Soil pH

Soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions (H
₃O⁺
). It range from 0 to 14, with 0 being most acidic, 14 being highly basic, and 7 being neutral. Soil pH affects plant nutrient availability by controlling the chemical form of the nutrient. The preferred soil pH varies between plants and can be modified via various soil amendments.

Global variation in soil pH. Red = acidic soil. Yellow = neutral soil. Blue = alkaline soil. Black = no data.

Classification of soil pH ranges

The United States Department of Agriculture Natural Resources Conservation Service, formerly Soil Conservation Service classifies soil pH ranges as follows:

Denomination	pH range
Ultra acid	<3.5
Extremely acid	3.5 - 4.4
Very strongly acid	4.5 - 5.0
Strongly acid	5.1 - 5.5
Moderately acid	5.6 -6.0
Slightly acid	6.1 -6.5
Neutral	6.6 - 7.3
Slightly alkaline	7.4 - 7.8
Moderately alkaline	7.9 - 8.4
Strongly alkaline	8.5 -9.0
Very strongly alkaline	>9.0

Nutrient availability in relation to soil pH

Nutrients needed in the largest amount by plants are referred to as macro-nutrients and include nitrogen (N), phosphorus (P), and potassium (K). In addition to carbon (C), N and P comprise the largest fraction of plant tissue. C is assimilated as carbon dioxide (CO
₂) from air, but supplying N, P and K in sufficient amounts is necessary for a thriving garden. In addition to macronutrients, plants also need trace nutrients. Trace nutrients are not major components of plant tissue, but, for example, make up key components of vitamins. Both macro and trace nutrient availability is controlled by soil pH. Organic gardening requires no fertilizer amendments but relies on both macro and trace nutrients being supplied by degradation of plant material.

Nutrient availability in relation to soil pH

N is supplied as ammonium (NH
₄) or nitrate (NO
₃) in fertilizer amendments, and dissolved N will have the highest concentrations in soil with pH 6-8. Concentrations of available N are less sensitive to pH than concentration of available P. In order for P to be available for plants, soil pH needs to be in the range 6.0 and 7.5. If pH is lower than 6, P starts forming insoluble compounds with iron (Fe) and aluminium (Al) and if pH is higher than 7.5 P starts forming insoluble compounds with calcium (Ca).

Determining pH

Methods of determining pH include:

• Observation of soil profile. Strongly acidic soils often have poor incorporation of the organic surface layer with the underlying mineral layer. The mineral horizons are distinctively layered in many cases, with a pale eluvial (E) horizon beneath the organic surface; this E is underlain by a darker B horizon in a classic podzol horizon sequence. This is a very rough gauge of acidity as there is no correlation between thickness of the E and soil pH. E horizons a few feet thick in Florida usually have pH just above 5 (merely "strongly acid") while E horizons a few inches thick in New England are "extremely acid" with pH readings of 4.5 or below.[1][2] [3] In the southern Blue Ridge Mountains there are "ultra acid" soils, pH below 3.5, which have no E horizon.[4]
• Observation of predominant flora. Calcifuge plants (those that prefer an acidic soil) include Erica, Rhododendron and nearly all other Ericaceae species, many birch (Betula), foxglove (Digitalis) , gorse (Ulex spp.), and Scots Pine (Pinus sylvestris). Calcicole (lime loving) plants include ash trees (Fraxinus spp.), honeysuckle (Lonicera), Buddleja, dogwoods (Cornus spp.), lilac (Syringa) and Clematis species.
• Use of an inexpensive pH testing kit based on barium gay sulphate in powdered form, where in a small sample of soil is mixed with water which changes colour according to the acidity/alkalinity.
• Use of litmus paper. A small sample of soil is mixed with distilled water, into which a strip of litmus paper is inserted. If the soil is acidic the paper turns red, if alkaline, blue.
• Use of a commercially available electronic pH meter, in which a rod is inserted into moistened soil and measures the concentration of hydrogen ions.

Examples of plant pH preferences

• pH 4.5-5.0 Blueberry, Bilberry, Heather, Cranberry, Orchid, Azalea, for blue Hydrangea (less acidic for pink) , Sweet Gum, Pin Oak.
• pH 5.0 - 5.5 Parsley, Potato, Heather, Conifers, Pine, Sweet Potato, Maize, Millet, Oars, Tye, Radish, Ferns, Iris, Orchids, Rhododendron, Camellia, Daphne and Boronia.
• pH 5.5 - 6.0 Bean, Brussels Sprouts, Carrot, Choko, Endive, Kohl Rabi, Peanuts, Rhubarb, Soybean, Crimson Clover, Aster, Begonia, Canna, Daffodil, Jonquil, Larkspur, Petunia, Primrose, Violet and most bulbs.
• pH 6.0 - 6.5 Broccoli, Cabbage, Cauliflower, Cucumber, Egg Plant, Pea, Sweet Corn, Pumpkin, Squash, Tomato, Turnip, Red Clover, Sweet Clover, White Clover, Candytuft, Gladiolus, Iceland Poppy, Pansy, Rose, Snapdragon, Viola, Wallflower, Zinnea and Strawberry.
• pH 6.5 - 7.0 Asparagus, Beet, Celery, Lettuce, Melons, Onion, Parsnip, Spinach, Lucerne, Carnation, Chrysanthemum, Dahlia, Stock, Sweet Pea and Tulip.
• pH 7.1 - 8.0 Lilac, brassica

Changing soil pH

Increasing pH of acidic soil

Acidic soil has low pH. The most common amendment to increase soil pH is lime, usually in the form of finely ground agricultural lime. The particle size is important, since smaller particles will dissolve quicker and therefore change pH quicker. The amount of lime needed to change pH a certain amount is usually listed on the package. However, if you have a lot of organic matter in the soil you may need more lime than if you have less organic matter.

Decreasing pH of basic soil

• Iron sulphates or aluminium sulphate.
• Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulphate and monopotassium phosphate
• Decayed vegetable matter, compost, stable manure

See also

• Alkali soils
• Fertilizer
• Liming (soil)
• Organic gardening
• Soil conservation

External links

• "A Study of Lime Potential, R.C. Turner, Research Branch, Canadian Department of Agriculture, 1965"