Soil pH

Soil pH is a measure of the acidity or basicity of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the negative logarithm of the activity of hydronium ions in a solution. In soils, it is measured in a slurry of soil mixed with water, and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils and very strongly alkaline soils 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.

Classification of soil pH ranges

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

Semantic description	pH range
Ultra acidic	< 3.5
Extremely acidic	3.5–4.4
Very strongly acidic	4.5–5.0
Strongly acidic	5.1–5.5
Moderately acidic	5.6–6.0
Slightly acidic	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

0 to 6=acidic
7=neutral
8 and above=alkaline

Determining pH

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 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 include Erica, Rhododendron and nearly all other Ericaceae species, many birch, foxglove, gorse, and Scots Pine. Calcicole plants include ash trees, honeysuckle, Buddleja, dogwoods, lilac 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. Red cabbage is similarly affected.
Use of a commercially available electronic pH meter, in which a glass or solid-state electrode is inserted into moistened soil or a mixture 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.

Factors affecting soil pH

The pH of a natural soil depends on 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 over time as the products of weathering are leached by water moving laterally or downwards through the soil. In dry climates, however, soil weathering and leaching are less intense and soil pH is often neutral or alkaline.

Effect of soil pH on plant growth

Acid soils

High levels of aluminium occur near mining sites; small amounts of aluminium are released to the environment at the coal-fired power plants or incinerators. Aluminium in the air is washed out by the rain or normally settles down but small particles of aluminium remain in the air for a long time.
Acidic precipitation is the main natural factor to mobilize aluminium from natural sources and the main reason for the environmental effects of aluminium; however, the main factor of presence of aluminium in salt and freshwater are the industrial processes that also release aluminium into air. Plants grown in acid soils can experience a variety of stresses including aluminium , hydrogen , and/or manganese toxicity, as well as nutrient deficiencies of calcium and magnesium .
Aluminium toxicity is the most widespread problem in acid soils. Aluminium is present in all soils to varying degrees, 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 can exist in many different forms and is a responsible agent for limiting growth in various parts of the world. Aluminium tolerance studies have been conducted in different plant species to see viable thresholds and concentrations exposed along with function upon exposure. 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 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 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:
As discussed above, aluminium toxicity has direct effects on plant growth; however, by limiting root growth, it also reduces the availability of plant nutrients. Because roots are damaged, nutrient uptake is reduced, and deficiencies of the macronutrients are frequently encountered in very strongly acidic to ultra-acidic soils. When aluminum levels increase in the soil, it decreases the pH levels. This does not allow for trees to take up water, meaning they cannot photosynthesize, leading them to die. The trees can also develop yellowish colour on their leaves and veins.
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.
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, and decreased at higher and lower pH. Interactions of phosphorus with pH in the moderately to slightly acidic range are, however, far more complex than is suggested by this view. 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; while in dry conditions, plant-available water is rapidly depleted and the soils become hard and cloddy. The higher the pH in the soil, the less water available to be distributed to the plants and organisms that depend on it. With a decreased pH, this does not allow for plants to uptake water like they normally would. This causes them to not be able to photosynthesize.
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.

Plant pH preferences

In general terms, different plant species are adapted to soils of different pH ranges. For many species, the suitable soil pH range is fairly well known. Online databases of plant characteristics, such as USDA PLANTS and Plants for a Future can be used to look up the suitable soil pH range of a wide range of plants. Documents like Ellenberg's indicator values for British plants can also be consulted.
However, a plant may be intolerant of a particular pH in some soils as a result of a particular mechanism, and that mechanism may not apply in other soils. For example, a soil low in molybdenum may not be suitable for soybean plants at pH 5.5, but soils with sufficient molybdenum allow optimal growth at that pH. Similarly, some calcifuges can tolerate calcareous soils if sufficient phosphorus is supplied. Another confounding factor is that different varieties of the same species often have different suitable soil pH ranges. Plant breeders can use this to breed varieties that can tolerate conditions that are otherwise considered unsuitable for that species – examples are projects to breed aluminium-tolerant and manganese-tolerant varieties of cereal crops for food production in strongly acidic soils.
The table below gives suitable soil pH ranges for some widely cultivated plants as found in the USDA PLANTS Database. Some species tolerate only a narrow range in soil pH, whereas others tolerate a very wide pH range.
In natural or near-natural plant communities, the various pH preferences of plant species at least partly determine the composition and biodiversity of vegetation. 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 to strongly alkaline 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.