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The soil pH is a measure of the acidity or alkalinity in soils. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+
or, more precisely, H
aq) in a solution. In soils, it is measured in a slurry of soil mixed with water (or a salt solution), and normally falls between 3 and 10, with 7 being neutral. A pH below 7 is acidic and above 7 is alkaline. Ultra-acidic soils (pH<3.5) and very strongly alkaline soils (pH>9) are rare.
Soil pH is considered a master variable in soils as it affects many chemical processes. It specifically affects plant nutrient availability by controlling the chemical forms of the different nutrients and influencing the chemical reactions they undergo. The optimum pH range for most plants is between 5.5 and 7.5; however, many plants have adapted to thrive at pH values outside this range.
- 1 Classification of soil pH ranges
- 2 Factors affecting soil pH
- 3 Effect of soil pH on plant growth
- 4 Water availability in relation to soil pH
- 5 Determining pH
- 6 Examples of plant pH preferences
- 7 Changing soil pH
- 8 See also
- 9 References
- 10 External links
Classification of soil pH ranges
|Ultra acidic||< 3.5|
|Very strongly acidic||4.5–5.0|
|Very strongly alkaline||> 9.0|
Factors affecting soil pH
The pH of a natural soil depends in the mineral composition of the parent material of the soil, and the weathering reactions undergone by that parent material. In warm, humid environments, soil acidification occurs (soil pH decreases) over time as the products of weathering are leached by the flow of water through the soil. In dry climates, however, soil weathering and leaching are less intense and soil pH is often neutral or alkaline.
Sources of acidity
- 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 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).
- Decomposition of organic matter by microorganisms releases CO2 which when mixed with soil water can form carbonic acid (H2CO3).
- Acid rain: The burning of fossil fuels releases oxides of sulfur and nitrogen into the atmosphere, these react with water in the atmosphere to form sulfuric and nitric acid in rain.
- Oxidative weathering: Oxidation of some primary minerals, especially sulphides and those containing Fe2+ generate acidity. This process is often accelerated by human activity:
Sources of alkalinity
- Weathering of silicate, aluminosilicate and carbonate minerals containing Na+, Ca2+, Mg2+ and K+;
- Addition of silicate, aluminosilicate and carbonate minerals to in soils; this may happen by deposition of material eroded elsewhere by wind or water, or by mixing of the soil with less weathered material (such as the addition of limestone to acid soils);
- Addition of water containing dissolved bicarbonates (as occurs when irrigating with high-bicarbonate waters).
The accumulation of alkalinity in a soil (as Na, K, Ca and Mg bicarbonates and carbonates) occurs when there is insufficient water flowing through the soils to leach soluble salts. This may be due to arid conditions, or poor internal soil drainage; in these situations most of the water that enters the soil is transpired (taken up by plants) or evaporates, rather than flowing through the soil.
The soil pH is usually increased when total alkalinity increases, but the balance of the added cations also has a marked effect on the soil pH – for example, increasing the amount of sodium in an alkaline soil will tend to induce dissolution of calcium carbonate, which will increase the pH. Calcareous soils may vary in pH from 7.0 to 9.5, depending on the degree to which Ca2+ or Na+ dominate the soluble cations.
Effect of soil pH on plant growth
Plants grown in acid soils can experience a variety of stresses including aluminium (Al), hydrogen (H), and/or manganese (Mn) toxicity, as well as nutrient deficiencies of calcium (Ca) and magnesium (Mg).
Aluminium toxicity is the most widespread problem in acid soils. Aluminium is present in all soils, but dissolved Al3+ is toxic to plants; Al3+ is most soluble at low pH; above pH 5.0, there is little Al in soluble form in most soils. Aluminium is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis. Aluminium inhibits root growth; lateral roots and root tips become thickened and roots lack fine branching; root tips may turn brown. In the root, the initial effect of Al3+ is the inhibition of the expansion of the cells of the rhizodermis, leading to their rupture; thereafter it is known to interfere with many physiological processes including the uptake and transport of calcium and other essential nutrients, cell division, cell wall formation, and enzyme activity.
Proton (H+ ion) stress can also limit plant growth. The proton pump, H+-ATPase, of the plasmalemma of root cells works to maintain the near-neutral pH of their cytoplasm. A high proton activity (pH within the range 3.0–4.0 for most plant species) in the external growth medium overcomes the capacity of the cell to maintain the cytoplasmic pH and growth shuts down.
In soils with a high content of manganese-containing minerals, Mn toxicity can become a problem at pH 5.6 and lower. Manganese, like aluminium, becomes increasingly soluble as pH drops, and Mn toxicity symptoms can be seen at pH levels below 5.6. Manganese 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
Soil pH affects the availability of some plant nutrients:
Aluminium toxicity reduces the availability of all nutrients by limiting root growth; this is largely limited to soil pH<5.0. Because roots are damaged, it becomes more difficult for plants to take up all nutrients, and deficiencies of the macronutrients (nitrogen, phosphorus, potassium, calcium and magnesium) are frequently encountered in very strongly acidic to ultra-acidic soils (pH<5.0).
Molybdenum availability is increased at higher pH; this is because the molybdate ion is more strongly sorbed by clay particles at lower pH.
Zinc, iron, copper and manganese show decreased availability at higher pH (increased sorbtion at higher pH).
The effect of pH on phosphorus availability varies considerably, depending on soil conditions and the crop in question. The prevailing view in the 1940s and 1950s was that P availability was maximized near neutrality (soil pH 6.5–7.5), and decreased at higher and lower pH. Interactions of phosphorus with pH in the moderately to slightly acidic range (pH 5.5–6.5) are, however, far more complex than this. Laboratory tests, glasshouse trials and field trials have indicated that increases in pH within this range may increase, decrease, or have no effect on P availability to plants.
Water availability in relation to soil pH
Strongly alkaline soils are sodic and dispersive, with slow infiltration, low hydraulic conductivity and poor available water capacity. Plant growth is severely restricted because aeration is poor when the soil is wet; in dry conditions, plant-available water is rapidly depleted and the soils become hard and cloddy (high soil strength).
Many strongly acidic soils, on the other hand, have strong aggregation, good internal drainage, and good water-holding characteristics. However, for many plant species, aluminium toxicity severely limits root growth, and moisture stress can occur even when the soil is relatively moist.
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. Presence of a caliche 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, 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, where in a small sample of soil is mixed with indicator solution 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 glass or solid-state electrode is inserted into moistened soil and measures the hydrogen ion activity.
Examples of plant pH preferences
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- pH 4.5–5.0: Ericaceae (azalea, bilberry, blueberry, cranberry; heather[disambiguation needed]), hydrangea for blue, (less acidic for pink), liquidambar or sweet gum, orchid, pin oak
- pH 5.0–5.5: Boronia, daphne; Ericaceae: (camellia, heather[disambiguation needed], rhododendron), ferns, iris, orchids, parsley, conifers (e.g., pine); Poaceae: (maize, millet, rye, oat), radish; Solanales: (potato, sweet potato); Bromeliaceae (pineapple).
- pH 5.5–6.0: Asteraceae: (aster, endive); Brassicaceae: (brussels sprout, kohlrabi), carrot; Cucurbitales: (begonia, chayote or choko); Fabaceae: (bean, crimson clover, peanut, soybean), petunia, rhubarb, violet, most bulbs (canna, daffodil, jonquil), larkspur[disambiguation needed], primrose[disambiguation needed]
- pH 6.0–6.5 antirrhinum or snapdragon, Brassicaceae: (broccoli, cabbage, candytuft, cauliflower, turnip, wallflower); Cucurbitaceae: (cucumber, pumpkin, squash); Fabaceae: (pea, red clover, white clover), gladiolus, Iceland poppy; Rosales: (cannabis, rose, strawberry); Solanaceae: (eggplant or aubergine, tomato), sweet corn; Violaceae: (pansy, viola), zinnia or zinnea
- pH 6.5–7.0: Amaranthaceae: (beet, spinach); Apiaceae: (celery, parsnip); Asparagales: (asparagus, onion); Asteraceae: (chrysanthemum, dahlia, lettuce), carnation; Fabaceae: (alfalfa, sweet pea), melon, stock[disambiguation needed], tulip
- pH 7.1–8.0 lilac
Changing soil pH
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. 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, 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 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 neutralizes active acidity in the soil by removing free hydrogen 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 alkaline soil
The pH of an alkaline soil can be reduced by adding acidifying agents or organic materials such as the ones listed below. Acidifying fertilizers, such as those containing ammonium, can help to reduce the pH of a soil. However, if a high pH soil has a calcium carbonate content of more than 5%, it can be very costly and not very effective to reduce with acids. In this case, it is more efficient to add phosphorus, iron, copper, and instead because the plants will be deficient in these nutrients in calcareous soils.
- Amend the soil with organic matter. On average, soils with higher organic matter contents have lower pH. Peat or sphagnum peat moss are highly acidic and will lower soil pH more than other organic amendments.
- Add elemental sulfur (90 or 99% sulfur material) annually at a rate of 6 to 10 pounds per 1000 square feet of area. Elemental sulfur slowly oxidizes in soil to form sulfuric acid. Test the soil occasionally and stop adding sulfur when pH has reached desirable levels.
- Use acidifying fertilizers such as ammonium sulfate and other products with label designations indicating an acidic reaction in the soil. With repeated use these materials may reduce soil pH.
- Plant on raised beds in a sandy medium amended with peat moss or another source of acidic organic matter. An alternative is to plant in boxes or ½ barrels heavily amended with acidic forms of organic matter.
- Acid mine drainage
- Acid sulfate soil
- Cation-exchange capacity
- Liming (soil)
- Organic horticulture
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