Soil pH

Soil pH is a measurement of the acidity or alkalinity of a soil.  On the pH scale, 7.0 is neutral. Below 7.0 acid, and above 7.0is basic or alkaline.  A pH range of 6.8 to 7.2 is termed near neutral.  Areas of the world with limited rainfall typically have alkaline soils while areas with higher rainfall typically have acid soils.

 Soils with a pH above 7.5 generally have a high calcium carbonate content, known as free lime.  In some mountain soils and older gardens that have been highly irrigated and cultivated for many years the pH may be in the neutral range or slightly acid.

Many gardening books list the preferred pH for common plants (generally 6.0 to 7.2).  For most plants, however, what is preferred and what is tolerated are not related.  Most garden and landscape plants tolerate a pH up to 7.5 to 7.8 with little problem.  The exception is acid-loving plants, like blueberries, azaleas, and rhododendrons that need acid soil.  Blue hydrangeas also require a pH lower than 5.0 to induce the blue flower color.  [Figure 1]

Figure 1. pH Ranges and influence on plant growth potential - 7.0 is neutral. Above 7.0 is alkaline Below 7.0 is acid.

pH and Nutrient Availability

Soil pH is an important chemical property because it affects the availability of nutrients to plants and the activity of soil microorganisms.  The influence of pH on nutrient availability is illustrated in the Figure 2.  Iron chlorosis is common in Colorado due to alkaline soil pH.  Phosphorus will become less available in highly alkaline soils.  Zinc deficiencies are occasionally observed in sensitive field crops, like corn and beans. [Figure 2]

Figure 2.  Availability of nutrients based on soil pH

Managing Alkaline Soils

In Colorado soils with moderate to high alkalinity (pH above 7.5), manage the soil by giving extra attention to increasing the organic matter, using organic mulches, and light frequent irrigation.  Plants are less tolerant of dry soil conditions when the pH is high.

Soils with a pH above 7.3 and/or with free lime cannot be adequately amended for acid-loving plants like blueberries, azaleas, and rhododendrons.

In near-neutral pH soils rich with organic matter and without free-lime, gardeners may find a slight decrease in soil pH over many decades.  This occurs as irrigation leaches out some naturally occurring elements (calcium and magnesium) contributing to the higher pH.  The growth of plants that secrete weak acids into the soil may also contribute to a gradual pH change.

Lowering the pH

Textbooks talk of sulfur applications to lower a soil’s pH.  This is effective in many parts of the country.  However it is not effective in many Colorado soils due to high levels of “free lime” (calcium carbonate) found in the soils. 

To test for free lime, place a heaping tablespoon of crumbled dry soil in a cup.  Moisten it with vinegar.  If the soil-vinegar mix bubbles, the soil has free lime.  On soils with free lime, a gardener will not effectively lower the pH. 

On soils without free lime, the following products may help lower the pH.

• Elemental sulfur is one chemical that can be used to lower soil pH.  The soil type, existing pH, and the desired pH are used to determine the amount of elemental sulfur needed, (see Table 1).  Incorporate sulfur to a depth of six inches.  It may take several months to over a year to react with the soil, lowering the pH.  Test soil pH again 3 to 4 months after initial application.  If the soil pH is not in the desired range, reapply.

Table 1. Pounds of Sulfur Needed to Lower Soil pH 1
Material	pH Change	Pounds per 100 Square Feet 2
Sulfur	7.5 to 6.5 8.0 to 6.5 8.5 to 6.5	1.5 3.5 4.0
Iron sulfate	7.5 to 6.5 8.0 to 6.5 8.5 to 6.5	12.5 29.0 33.2
1 Effective only on soils without free lime, do the vinegar test! 2 Higher rates will be required on fine-textured clayey soils and soils with a pH of 7.3 and above

• Iron sulfate can also be used to acidify soils.  This material reacts much faster than elemental sulfur, usually within three to four weeks following application.  Do not apply more than nine pounds per 100 square feet in a single application.  If higher rates are required, split applications to avoid excessive levels of soluble salts.  (See Table 2)


• Aluminum sulfate will also lower pH, but 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 help provide a favorable rooting environment for the establishment of acid-loving plants in near neutral soils.  Incorporate peat at the rate of one to two cubic feet per plant.  The positive effects of acid peat will last a few years, but unless other measures are used, the pH of the soil will eventually increase.  The pH will be driven up with the high calcium in our irrigation water.  Soil with a pH above 7.3 and/or with free lime cannot be adequately amended for acid-loving plants.


• Fertilizers – Use of ammonium sulfate or urea as nitrogen fertilizer sources will also have a small effect on lowering soil pH in soils without free lime.  However, do not use these fertilizers at rates greater than those required to meet the nitrogen needs of the plants.  For example, ammonium sulfate fertilizer, 21-0-0, at ten pounds per 1000 square feet (maximum rate for crop application) may lower the pH from 7.3 to 7.2.
  
  Fertilizers that contain nitrogen in the nitrate form will have a slight effect to increase the pH.

Raising the pH on Acid Soil

On acid soils, the pH can be raised by adding lime (calcium carbonate). The amount to add depends on the cation exchange capacity (nutrient-holding capacity) of the soil, which is based on the soil’s clay content. Soil higher in clay will have a higher cation exchange capacity and will require more materials to raise the pH.

A laboratory test called buffer index measures the responsiveness of the soil to lime applications. The soil test will give recommendations on application rates based on the buffer index rather than just the pH. Table 3 gives an estimated amount of lime to apply to raise a soil’s pH.

Table 3. Lime Application Rates to Raise Soil pH to Approximately 7.0 for Turf
Existing Soil pH	Lime Application Rate (pounds per 1,000 square feet)
	Sandy	Loamy	Clayey
5.5 to 6.0 5.0 to 5.5 3.4 to 5.0 3.5 to 4.5	20 30 40 50	25 40 55 70	35 50 80 80
Lime application rates shown in this table are for dolomite, ground, and pelletized limestone and assume a soil organic matter level of approximately 2% or less. On soils with 4 to 5% organic matter, increase limestone application rates by 20%. Individual applications to turf should not exceed 50 pounds of limestone per 1,000 square feet. Avoid the use of hydrated or burned lime because it is hazardous to both humans and turf (can seriously burn skin and leaves). If hydrated lime is used, crease application rates in the above table by 50% and apply no more than 10 pounds of hydrated or burned line per 1000 square feet of turf.

Lime is commonly sold as ground agricultural limestone. It varies in how fine it has been ground. The finer the grind, the more rapidly it becomes effective in lowering pH. Calcitic lime mostly contains calcium carbonate (CaC03). Dolomitic lime contains both calcium carbonate and dolomite [MgCa(CO3)2]. On most soils, both are generally satisfactory. However, on sandy soils low in organic matter, dolomitic lime may supplement low magnesium levels.