Soil pH

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Soil pH is the measure of the acidity or alkalinity (alkalinity) of a soil. pH is defined as the negative logarithm (base 10) of the hydronium ion activity ( H < br> or more precisely, H
₃ O
_aq ) in a solution. On the ground, it is measured in a ground slurry mixed with water (or salt solution, such as 0.01 M CaCl
₂ ), and usually falls between 3 and 10, with 7 neutrals. Acidic soils have a pH below 7 and alkaline soils have a pH above 7. Very acidic soils (pH & lt; 3,5) and very alkaline soils (pH & gt; 9) are uncommon.

soil pH is considered a major variable in the soil because it affects many chemical processes. Specifically affect the availability of plant nutrients by controlling the chemical forms of different nutrients and affecting their chemical reactions. 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.

Video Soil pH

Classification of soil pH range

The Natural Resources Conservation Service of the US Department of Agriculture classifies the range of soil pH as follows:

Maps Soil pH

Specify pH

The pH determination method includes:

• Observation of soil profile: Certain profile characteristics may be an indicator of acid, salt, or sodic conditions. Examples are:
  • The incorporation of a bad organic surface layer with an underlying mineral layer - this can indicate a very acidic soil;
  • The classic podzol horizon sequence, because podzol is very acidic: in this soil, the eluvial pale horizon (E) lies beneath the organic surface layer and obscures the dark horizon B;
  • The presence of a caliche layer indicates the presence of calcium carbonate, which is present under basic conditions;
  • The column structure can be an indicator of sodic conditions.
• Observation of dominant flora. Calcifuge plants (which prefer acidic soils) include Erica , Rhododendron and almost all other Ericaceae species, many birch ( Betula ), foxglove (< i> Digitalis ), gorse ( Ulex spp.), and Pine Scots ( Pinus sylvestris ). Calcicole plants include ash ( Fraxinus spp.), Honeysuckle ( Lonicera ), Buddleja , dogwood ( Cornus spp.), lilac ( Syringa ) and species Clematis .
• Use of cheap pH test kits, where in small samples of soil mixed with indicator solutions that change color according to the acidity.
• The use of litmus paper. Small samples of soil mixed with distilled water, where a piece of litmus paper is inserted. If the acid soil paper turns red, if the base, blue.
• Use of commercially available pH measuring devices, where glass or solid-state electrodes are introduced into wetted soil or soil and water (suspension) mixtures; pH is usually read on a digital display screen.
• Recently, spectrophotometric methods have been developed to measure soil pH which involves adding indicator dyes to soil extracts. This compares favorably with the measurement of glass electrodes but offers substantial advantages such as lack of drift, liquid intersections and suspension effects.

Appropriate soil pH measurements are required for scientific research and monitoring. This usually requires laboratory analysis using standard protocols; an example of such a protocol is that in the USDA Land Survey Field and the Laboratory Method Manual. In this document the three-page protocol for ground pH measurement includes the following sections: Application; Summary Method; Interference; Security; Equipment; Reagent; and Procedures.

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Factors affecting soil pH

Natural soil pH depends on the mineral composition of the parent material, and the weathering reaction experienced by the parent material. In warm and humid environments, soil acidification occurs over time as weathering products are washed by water moving in the lateral or downward direction through the soil. However, in dry climates, weathering and soil leaching are less intense and soil pH is often neutral or alkaline.

Acid source

Many processes contribute to soil acidification. These include:

• Rainfall: Acid soils are most commonly found in areas with high rainfall. Rain water has a slightly acidic pH (usually around 5.7) because of the reaction with CO
  ₂ in the atmosphere that forms carbonic acid. When this water flows through the soil, it generates the release of the basic cations from the soil as bicarbonate; this increases the percentage of Al ³ and H relative to cation other.
• Respiration and decomposition of roots from organic matter by microorganisms releases CO
  ₂ which increases the carbonic acid ( H
  ₂ CO
  ₃ ) concentration and subsequent washing.
• Plant growth: Crops take nutrients in the form of ions (eg NO ^-
  ₃ , NH
  ₄ , Ca ² ₂ PO ^- ₄ ), and they often take more cations than anions. But plants must maintain a neutral charge at their roots. To compensate for an additional positive charge, they will release H ion from its root. Some plants also emit organic acids into the soil to acidify the zones around their roots to help dissolve insoluble metal nutrients in neutral pH, such as iron (Fe).
• Use of fertilizer: Ammonium ( NH _{NO <4). Fertilizer reacts in soil with nitrification process to form nitrate ( NO < span> ^-
  3} ), and in the release process H
  ion.
• Acid rain: Burning fossil fuels releases sulfur and nitrogen oxides into the atmosphere. It reacts with water in the atmosphere to form sulfuric acid and nitrate in the rain.
• Oxidative weathering: Oxidation of some primary minerals, especially sulfides and containing Fe ² , generating acidity. This process is often accelerated by human activity:
  • Mining weakness: Very severe acid conditions can form on soil near some of the mine spoils due to pyrite oxidation.
  • The naturally occurring acid sulphate soils in coastal environments and waterlogged estuarine can be very acidic when dried or dug.

Source of alkalinity

Total soil alkalinity increased by:

• Weathering silicate, aluminosilicate and carbonate minerals containing Na
  , Ca ²
  , Mg ;
• The addition of silicate minerals, aluminosilicate and carbonate to the soil; this can occur with the deposition of materials eroded elsewhere by wind or water, or by mixing the soil with lesser materials (such as the addition of limestone to acidic soil);
• The addition of water containing dissolved bicarbonate (as happened when irritating with high bicarbonate water).

Accumulation of alkalinity in soils (such as carbonates and bicarbonates of Na, K, Ca, and Mg) occurs when there is not enough water flowing through the soil to dissolve the soluble salt. This may be due to dry conditions, or poor internal ground drainage; in this situation most of the water entering the ground takes place (taken by the plants) or evaporates, rather than flowing through the soil.

Soil pH usually increases when total alkalinity increases, but the added cation balance also has a noticeable effect on soil pH. For example, increasing the amount of sodium in the base soil tends to cause the dissolution of calcium carbonate, which increases the pH. Calcareous soils may vary in pH from 7.0 to 9.5, depending on the degree to which Ca ²
or Na dominate the dissolved cations.

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The effect of soil pH on plant growth

acid soil

Plants grown in acid soils may undergo various stresses including aluminum toxicity (Al), hydrogen (H), and/or manganese (Mn), as well as calcium nutrient deficiency (Ca) and magnesium (Mg).

Aluminum poisoning is the most widespread problem in acid soils. Aluminum is present in all soils, but dissolved Al ³ is toxic to plants; Al ³ is very soluble at low pH; pH above 5.0, there is little Al in the soluble form in most of the soil. Aluminum is not a plant nutrient, and as such, is not actively taken by plants, but enters the roots of plants passively through osmosis. Aluminum inhibits root growth; the lateral root and root tip become thickened and the roots do not have a good branch; the root tip can turn brown. At root, the initial effect of Al ³ is the inhibition of expansion of rhizodermic cells, which causes their rupture; after which it is known to interfere with many physiological processes including the absorption and transport of calcium and other essential nutrients, cell division, cell wall formation, and enzyme activity.

Proton pressure (H ion) can also limit plant growth. Proton pumps, H -ATPase, from plasmalemma root cells work to keep their neutral cytoplasmic pH. High proton activity (pH in the range of 3.0-4.0 for most plant species) in external growth media overcomes cell capacity to maintain cytoplasmic pH and closing growth.

In soils with a high content of manganese-containing minerals, Mn toxicity can be a problem at pH 5.6 and lower. Manganese, such as aluminum, becomes increasingly soluble because of the pH down, and the symptoms of Mn toxicity can be seen at pH levels below 5.6. Manganese is an important plant nutrient, so the plants transport Mn to the leaves. The classic symptoms of Mn toxicity are wrinkled or cupping of leaves.

Availability of nutrients in relation to soil pH

The soil pH affects the availability of some plant nutrients:

As discussed above, aluminum toxicity has a direct effect on plant growth; However, by limiting root growth, it also reduces the availability of plant nutrients. Because the roots are damaged, nutrient uptake is reduced, and deficiency of macronutrients (nitrogen, phosphorus, potassium, calcium and magnesium) is common in very acidic to ultra-acid soils (pH & lt; 5.0).

The availability of molybdenum increases at higher pH; This is because the stronger molybdate ions are absorbed by the clay particles at a lower pH.

Zinc, iron, copper and manganese showed decreased availability at higher pH (higher sorbtion at higher pH).

The effect of pH on the availability of phosphorus varies greatly, depending on the condition of the soil and the plant concerned. The prevailing view in the 1940s and 1950s was that the availability of P was maximized near neutrality (soil pH 6.5-7.5), and decreased at higher and lower pH. The interaction of phosphorus with a pH in the medium to slightly acid range (pH 5.5-6.5), however, is much more complex than suggested by this view. Laboratory tests, greenhouse trials and field trials show that an increase in pH in this range may increase, decrease, or not affect P availability to plants.

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Water availability in relation to soil pH

The highly alkaline earth is sodic and dispersed, with slow infiltration, low hydraulic conductivity and poor water capacity. Plant growth is very limited due to bad aeration when the soil is wet; in dry conditions, the water available at the plant quickly runs out and the soil becomes hard and expired (high soil strength).

Many highly acidic soils, on the other hand, have strong aggregation, good internal drainage, and good water retention characteristics. However, for many plant species, aluminum poisoning severely limits root growth, and moisture stress can occur even when the soil is relatively moist.

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Plant pH preferences

In general, different plant species adapted to soil with different pH ranges. For many species, a suitable soil pH range is well known. Plant-based online databases, such as USDA PLANTS and Plants for a Future can be used to locate the appropriate pH range for different types of plants. Documents like Ellenberg's indicator values ​​for UK plants can also be consulted.

However, plants may be intolerant of certain pH in some soils as a result of certain mechanisms, and the mechanism may not apply in other soils. For example, low-molybdenum soils may not be suitable for soybean crops at pH 5.5, but soil with sufficient molybdenum allows optimum growth at the pH. Similarly, some calcifuges (high pH-soil intolerance) may tolerate calcareous soil if enough phosphorus is provided. Another confounding factor is that different varieties of the same species often have a corresponding soil pH range. Plant breeders can use this to breed varieties that can tolerate conditions that are unlikely to be suitable for the species - for example projects to develop sunflower-tolerant and tolerant varieties of cereals for food production in highly acidic soils.

The table below provides a suitable soil pH range for some widely cultivated crops such as those found in USDA PLANTS database . Some species (such as Pinus radiata and Opuntia ficus-indica) tolerate only a narrow range in soil pH, while others (such as Vetiveria zizanioides ) tolerate a very wide pH range.

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Changing the soil pH

Increase acid soil pH

Soil finely ground lime is often applied to acid soils to increase soil pH (calcification). The amount of limestone or lime needed to change the pH is determined by the lime mesh size (how fine the soil) and the soil buffer capacity. The high mesh size (60 mesh = 0.25 mm, 100 mesh = 0.149 mm) indicates soil chalk that will react quickly with soil acidity. The capacity of the soil buffer depends on the content of clay, clay type, and the amount of organic material present, and may be related to the capacity of the soil cation exchange. High clay soils will have a higher buffer capacity than the soils with less clay, and soils with high organic materials will have a higher buffering capacity compared to low organic materials. Soil with a higher buffering capacity requires more lime to achieve an equivalent pH change.

Amendments other than agricultural lime can be used to increase soil pH including wood ash, industrial calcium oxide (burnt lime), magnesium oxide, slag base (calcium silicate), and oyster shells. These products increase soil pH through various acid-base reactions. Calcium silicate neutralizes the active acidity in soil by reacting with H ion to form monosilicic acid (H ₄ SiO ₄ ), neutral solute.

Reduce alkaline earth pH

Alkaline earth pH can be reduced by adding acidity or acidic organic matter. The sulfur element (90-99% S) has been used at the application level of 300-500 kg/ha - is slowly oxidizing in the soil to form sulfuric acid. Acidic fertilizers, such as ammonium sulfate, ammonium nitrate and urea, can help reduce the pH of a soil because of the oxidation of ammonium to form nitric acid. Acidification of organic materials including peat or sphagnum peat.

However, in soils with high pH with high calcium carbonate content (more than 2%), it can be very expensive and/or ineffective to try to reduce pH with acid. In such cases, it is often more efficient to add phosphorus, iron, manganese, copper and/or zinc instead, since this nutritional deficiency is the most common reason for poor plant growth in calcareous soils.

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See also

• Drainage mine drainage
• acid sulfuric soil
• Cation exchange capacity
• Fertilizer
• Calcification (ground)
• Organic horticulture

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References

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External links

• "Lime Potential Study, R.C. Turner, Research Branch, Department of Agriculture of Canada, 1965"
• "Adjust and Measure Soil PH, Soil PH and Nutrition for Home Gardening"

Source of the article : Wikipedia