Soil pH

Soil pH is an indication of the alkalinity or acidity of soil. It is based on the measurement of pH, which is based in turn on the activity of hydrogen ions (H⁺) in a water or salt solution.

When in balance (pH 7) the soil is said to be neutral. The pH scale covers a continuum ranging from 0 (very acidic) to 14 (very alkaline or basic). It is however uncommon to find soils at either extreme of this range. Under many conditions soils tend to become more acid or alkaline over time if steps are not taken to maintain a balance.

Soil pH is an important consideration for farmers and gardeners for several reasons, including the fact that many plants and soil life forms prefer either acid or alkaline conditions, that some diseases tend to thrive when the soil is alkaline or acidic, and that 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. Some plants however prefer more acidic (e.g., potatoes, strawberries) or alkaline (brassicas) conditions.

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	Acid					Neutral			Alkali
	4	4.5	5	5.5	6	6.5	7	7.5	8	8.5	9	9.5	10
nitrogen, N
phosphorus, P
potassium, K
calcium, Ca
magnesium, Mg
sulfur, S
iron, Fe
manganese, Mn
boron, B
copper, Cu
zinc, Zn
molybdenum, Mo

The above table gives a guide to the availability of several nutrients at various pH values

During the acidification process the decrease in pH result in a release of positively charged ions (cations) from the cation exchange surfaces (organic matter and clay minerals). In the short term acidification thus increases the concentration of potassium (K), magnesium (Mg), and calcium (Ca) in soil solution. 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 wetted), 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. The term plant available nutrients usually include pools other than soil solution but which are supposed to replenish soil solution pretty fast e.g. through cation exchange. One reason for including such pools is the plants capability of releasing organic acids which increase the total soil solution concentration of some cation nutrients that are important for the plant.

It is thus important to realise 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 increase 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 sulphate, nitrate and to some extent ammonia decrease pH of soil solution essentially without affecting Ca, Mg and K concentrations whereas deposition of seasalt 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 defficiency 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 7.0 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.

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 helds in colloidal soil reserves, leading to soil acidity. Clay soils often contain Fe and hydroxy Al, 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 or Sulfur may be added to reclaim the soil.

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 a healthy, living soil. Earthworms, whose feeding and tunnelling activities aerate the soil and speed the decay of organic matter, immeasurably benefitting the soil, also prefer these 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[]

pH is not constant in soil or water, but varies on a seasonal or even daily basis due to factors such as rainfall, biological growth within the soil, and temperature changes. Rather, 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 varys 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.

Methods of determining pH include:

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, wherein 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 acid 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.

Altering 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). These are natural forms of calcium carbonate which are extracted in the UK from areas such as the Mendips and Salisbury Plain. This is 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).
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, known as gypsum can be used to amend soil acidity and is also useful for lightening the structure of heavy clays. Gypsum can be purchased or can sometimes be obtained from old domestic plaster.

The pH of an alkaline soil is lowered by adding sulfur, iron sulfate or aluminum 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.