Soil pH

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, because the solubility of most nutrients necessary for healthy plant growth is highest at pH 6.3-6.8. Some plants however prefer more acidic (e.g., potatoes, strawberries) or alkaline (brassicas) conditions.

When the pH falls below 5.5, most major plant-nutrient minerals (those needed in substantial quantities to promote healthy plant growth include nitrogen (N), phosphorus (P), potassium (K), sulfur (S), magnesium (Mg), and calcium (Ca)) and some micronutrients (elements important to plant growth in very small amounts) become insoluble and hence unavailable for uptake by plant roots.

Many cationic (positively charged) nutrients 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.

This table indicates the availability of several nutrients at various pH values:

	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

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 fertilisers, 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 (heath), 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