Trees for Acid Soils

18 Jan 2006

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

~~~~~~~~~~~~~

Putting the right plant in the right place is one of the golden rules of horticulture. Many problems are avoided by following this simple rule. What makes a plant and a place right is multi-faceted but one important consideration is the pH.

Acid soils are all soils that test lower than pH 7.0, which includes most soils east of the Mississippi River. Soils with a pH below 6.0 may need special treatment or careful plant selection. When soils become very acid, however, with a pH below 4.5, problems develop for many common landscape trees and shrubs. These strongly acid soils need either an amendment to raise the pH or more careful plant selection.

pH measures the available hydrogen ions in a solution, and describes the relative level of acidity or alkalinity measured on a scale of 1 (acid) to 14 (basic), with 7.0 as neutral. Vinegar is a mild acid; battery acid (sulfuric acid) is a strong acid. Baking soda is a mild base when mixed with water; lye (Drano) is a strong base. Pure water is neither basic nor acidic, and has a pH of 7.0.

The pH scale is logarithmic, with 14 gradations. Each increment of 1.0 actually represents a difference of ten times either more basic (alkaline) or acidic (i.e., 6.0 is ten times more acid than 7.0, 5.0 is a hundred times more acid than 7.0, and 4.0 is a thousand times more acid than 7.0). Acid soils are often called "sour" soils and basic soils are often called "sweet" soils.

Soil nutrients are available to plants for uptake and use only when they are dissolved in water as ions. Nutrient availability differs depending on the pH of the soil solution. A soil pH range of 6.0 to 7.0 provides the greatest compromise of nutrient availability for plants. Moving away from the pH 6.0 to 7.0 range makes some nutrients more available, while decreasing the availability of others. A decreasing or more acid pH not only makes some nutrients less available, but it may also cause others, such as aluminum and manganese, to become available in toxic concentrations.

Acid soils may be due to natural conditions or human disturbance of the soil. The same methods for adjusting acidity are recommended for either situation. The primary difference is that naturally acidic soil will eventually return to that state after pH adjustment, while pH adjustments to disturbed soils present a more long term change. Naturally acid soils occur when bedrock from which the soil evolved provides the natural elements for acidity, when rainfall leaches or washes away basic elements in the soil, or when land has been naturally forested. All of these situations are present in Virginia soils.

Virginia soil pH ranges from 4.0 to 8.0 with most in the 5.1 to 5.5 range. Virginia rainfall is typically high, averaging 40 to 45 inches per year. Over time this rainfall has carried away water-soluble basic elements (i.e., calcium, magnesium, potassium, sodium), thus resulting in acid soils. High rainfall has also supported Virginia forests. In contrast, low rainfall tends to produce natural grasslands with minimal leaching and thus alkaline (basic) soils.

Human-made acidic soils occur as a result of mining operations, farming, construction site development, and similar events that remove topsoil and expose the naturally acidic subsoil, or that deplete the soil of basic-forming elements. What is left is usually lacking in organic matter and available nutrients. While pH recovery can occur naturally, it will be slow, as will be plant reestablishment.

Landscape fertilization practices can also affect soil acidity. Repeated use of high ammonium or urea fertilizers, especially to turf areas that cover tree roots, can compound acidity problems. In situations where the soil is too acid, it may be better to use more basic fertilizers, many of which are nitrate based. Water used for landscape irrigation can also contribute to acidity problems and, therefore, it is valuable to have both your soil and your water tested before taking corrective measures.

Adding ground limestone, either calcium carbonate or calcium-magnesium carbonate (dolomitic lime) raises the pH of acid soils. Quicklime and slaked lime are also used and act faster, but are more expensive and disagreeable to use. The amount of limestone needed to change the pH of the top 8 inches of soil from 4.5 to a desired 6.5 varies according to soil type and ranges from approximately 150 lbs/1000 ft2 for a sandy soil to 250-300 lbs/1000 ft2 for a clay loam. The rate needed will be provided as part of the soil test results from a soil-testing lab, if the soil type and the intended use of the land are listed when the soil sample is submitted.

Ideally, lime should be worked into the soil to a depth of at least 8 inches. This is possible when preparing a new site with bare ground. Lime incorporation can be combined with deep tilling and the addition of organic matter (compost/manures) and fertilizer to revitalize a site that has been stripped or over-farmed. Lime incorporated into the soil will increase the pH to the target level (i.e., 6.5) in about two years.

Raising the pH of an established landscape, however, is much harder because incorporating lime into the soil may damage existing plant roots. If the total quantity of lime recommended exceeds 50-lbs/1000 ft2, split surface applications into a series of several smaller applications over time. Unfortunately, surface applications take longer to significantly change the pH below a 4-inch depth. Adding lime to the surface without incorporating it is not a desirable method for correcting acidity deep in the soil.

Many trees tolerant of acid conditions (including strongly acid conditions below pH 4.5) are available for Virginia landscapes. These plants may often be a better solution than attempting to adjust the soil to a higher pH, particularly where the native soil condition is by nature acidic. Here is a list of trees to consider if your soil test shows you have acidic soil. Note that most trees native to Virginia are adapted to at least slightly acid soils. An asterisk (*) indicates a species native to Virginia.

pH 5.0 to 6.5 and above

Red maple*	Acer rubrum
Downy serviceberry*	Amelanchier arborea
Fringetree*	Chioanthus virginicus
Franklinia*	Franklinia alatamaha
Hollies*	Ilex species
American sweetgum*	Liquidambar styraciflua
Magnolias*	Magnolia species
Crabapples	Malus species
Norway spruce	Picea abies
Eastern white pine*	Pinus strobus
Scotch pine	Pinus sylvestris
White oak*	Quercus alba
Sassafras*	Sassafras albidum
Japanese snowbell	Styrax japonicus

pH 5.0 and below

River birch*	Betula nigra
Flowering dogwood*	Cornus florida
Japanese dogwood	Cornus kousa
Japanese cedar	Cryptomeria japonica
American beech*	Fagus grandifolia
Carolina silverbell*	Halesia tetraptera
Black gum*	Nyssa sylvatica
Sourwood*	Oxydendrum arboreum
Loblolly pine*	Pinus taeda
Virginia pine*	Pinus virginiana
Golden larch	Pseudolarix amabilis
Willow oak*	Quercus phellos

~~~~~~~~~~~~~

For more information about these and other landscape gardening topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.