Raising Soil pH and Soil Acidification

Gardens & Landscapes August 06, 2008|Print

Adapted from Carl J. Rosen, Peter M. Bierman, and Roger D. Eliason. Department of Soil, Water, and Climate. University of Minnesota.[1]


Ground limestone is the most common material used to raise soil pH. Limestone consists either of calcium carbonate (calcitic limestone) or calcium/magnesium carbonate (dolomitic limestone). On soils low in magnesium, dolomitic limestone is the preferred form.

Apply limestone only if a need is indicated by the results of soil testing and the requirements of the plants being grown. Overliming can reduce nutrient availability, especially of micronutrients such as iron, manganese, and zinc. Iron deficiency (chlorosis) of pin oak, for example, is common when soil pH is greater than 7.0.


Contents

Mineral Soils

Limestone recommendations for mineral soils are based on two factors:

  1. Soil pH measurement
  2. Determination of a buffer pH or buffer index value


The soil’s buffering capacity represents its ability to resist changes such as pH modification. The amount of clay and/or organic matter in a soil will determine its buffering capacity. The more clay and/or organic matter a soil has, the greater its buffering capacity and the more limestone or sulfur will be needed to effect a pH change. There are several commonly used methods of measuring a soil's buffering capacity. Some commonly used methods include the Mehlich buffer test, SMP buffer test, and Woodruff buffer test. While the techniques of these test are different, they are used to interpret a soil's buffering capacity in similar ways.

The soil pH value reported in a soil test indicates whether limestone is needed, but it cannot be used to determine how much lime is needed. The amount of limestone to apply depends upon reserve soil acidity, which is measured by a buffer pH test. The lower the buffer pH or buffer index, the larger is the quantity of limestone required to raise soil pH to the desired level. Find a local laboratory, using the following hyperlink, to provide a calibrated buffer index that is suitable for determining liming rates for your local soils.

  • Soil Testing Laboratories

Most liming materials take several months to react with the soil, so for best results they should be applied and incorporated to a depth of 6 inches well before planting time. For established lawns or turfgrass areas, perennial vegetables and flowers, small fruits, fruit trees, and ornamental trees and shrubs, topdress applications of limestone are used. These surface applications may take longer to change the soil pH than direct soil incorporation prior to planting. Also, when large quantities of limestone need to be applied to established planting sites, they are made at rates of 5 to 7 pounds of ground limestone per 100 square feet spaced at least at one-month intervals. The best time to apply limestone to established turf is following core aeration for thatch control or compaction. Spread the limestone uniformly around other types of established plants. If possible, work it lightly into the soil, while avoiding the disturbance of plant roots.

Soil Acidification

Neutral to Slightly Acid Soils

For most plants, soil acidification is unnecessary, but lowering soil pH is frequently required to grow blueberries, 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 the soil pH will migrate towards values typical of native soils. In some areas of the country it will tend to increase, especially if the water supply used for irrigation is alkaline, as is the case in municipal water supplies in some parts of the country. In other areas, such as the northeast, many soils are naturally acidic and would seldom require sulfur additions.

Table 1. Image courtesy of University of Minnesota Extension.
Table 1. Image courtesy of University of Minnesota Extension.


Image courtesy of University of Minnesota Extension.
Image courtesy of University of Minnesota Extension.

Elemental sulfur is one amendment that can be used to lower soil pH. The soil texture, amount of soil organic matter, present pH, and the desired pH are all used to determine the amount of elemental sulfur needed. Table 1 provides guidelines to lower the pH of a soil with elemental sulfur by one unit on an area basis and volume basis. Table 2 provides guidelines for elemental sulfur use to lower soil pH to 4.5. 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 6 to 9 months after the initial application. If soil pH is not in the desired range, reapply according to the recommendations in Tables 1 or 2. Most homeowners prefer to use granulated elemental sulfur.

Aluminum sulfate or iron sulfate can also be used to acidify soils. These materials react much faster than elemental sulfur, usually within three to four weeks following application. Multiply the rate of elemental sulfur recommended in Table 1 by 6 to determine the rate of iron sulfate or aluminum sulfate required for an equivalent pH reduction. Do not apply more than 9 lb/100 sq. ft. of iron sulfate or aluminum sulfate in a single application. If higher rates are required, split applications are recommended to avoid excessive levels of soluble salts. Use iron sulfate or aluminum sulfate cautiously because of the potential for iron or aluminum toxicity to plant roots and soil organisms.

Acid sphagnum peat incorporated into the soil prior to planting will provide a more favorable root environment for the establishment of acid-loving plants in high-pH soils. Incorporate acid sphagnum peat moss at the rate of 1 to 2 cubic feet per plant, or when possible incorporate an inch or two of acid sphagnum peat moss into the top 6 inches of soil before planting. According to one source,[2] an application of 2 inches of sphagnum peat moss incorporated into the top 6 inches of soil will lower the soil pH by one unit or more within a growing season. The positive effects of sphagnum peat can last two or more 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 will increase soil pH and should not be used for acid loving plants.

Alkaline Soils

Fine-textured clay and clay loam soils and high lime soils with a pH greater than about 7.3 require higher rates of acidifying amendments than given in Table 1. 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.

Credits and Literature Cited

  1. ? Rosen, C. et al. 2004. Soil pH Modification. Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants. http://www.extension.umn.edu/distribution/horticulture/components/1731-03.html (accessed January 15, 2008). University of Minnesota.
  2. ? Rose, M. and H. Wang. 1996. Composts in the Landscape: Effects on Soil pH and Plant Growth.
    http://ohioline.osu.edu/sc157/sc157_14.html (accessed February 15, 2008). The Ohio State University, Ohio Agricultural Research and Development Center. Special Circular 152. Ornamental Plants: Annual Reports and Research Summaries. pp. 7-11.

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