Soil pH
[1] pH is defined as the negative logarithm (base 10) of the activity of hydronium ions (H+ or, more precisely, H3O+aq) in a solution.
Precise, repeatable measures of soil pH are required for scientific research and monitoring.
to the soil suspension that was prepared for the water pH, the final soil-solution ratio is 1:2 0.01 M
A 20-g soil sample is mixed with 20 mL of reverse osmosis (RO) water (1:1 w:v) with occasional stirring.
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.
[16] 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 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.
The proton pump, H+-ATPase, of the plasmalemma of root cells works to maintain the near-neutral pH of their cytoplasm.
[23] In soils with a high content of manganese-containing minerals, Mn toxicity can become a problem at pH 5.6 and lower.
Because roots are damaged, nutrient uptake is reduced, 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).
[28] The effect of pH on phosphorus availability varies considerably, depending on soil conditions and the crop in question.
[30][31] Strongly alkaline soils are sodic and dispersive, with slow infiltration, low hydraulic conductivity and poor available water capacity.
[33] The higher the pH in the soil, the less water available to be distributed to the plants and organisms that depend on it.
However, for many plant species, aluminium toxicity severely limits root growth, and moisture stress can occur even when the soil is relatively moist.
[39] 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.
[41][42] Although widely reported and supported by experimental results,[43][44] the observed increase of plant species richness with pH is still in need of a clearcut explanation.
[52][53] 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.
[54] 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.
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).
[62] On the opposite side, earthworms exert a buffering effect on soil pH through their excretion of mucus, endowed with amphoteric properties.
The amount of limestone or chalk 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.
[66] Soils with higher buffering capacity require a greater amount of lime to achieve an equivalent change in pH.
[68] Amendments other than agricultural lime that can be used to increase the pH of soil include wood ash, industrial calcium oxide (burnt lime), magnesium oxide, basic slag (calcium silicate), and oyster shells.
[69] The pH of an alkaline soil can be reduced by adding acidifying agents or acidic organic materials.
[70] However, in high-pH soils with a high calcium carbonate content (more than 2%), attempting to reduce the pH with acids can be very costly and ineffective.
In such cases, it is often more efficient to add phosphorus, iron, manganese, copper, or zinc instead because deficiencies of these nutrients are the most common reasons for poor plant growth in calcareous soils.
