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Soil pH

Related subjects: Geology and geophysics

Background Information

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Soil pH is a measure of the soil acidity or Soil alkalinity. An acid solution has a pH value less than 7. While a basic solution always has a pH larger than 7, an alkaline solution (i.e. a solution with positive acid neutralizing capacity) does not necessarily have a pH larger than 7. For details on the relation between pH and ANC, see acid neutralizing capacity.

Soil pH is an important consideration for farmers and gardeners for several reasons:

  • Many plants and soil life forms prefer either alkaline or acidic conditions
  • Some diseases tend to thrive when the soil is alkaline or acidic
  • The pH can affect the availability of nutrients in the soil.

Nutrient availability in relation to soil pH

The majority of food crops prefer a neutral or slightly acidic soil(pH 7). Some plants however prefer more acidic (e.g., potatoes, strawberries) or alkaline ( brassicas) conditions. Once the cation exchange surface has become depleted of these ions, however, the concentration in soil solution can be quite low and is largely determined by the weathering rate. The weathering rate in turn is dependent on such things as mineralogy (e.g. presence of easily weathered minerals), surface area (i.e. the soil texture), soil moisture (i.e. how large a fraction of the mineral surface area that is wet), pH, concentration of base cations such as Ca, Mg and K as well as concentration of aluminium. The amount of plant available nutrients is a much more difficult issue than soil solution concentrations.

It is thus important to realize that there exists no simple relation between soil solution concentration of Ca, Mg and K and reasonable pH-values. The reason for this is that Ca, Mg and K are base cations, i.e. cations of strong bases, and strong bases are fully dissociated at the pH-ranges occurring in most natural waters. However, as the soil solution pH is dependent on mineral weathering, and mineral weathering increases pH by releasing Ca, Mg and K, a soil which is rich in easily weatherable minerals tends to have both a higher pH and higher soil solution concentration of Ca, Mg and K. On the other hand deposition of sulfate, nitrate and to some extent ammonia decrease pH of soil solution essentially without affecting Ca, Mg and K concentrations whereas deposition of sea salt increases Ca, Mg and K concentrations without having much of an effect on soil solution pH.

When interpreting soil solution pH values it is essential to take into account the method by which pH has been measured. Depending on whether or not the water has been equilibrated with ambient CO2 pressure or not the pH reported from the same site may be either high or low. This is simply because the carbon dioxide pressure deep down in the soil might be 10–20 times higher than the ambient pressure due to decomposition of organic material. The higher carbon dioxide pressure result in more carbonic acid and hence a lower pH. Furthermore, soil solution can be extracted from the soil in many ways, e.g. by lysimeters, zero-tension lysimeters, centrifugation, extraction with CaCl2, overhead shaking of soil sample with added water, etc. The CaCl2 extraction method do not give the actual soil solution pH but rather a mix between soil solution pH and what is easily available e.g. through cation exchange. Also when mixing soil samples with water and using overhead shakers (or similar) the result is a mix between actual soil solution and cation exchange, although the hope is that the extracted water will be similar to the actual soil solution in most respects. If centrifugation or pressurised lysimeters are used, care must be taken that the extracted water do not include water that is not readily available (think wilting point and crystal water). Naturally, taking a sample introduces a disturbance of the system, which can e.g. result in a change in nutrient uptake and decomposition rates (e.g. due to cutting of fine roots when placing the lysimeter).

Many nutrient cations such as zinc (Zn2+), aluminium (Al3+), iron (Fe2+), copper (Cu2+), cobalt (Co2+), and manganese (Mn2+) are soluble and available for uptake by plants below pH 5.0, although their availability can be excessive and thus toxic in more acidic conditions. In more alkaline conditions they are less available, and symptoms of nutrient deficiency may result, including thin plant stems, yellowing ( chlorosis) or mottling of leaves, and slow or stunted growth.

pH levels also affect the complex interactions among soil chemicals. Phosphorus (P) for example requires a pH between 6.0 and 7.5 and becomes chemically immobile outside this range, forming insoluble compounds with iron (Fe) and aluminium (Al) in acid soils and with calcium (Ca) in calcareous soils.

How is acidic soil formed

To understand how acid soils are formed, take a simple walk through a woodland. Rainfall filters through trees and into the ground, where it dissolves limestone sediment and other alkaline minerals that help neutralize soil acidity. The woodland floor is carpeted in needles of conifers, leaves of hardwood trees, and other dead plant matter, most of which increase soil acidity as they decompose. Unless this woodland is on top of a huge deposit of alkaline material such as limestone or serpentine, the soil will tend to be acidic.

Soils and acidity

Under conditions in which rainfall exceeds evapotranspiration (leaching) during most of the year, the basic soil cations (Ca, Mg, K) are gradually depleted and replaced with cations held in colloidal soil reserves, leading to soil acidity. Clay soils often contain iron and aluminium hydroxides, which affect the retention and availability of fertilizer cations and anions in acidic soils.

Soil acidification may also occur by addition of hydrogen, due to decomposition of organic matter, acid-forming fertilizers, and exchange of basic cations for H+ by the roots.

Soil acidity is reduced by volatilization and denitrification of nitrogen. Under flooded conditions, the soil pH value increases. In addition, the following nitrate fertilizers -- calcium nitrate, magnesium nitrate, potassium nitrate and sodium nitrate -- also increase the soil pH value.

Some alkaline soils have calcium in the form of limestone that is not chemically available to plants. In this case sulfuric acid

Factors affecting soil pH

The pH value of a soil is influenced by the kinds of parent materials from which the soil was formed. Soils developed from basic rocks generally have higher pH values than those formed from acid rocks.

Rainfall also affects soil pH. Water passing through the soil leaches basic nutrients such as calcium and magnesium from the soil. They are replaced by acidic elements such as aluminium and iron. For this reason, soils formed under high rainfall conditions are more acidic than those formed under arid (dry) conditions.

Soil life and pH

A pH level of around 6.3-6.8 is also the optimum range preferred by most soil bacteria, although fungi, molds, and anaerobic bacteria have a broader tolerance and tend to multiply at lower pH values. Therefore, more acidic soils tend to be susceptible to souring and putrefaction, rather than undergoing the sweet decay processes associated with the decay of organic matter, which immeasurably benefit the soil. These processes also prefer near-neutral conditions.

pH and plant diseases

Many plant diseases are caused or exacerbated by extremes of pH, sometimes because this makes essential nutrients unavailable to crops or because the soil itself is unhealthy (see above). For example, chlorosis of leaf vegetables and potato scab occur in overly alkaline conditions, and acidic soils can cause clubroot in brassicas.

Determining pH

A map of the pH level is a mosaic, varying according to soil crumb structure, on the surface of colloids, and at microsites. The pH also exhibits vertical gradients, tending to be more acidic in surface mulches and alkaline where evaporation, wormcasts, and capillary action draw bases up to the soil surface. It also varies on a macro level depending on factors such as slope, rocks, and vegetation type. Therefore the pH should be measured regularly and at various points within the land in question.dd c Methods of determining pH include:

  • Observation of soil profile. 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. This is a very rough gauge of acidity as there is no correlation between thickness of the E and soil pH. E horizons a few feet thick in Florida usually have pH just above 5 (merely "strongly acid") while E horizons a few inches thick in New England are "extremely acid" with pH readings of 4.5 or below. [3]In the southern Blue Ridge Mountains there are "ultra acid" soils, pH below 3.5, which have no E horizon.
  • Observation of predominant flora. Calcifuge plants (those that prefer an acidic soil) include Erica, Rhododendron and nearly all other Ericaceae species, many Betula ( birch), Digitalis ( foxgloves), gorse, and Scots Pine. Calcicole (lime loving) plants include Fraxinus (Ash), Honeysuckle (Lonicera), Buddleia, Cornus spp ( dogwoods), Lilac(Syringa) and Clematis spp.
  • Observation of symptoms that might indicate acidic or alkaline conditions, such as occurrence of the plant diseases mentioned above or salinisation of alkaline soils. The house hydrangea (Hydrangea macrophylla) produces pink flowers at pH values of 6.8 or higher, and blue flowers at pH 6.0 or below.
  • Use of an inexpensive pH testing kit based on barium sulfate in powdered form, where in a small sample of soil is mixed with water 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 rod is inserted into moistened soil and measures the concentration of hydrogen ions.

Increasing soil pH

The aim when attempting to adjust soil acidity is not so much to neutralise the pH as to replace lost cation nutrients, particularly calcium. This can be achieved by adding limestone to the soil, which is available in various forms:

  • Agricultural lime (ground limestone or chalk) is used for soil liming. These natural forms of calcium carbonate are probably the cheapest form of lime for gardening and agricultural use and can be applied at any time of the year. These forms are slow reacting, thus their effect on soil fertility and plant growth is steady and long lasting. Ground lime should be applied to clay and heavy soils at a rate of about 500 to 1,000 g/m² (1 to 2 lb/yd² or 4,500 to 9,000 lb/ac). The concept of "corrected lime potential" to define the degree of base saturation in soils became the basis for procedures now used in soil testing laboratories to determine the "lime requirement" of soils.
  • Quicklime and slaked lime: The former is produced by burning rock limestone in kilns. It is highly caustic and cannot be applied directly to the soil. Quicklime reacts with water to produce slaked, or hydrated, lime, thus quicklime is spread around agricultural land in heaps to absorb rain and atmospheric moisture and form slaked lime, which is then spread on the soil. Quicklime should be applied to heavy clays at a rate of about 400 to 500 g/m² (0.75 to 1 lb/yd² or 3,600 to 4,500 lb/ac), hydrated lime at 250 to 500 g/m² (0.5 to 1 lb/yd²). However, quicklime and hydrated lime are very fast acting and are not suitable for inclusion in an organic system. Their use is prohibited under the standards of both The Soil Association and the Henry Doubleday Research Association.
  • Calcium sulfate (gypsum) cannot be used to amend soil acidity. It is a common myth that gypsum affects soil acidity. However, gypsum does reduce aluminium toxicity. Because gypsum is more soluble than alkaline earth carbonates, it is recommended for the treatment of acidic subsoils.

Reducing Soil pH

See also Soil acidification

  • The pH of an alkaline soil is lowered by adding sulfur, iron sulfates or aluminium sulfate, although these tend to be expensive, and the effects short term.
  • Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulfate and monopotassium phosphate also lower soil pH.
  • Fertilizers often also effect a slightly more acidic soil since this increases uptake of nutrients.
  • Decayed vegetable matter, compost, stable manure, urea, etc.
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