH is to add enough elemental sulfur to the soil when planting to partially change the pH. By incorporating most of the needed sulfur into the soil, and allowing time for the pH to change, you can get the pH close to the optimal level. Then retest and add a second, smaller surface application the following year to reach the target pH.

How do I modify the acidity of my soil if plants are already established?

soil-FIGURE-2 If you have established plants, you must be very careful when modifying pH. To lower pH after planting, you can use the same materials as you would use prior to planting. However, for the sulfur additives to have the desired effect, there must be soil-to-sulfur contact, adequate moisture, proper temperature and sufficient microbes. This requires the additives be mixed into the soil. Be careful; total incorporation of acid-generating materials risks root damage to shallow-rooted plants.

feeding bar One approach to more safely adjust soil pH post-planting is to create a feeding bar. Feeding bars act as a localized source from which the plant can supplement its needed nutrients, without the grower having to disrupt the majority of the plant’s roots. To make a feeding bar dig a 6 to 12 inch deep hole (or multiple holes for large plants) in the soil at about the drip line’s distance from the plant’s stem. Place sulfur mixed with soil into the hole and backfill with organic matter or top soil. In the remaining rooting area, add acidifying materials, incorporate them with great caution, and cover them with organic matter. Be very careful not to damage the roots of your acid-loving plants when incorporating the acid-generating materials into the soil surface. Most acid-loving plants will also benefit from an annual mulching with acidic organic materials.

For example, assume you have an existing stand of blueberries in loamy soil with a pH of 5.5. Since blueberries prefer a soil pH around 4.2, our needed change is 1.3 pH units. With a 100 ft2 bed, we will need to add between 2.4 and 3.5 pounds of sulfur to change the pH (from Table 1). In this scenario, add 2 pounds of sulfur to a freshly raked soil surface, carefully incorporate the materials into the soil, and cover the surface with pine bark mulch. Next dig a small post hole 6 to 12 inches deep at or near the drip line of the plant. Fill the hole with 0.5 pound of sulfur mixed with native soil, and top it with pine bark mulch. The following year, take a soil sample from inside the drip line, away from the feeding bar, and measure the pH. Fertilize the blueberries with ammonium-containing fertilizer, and add other soil-acidifying materials to the soil’s surface below the pine bark mulch, inside the drip line, as needed to reach the target pH. If the pH is very high, a second feeding bar may be needed.

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