Soil pH
The soil pH is a measure of the acidity or basicity in soils. pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 is defined as the negative logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

 (base 10) of the activity
Activity (chemistry)
In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...

 of hydrogen ion
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....

s in solution. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH range for most plants is between 6 and 7.5, however many plants have adapted to thrive at pH values outside this range.

Classification of soil pH ranges

The United States Department of Agriculture Natural Resources Conservation Service
Natural Resources Conservation Service
The Natural Resources Conservation Service , formerly known as the Soil Conservation Service , is an agency of the United States Department of Agriculture that provides technical assistance to farmers and other private landowners and managers.Its name was changed in 1994 during the Presidency of...

, formerly Soil Conservation Service classifies soil pH ranges as follows:

Denomination pH range
Ultra acid <3.5
Extreme acid 3.5 - 4.4
Very strong acid 4.5 - 5.0
Strong acid 5.1 - 5.5
Moderate acid 5.6 -6.0
Slight 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

Soil Acidity

Acidity in soils comes from H+ and Al3+ ions in the soil solution and sorbed to soil surfaces. While pH is the measure of H+ in solution, Al3+ is important in acid soils because between pH 4 and 6, Al3+ reacts with water (H2O) forming AlOH2+, and Al(OH)2+, releasing extra H+ ions. Every Al3+ ion can create 3 H+ ions.
Many other processes contribute to the formation of acid soils including rainfall, fertilizer use, plant root activity and the weathering of primary and secondary soil minerals. Acid soils can also be caused by pollutants such as acid rain and mine spoilings.
  • Rainfall: Acid soils are most often found in areas of high rainfall. Excess rainfall leaches base cation from the soil, increasing the percentage of Al3+ and H+ relative to other cations. Additionally, rainwater has a slightly acidic pH of 5.7 due to a reaction with CO2 in the atmosphere that forms carbonic acid.
  • Fertilizer use: Ammonium (NH4+) fertilizers react in the soil in a process called nitrification
    Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. Degradation of ammonia to nitrite is usually the rate limiting step of nitrification. Nitrification is an important step in the nitrogen cycle in soil...

     to form nitrate (NO3-), and in the process release H+ ions.
  • Plant root activity: Plants take up nutrients in the form of ions (NO3-, NH4+, Ca2+, H2PO4-, etc.), and often, they take up more cations than anions. However plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release H+ ions from the root. Some plants will also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe).
  • Weathering
    Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

     of minerals: Both primary and secondary minerals that compose soil contain Al. As these minerals weather, some components such as Mg, Ca, and K, are taken up by plants, others such as Si are leached from the soil, but due to chemical properties, Fe and Al remain in the soil profile. Highly weathered soils are often characterized by having high concentrations of Fe and Al oxides.
  • Acid Rain
    Acid rain
    Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

    : When atmospheric water reacts with sulfur and nitrogen compounds that result from industrial processes, the result can be the formation of sulfuric and nitric acid in rainwater. However the amount of acidity that is deposited in rainwater is much less, on average, than that created through agricultural activities.
  • Mine Spoil
    Acid mine drainage
    Acid mine drainage , or acid rock drainage , refers to the outflow of acidic water from metal mines or coal mines. However, other areas where the earth has been disturbed may also contribute acid rock drainage to the environment...

    : Severely acidic conditions can form in soils near mine spoils due to the oxidation of pyrite.

Sources of Basicity

Basic soils have a high saturation of base cations (K+, Ca2+, Mg2+ and Na+). This is due to an accumulation of soluble salts are classified as either saline soil, sodic soil, saline-sodic soil or alkaline soil. All saline and sodic soils have high salt concentrations, with saline soils being dominated by Ca and Mg salts and sodic soils being dominated by Na. Alkaline soils are characterized by the presence of carbonates.

Acid affected soils

Plants grown in acid soils can experience a variety of symptoms including Al, H, and/or Mn toxicity, as well as potential nutrient deficiencies of Ca and Mg.

Al toxicity is the most widespread problem in acid soils. Al is present in all soils, but dissolved Al3+ is toxic to plants; Al3+ is most soluble at low pH, above pH 5.2 little aluminum is in soluble form in most soils. Al is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis. Al damages roots in several ways: In root tips and Al interferes with the uptake of Ca, an essential nutrient, as well as bind with phosphate and interfere with production of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 and DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

, both of which contain phosphate. Al can also restrict cell wall expansion causing roots to become stunted.

Below pH 4, H+ ions themselves damage root cell membranes.

In soils with high content of Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 (Mn) containing minerals, Mn toxicity can become a problem at pH 5.6 and below. Mn, like aluminum becomes increasingly more soluble as pH drops, and Mn toxicity symptoms can be seen at pH's below 5.6. Mn is an essential plant nutrient, so plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves '

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), calcium (Ca), Magnesium (Mg) and Sulfur (S). In addition to macronutrients, plants also need trace nutrients. Trace nutrients are not major components of plant tissue, but are required for growth. These include Iron, (Fe), manganese (Mn), zinc (Zn), copper (Cu), Cobalt (Co), Molybdenum (Mo), and Boron (Bo). Both macro and trace nutrient availability is controlled by soil pH. In slightly to moderately alkaline soils, molybdenum and macro-nutrient (except P) availability is increased, but P, Fe, Mn, Zn Cu, and Co levels are reduced so low they may affect plant growth. In acid soils, micro-nutrient availability (except Mo and Bo) is increased. Nitrogen is supplied as ammonium or nitrate 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).

Most nutrient deficiencies can be avoided between a pH range of 5.5 to 6.5, provided that soil minerals and organic matter contain the essential nutrients to begin with.

Determining pH

Methods of determining pH include:
  • Observation of soil profile: Certain profile characteristics can be indicators of either acid, saline, or sodic conditions. 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. In the southern Blue Ridge Mountains
    Blue Ridge Mountains
    The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. The mountain range is located in the eastern United States, starting at its southern-most...

     there are "ultra acid" soils, pH below 3.5, which have no E horizon. Presence of a caliche
    Caliche may refer to:*Caliche, a hardened deposit of calcium carbonate.*Caliche slang, a collection of slang words unique to Salvadoran Spanish....

     layer indicates the presence of calcium carbonates, which are present in alkaline conditions. Also, columnar structure can be an indicator of sodic condition.
  • Observation of predominant flora. Calcifuge plants (those that prefer an acidic soil) include Erica
    Erica ,the heaths or heathers, is a genus of approximately 860 species of flowering plants in the family Ericaceae. The English common names "heath" and "heather" are shared by some closely related genera of similar appearance....

    , Rhododendron
    Rhododendron is a genus of over 1 000 species of woody plants in the heath family, most with showy flowers...

    and nearly all other Ericaceae
    The Ericaceae, commonly known as the heath or heather family, is a group of mostly calcifuge flowering plants. The family is large, with roughly 4000 species spread across 126 genera, making it the 14th most speciose family of flowering plants...

     species, many birch
    Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...

     (Betula), foxglove (Digitalis
    Digitalis is a genus of about 20 species of herbaceous perennials, shrubs, and biennials that are commonly called foxgloves. This genus was traditionally placed in the figwort family Scrophulariaceae, but recent reviews of phylogenetic research have placed it in the much enlarged family...

    ) , gorse
    Gorse, furze, furse or whin is a genus of about 20 plant species of thorny evergreen shrubs in the subfamily Faboideae of the pea family Fabaceae, native to western Europe and northwest Africa, with the majority of species in Iberia.Gorse is closely related to the brooms, and like them, has green...

     (Ulex spp.), and Scots Pine
    Scots Pine
    Pinus sylvestris, commonly known as the Scots Pine, is a species of pine native to Europe and Asia, ranging from Scotland, Ireland and Portugal in the west, east to eastern Siberia, south to the Caucasus Mountains, and as far north as well inside the Arctic Circle in Scandinavia...

     (Pinus sylvestris). Calcicole (lime loving) plants include ash trees (Fraxinus spp.), honeysuckle
    Honeysuckles are arching shrubs or twining vines in the family Caprifoliaceae, native to the Northern Hemisphere. There are about 180 species of honeysuckle, 100 of which occur in China; Europe, India and North America have only about 20 native species each...

     (Lonicera), Buddleja
    Buddleja, often misspelled Buddleia but commonly known as the Butterfly Bush, is a genus of flowering plants. The generic name bestowed by Linnaeus honours the Reverend Adam Buddle , a botanist and rector in Essex, England, but who could never have seen a plant of the genus.-Classification:The...

    , dogwoods (Cornus spp.), lilac (Syringa) and Clematis
    Clematis is a genus of about 300 species within the buttercup family Ranunculaceae. Their garden hybrids have been popular among gardeners beginning with Clematis × jackmanii, a garden standby since 1862; more hybrid cultivars are being produced constantly. They are mainly of Chinese and Japanese...

  • Use of an inexpensive pH testing kit based on barium sulphate
    Barium sulfate
    Barium sulfate is the inorganic compound with the chemical formula BaSO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and materials prepared from it...

     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
    PH meter
    A pH meter is an electronic instrument used for measuring the pH of a liquid...

    , 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
    Blueberries are flowering plants of the genus Vaccinium with dark-blue berries and are perennial...

    , Bilberry
    Bilberry is any of several species of low-growing shrubs in the genus Vaccinium , bearing edible berries. The species most often referred to is Vaccinium myrtillus L., but there are several other closely related species....

    , Heather, Cranberry
    Cranberries are a group of evergreen dwarf shrubs or trailing vines in the subgenus Oxycoccus of the genus Vaccinium. In some methods of classification, Oxycoccus is regarded as a genus in its own right...

    , Orchid, Azalea
    Azaleas are flowering shrubs comprising two of the eight subgenera of the genus Rhododendron, Pentanthera and Tsutsuji . Azaleas bloom in spring, their flowers often lasting several weeks...

    , for blue Hydrangea
    Hydrangea is a genus of about 70 to 75 species of flowering plants native to southern and eastern Asia and North and South America. By far the greatest species diversity is in eastern Asia, notably China, Japan, and Korea...

     (less acidic for pink) , Sweet Gum, Pin Oak
    Pin oak
    Quercus palustris, the Pin oak or Swamp Spanish oak, is an oak in the red oak section Quercus sect. Lobatae.-Distribution:...

  • pH 5.0 - 5.5 Parsley
    Parsley is a species of Petroselinum in the family Apiaceae, native to the central Mediterranean region , naturalized elsewhere in Europe, and widely cultivated as an herb, a spice and a vegetable.- Description :Garden parsley is a bright green hairless biennial herbaceous plant in temperate...

    , Potato, Heather
    The Ericaceae, commonly known as the heath or heather family, is a group of mostly calcifuge flowering plants. The family is large, with roughly 4000 species spread across 126 genera, making it the 14th most speciose family of flowering plants...

    , Conifers, Pine
    Pines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...

    , Sweet Potato, Maize, Millet, Oars, Tye, Radish
    The radish is an edible root vegetable of the Brassicaceae family that was domesticated in Europe, in pre-Roman times. They are grown and consumed throughout the world. Radishes have numerous varieties, varying in size, color and duration of required cultivation time...

    , Fern
    A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

    s, Iris, Orchids, Rhododendron, Camellia, Daphne and Boronia.
  • pH 5.5 - 6.0 Bean, Brussels Sprouts, Carrot
    The carrot is a root vegetable, usually orange in colour, though purple, red, white, and yellow varieties exist. It has a crisp texture when fresh...

    , 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, Cannabis, Cauliflower, Cucumber, Egg Plant, Pea, Sweet Corn, Pumpkin, Squash, Tomato, Turnip, Red Clover, Sweet Clover, White Clover, Candytuft, Gladiolus, Iceland Poppy, Pansy, Rose
    A rose is a woody perennial of the genus Rosa, within the family Rosaceae. There are over 100 species. They form a group of erect shrubs, and climbing or trailing plants, with stems that are often armed with sharp prickles. Flowers are large and showy, in colours ranging from white through yellows...

    , Snapdragon, Viola, Wallflower, Zinnea and Strawberry.
  • pH 6.5 - 7.0 Asparagus, Beet, Celery, Lettuce, Melons, Onion
    The onion , also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion The onion...

    , Parsnip, Spinach, Lucerne, Carnation
    Dianthus caryophyllus is a species of Dianthus. It is probably native to the Mediterranean region but its exact range is unknown due to extensive cultivation for the last 2,000 years. It is the wild ancestor of the garden carnation.It is a herbaceous perennial plant growing to 80 cm tall...

    , Chrysanthemum, Dahlia, Stock, Sweet Pea and Tulip
    The tulip is a perennial, bulbous plant with showy flowers in the genus Tulipa, which comprises 109 species and belongs to the family Liliaceae. The genus's native range extends from as far west as Southern Europe, North Africa, Anatolia, and Iran to the Northwest of China. The tulip's centre of...

  • pH 7.1 - 8.0 Lilac, brassica

Increasing pH of acidic soil

The most common amendment to increase soil pH is lime (CaCO3 or MgCO3), usually in the form of finely ground agricultural lime
Agricultural lime
Agricultural lime, also called aglime, agricultural limestone, garden lime or liming, is a soil additive made from pulverized limestone or chalk. The primary active component is calcium carbonate...

. The amount of lime needed to change pH is determined by the mesh size of the lime (how finely it is ground)and the buffering capacity of the soil. A high mesh size (60 - 100) indicates a finely ground lime, that will react quickly with soil acidity. Buffering capacity of soils is a function of a soils cation exchange capacity
Cation exchange capacity
In soil science, cation-exchange capacity is the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, for exchanging with the soil solution. CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect...

, which is in turn determined by the clay content of the soil, the type of clay and the amount of organic matter present. Soils with high clay content, particularly shrink-swell
Shrink-swell capacity
The Shrink-swell capacity of clay refers to the extent to which a clay will expand when wet and retract when dry. Soil that is problematic due to high capacity is known as shrink-swell soil, or expansive soil.-Description:...

 clay, will have a higher buffering capacity than soils with little clay. Soils with high organic matter will also have a higher buffering capacity than those with low organic matter. Soils with high buffering capacity require a greater amount of lime to be added than a soil with a lower buffering capacity for the same incremental change in pH.

Other amendments that can be used to increase the pH of soil include wood ash, industrial CaO (burnt lime), and oyster shells. White firewood ash includes metal salts which are important for processes requiring ions such as Na+ (Sodium), K+ (Potassium), Ca2+ (Calcium), which may or may not be good for the select flora, but decreases the acidic quality of soil.

These products increase the pH of soils through the reaction of CO32- with H+ to produce CO2 and H2O.
Calcium silicate
Calcium silicate
Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and sometimes formulated 2CaO.SiO2. It is one of group of compounds obtained by reacting calcium oxide and silica in various ratios e.g. 3CaO.SiO2, Ca3SiO5; 2CaO.SiO2, Ca2SiO4; 3CaO.2SiO2, Ca3Si2O7 and...

 neutralizes active acidity in the soil by removing free hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 ions, thereby increasing pH. As its silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute.

Decreasing pH of basic soil

  • Iron sulphates or aluminium sulphate
    Aluminium sulfate
    Aluminium sulfate, alternatively spelt aluminum sulfate, aluminium sulphate, or aluminum sulphate; is a chemical compound with the formula Al23...

     as well as elemental sulfur (S) reduce pH through the formation of sulfuric acid.
  • Urea
    Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

    , urea phosphate
    Urea phosphate
    Urea phosphate is an organic compound of carbon, hydrogen, nitrogen, oxygen and phosphorus. Its formula is CO2.H3PO4. It is made by reacting urea with phosphoric acid....

    , ammonium nitrate, ammonium phosphate
    Ammonium phosphate
    Ammonium phosphate is the salt of ammonia and phosphoric acid. It has the formula 3PO4 and consists of ammonium cations and phosphate anion. It is obtained as a crystalline powder upon mixing concentrated solutions of ammonia and phosphoric acid, or on the addition of excess of ammonia to the...

    s, ammonium sulphate
    Ammonium sulfate
    Ammonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...

     and monopotassium phosphate
    Monopotassium phosphate
    Monopotassium phosphate -- 24 -- is a soluble salt which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent...

     fertilizers have a
  • organic matter in the form of plant litter, compost, and manure will decrease soil pH through the decomposition process. Certain acid organic matter such as pine needles, pine sawdust and acid peat are effective at reducing pH.

See also

  • Alkali soils
    Alkali soils
    Alkali, or alkaline, soils are clay soils with high pH , a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate...

  • Fertilizer
    Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...

  • Liming (soil)
  • Organic gardening
  • Soil conservation
    Soil conservation
    Soil conservation is a set of management strategies for prevention of soil being eroded from the Earth’s surface or becoming chemically altered by overuse, acidification, salinization or other chemical soil contamination...

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.