pH for the Garden OH 34
Leonard Perry, Extension Professor
What is soil pH?
Soil pH is the measure of the acidity (sourness) or alkalinity (sweetness) of a soil. A simple numerical scale is used to express pH. The scale goes from 0.0 To 14.0, with 0.0 being most acid, and 14.0 being most alkaline. The value, 7.0 is neutral--i.e., neither acid or alkaline.
Why is pH important?
Soil pH is important because it influences several soil factors affecting plant growth, such as (1) soil bacteria, (2) nutrient leaching, (3) nutrient availability, (4) toxic elements, and (5) soil structure. Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria operate best in the pH range of 5.5 to 7.0. Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5. Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5. Aluminum may become toxic to plant growth in certain soils with a pH below 5.0. The structure of the soil, especially of clay, is affected by pH. In the optimum pH range (5.5 to 7.0) clay soils are granular and are easily worked, whereas if the soil pH is either extremely acid or extremely alkaline, clays tend to become sticky and hard to cultivate.
A pH determination (soil test) will tell whether your soil will produce good plant growth or whether it will need to be treated to adjust the pH level. For most plants, the optimum pH range is from 5.5 to 7.0, but some plants will grow in more acid soil or may require a more alkaline level.
The pH is not an indication of fertility, but it does affect the availability of fertilizer nutrients. A soil may contain adequate nutrients yet growth may be limited by a very unfavorable pH. Likewise, builder's sand, which is virtually devoid of nutrients, may have an optimum pH for plant growth.
How to correct pH
Normally, lime or dolomite is used to increase the pH, or "sweeten" the soil. Lime contains mainly calcium carbonate and dolomite contains both calcium carbonate and magnesium carbonate. Ground limestone and dolomite are less likely to "burn" plant roots than hydrated lime and are therefore recommended for home use. The amount of these materials necessary to change the pH will depend on the soil. The greater the amount of organic matter or clay in a soil, the more lime or dolomite required to change the pH. Table 1 shows the amount of lime needed to raise the pH.
|Sandy loam soils||Silt loam soil||Silty-clay loam soils|
|To pH 6.0||To pH 6.5||To pH 6.0||To pH 6.5||To pH 6.0||To pH 6.5|
|From pH 6.0||None||2.5||None||4.0||None||5.5|
|From pH 5.5||2.5||4.5||4.0||8.5||5.5||11.5|
|From pH 5.0||4.5||7.0||8.5||12.5||11.5||17.0|
|From pH 4.8||5.5||8.0||9.5||14.0||14.0||19.5|
If a soil is tested as too alkaline, determine if this is due to recent application of lime or whether it is due to an inherent characteristic of the soil. It is quite difficult, if not impossible, to change appreciably the pH of naturally alkaline soil by use of acid-forming materials. If a high pH is due to applied lime or other alkaline additives, ammonium sulfate, sulfur, or similar acid-forming materials can be applied. Table 2 shows the amounts of sulfur needed to lower the pH.
|To pH 6.5||To pH 6.0||To pH 5.5||To pH 5.0|
|From pH 8.0||3.0||4.0||5.5||7.0|
|From pH 7.5||2.0||3.5||5.0||6.5|
|From pH 7.0||1.0||2.0||3.5||5.0|
|From pH 6.5||None||1.0||2.5||4.0|
Not more than 1 pound of sulfur per 100 square feet should be used in one application. Repeat applications of sulfur should not be made more often than once every 8 weeks. Sulfur oxidizes in the soil and mixes with water to form a strong acid that can burn the roots of plants and should be used with caution.
|Shade and flowering trees|
|Ash, European mtn.||6.0-7.0|
|Rose, hybrid tea||5.5-7.0|
|Viburnum, double file||6.5-7.5|
|Blueberry, high bush||4.5-5.5|
(Portions of this publication were adapted from J. Nesmith and E.W. McElwee, Florida Cooperative Extension Service Circular 352)
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Lawrence Forcier, Director, UVM Extension System, Burlington, Vermont. University of Vermont Extension System and U.S. Department of Agriculture cooperating, offer education and employment to everyone, without regard to race, color, national origin, sex, religion, age, disability, political beliefs, and marital or familial status.
Last reviewed 2003