User:Keegan Peet/Soil pH

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Determining pH

[edit] Methods of determining pH include:

• Observation of soil profile: certain profile characteristics can be indicators of either acid, saline, or sodic conditions. Examples are:
  • Poor incorporation of the organic surface layer with the underlying mineral layer – this can indicate strongly acidic soils;
  • The classic podzol horizon sequence, since podzols are strongly acidic: in these soils, a pale eluvial (E) horizon lies under the organic surface layer and overlies a dark B horizon;
  • Presence of a caliche layer indicates the presence of calcium carbonates, which are present in alkaline conditions;
  • 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.
• 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 basic, blue.
• Certain other fruit and vegetable pigments also change color in response to changing pH. Blueberry juice turns more reddish if acid is added, and becomes indigo if titrated with sufficient base to yield a high pH. This can be attributed to the influence of pH on the anthocyanin content in blueberries. The anthocyanin content in blueberries, along with many other fruits and vegetables, is responsible for the colorations of produce, and its stability can vary depending on pH conditions.
• Use of a commercially available electronic pH meter, in which a glass or solid-state electrode is inserted into moistened soil or a mixture (suspension) of soil and water; the pH is usually read on a digital display screen.
• In the 2010s, spectrophotometric methods were developed to measure soil pH involving addition of an indicator dye to the soil extract. These compare well to glass electrode measurements but offer substantial advantages such as lack of drift, liquid junction and suspension effects.

Precise, repeatable measures of soil pH are required for scientific research and monitoring. This generally entails laboratory analysis using a standard protocol; an example of such a protocol is that in the USDA Soil Survey Field and Laboratory Methods Manual. In this document the three-page protocol for soil pH measurement includes the following sections: Application; Summary of Method; Interferences; Safety; Equipment; Reagents; and Procedure.

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In natural or near-natural plant communities, the various pH preferences of plant species (or ecotypes) at least partly determine the composition and biodiversity of vegetation. pH levels and species richness are closely linked, with slightly acidic soil pH consistently creating conditions that support thriving biodiversity. While both very low and very high pH values are detrimental to plant growth, there is an increasing trend of plant biodiversity along the range from extremely acidic (pH 3.5) to strongly alkaline (pH 9) soils, i.e. there are more calcicole than calcifuge species, at least in terrestrial environments. Although widely reported and supported by experimental results, the observed increase of plant species richness with pH is still in need of a clearcut explanation. Competitive exclusion between plant species with overlapping pH ranges most probably contributes to the observed shifts of vegetation composition along pH gradients.

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Soil biota (soil microflora, soil animals) are sensitive to soil pH, either directly upon contact or after soil ingestion or indirectly through the various soil properties to which pH contributes (e.g. nutrient status, metal toxicity, humus form). According to the various physiological and behavioural adaptations of soil biota, the species composition of soil microbial and animal communities varies with soil pH. Along altitudinal gradients, changes in the species distribution of soil animal and microbial communities can be at least partly ascribed to variation in soil pH. The shift from toxic to non-toxic forms of aluminium around pH5 marks the passage from acid-tolerance to acid-intolerance, with few changes in the species composition of soil communities above this threshold, even in calcareous soils. Soil animals exhibit distinct pH preferences when allowed to exert a choice along a range of pH values, explaining that various field distributions of soil organisms, motile microbes included, could at least partly result from active movement along pH gradients. Like for plants, competition between acido-tolerant and acido-intolerant soil-dwelling organisms was suspected to play a role in the shifts in species composition observed along pH ranges. Adoption of mechanisms allows microbes to inhabit almost every pH level on earth, which gives rise to certain, pH-specific classifications for microorganisms (e.g. acidophiles, alkaliphiles, neutrophiles). Adaptations include preventative measures such as proton-neutralizing enzymes, specialized lipid membranes, or reactive measures such as restoration enzymes.

The opposition between acido-tolerance and acido-intolerance is commonly observed at species level within a genus or at genus level within a family, but it also occurs at much higher taxonomic rank, like between soil fungi and bacteria, here too with a strong involvement of competition. It has been suggested that soil organisms more tolerant of soil acidity, and thus living mainly in soils at pH less than 5, were more primitive than those intolerant of soil acidity. A cladistic analysis on the collembolan genus Willemia showed that tolerance to soil acidity was correlated with tolerance of other stress factors and that stress tolerance was an ancestral character in this genus. However the generality of these findings remains to be established.

At low pH, the oxidative stress induced by aluminium (Al³⁺) affects soil animals the body of which is not protected by a thick chitinous exoskeleton like in arthropods, and thus are in more direct contact with the soil solution, e.g. protists, nematodes, rotifers (microfauna), enchytraeids (mesofauna) and earthworms (macrofauna).

Effects of pH on soil biota can be mediated by the various functional interactions of soil foodwebs. It has been shown experimentally that the collembolan Heteromurus nitidus, commonly living in soils at pH higher than 5, could be cultured in more acid soils provided that predators were absent. Its attraction to earthworm excreta (mucus, urine, faeces), mediated by ammonia emission, provides food and shelter within earthworm burrows in mull humus forms associated with less acid soils

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Sources of acidity

[edit] Many processes contribute to soil acidification. These include:

• Rainfall: Average rainfall has a pH of 5.6 and is moderately acidic due to dissolved atmospheric carbon dioxide (CO

2) that combines with water to form carbonic acid (H 2CO 3). When this water flows through the soil it results in the leaching of basic cations as bicarbonates; this increases the percentage of Al3+ and H+ relative to other cations.

• Root respiration and decomposition of organic matter by microorganisms release CO 2 which increases the carbonic acid (H 2CO 3) concentration and subsequent leaching.
• Plant growth: Plants take up nutrients in the form of ions (e.g. NO− 3, NH+ 4, Ca2+ , H 2PO− 4), and they often 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 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).
• Fertilizer use: Ammonium (NH+ 4) fertilizers react in the soil by the process of nitrification to form nitrate (NO− 3), and in the process release H+ ions.
• 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. (Citation Added)
• Oxidative weathering: Oxidation of some primary minerals, especially sulfides and those containing Fe2+ , generate acidity. This process is often accelerated by human activity:
  • Mine spoil: Severely acidic conditions can form in soils near some mine spoils due to the oxidation of pyrite.
  • Acid sulfate soils formed naturally in waterlogged coastal and estuarine environments can become highly acidic when drained or excavated.

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• 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. (Citation Added)