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Acidification

The pH of a soil, the method that is used to indicate its degree of acidity, is one of the most important indicators of the suitability of soils for growing crops and a whole variety of natural and semi-natural plants. The pH is a measure of the concentration of hydrogen and hydroxyl ions in the soil solution, and is measured on a sample of soil at a specified water content or soil-water ratio. Soils vary considerably in their degree of acidity or pH and may be acid, neutral or alkaline, depending on the levels of hydrogen (H+) and hydroxyl (OH-) ions in soil solution. At pH 7, the midpoint of the pH range, a soil will have an equal number of hydrogen and hydroxyl ions. At values greater than pH 7 there will be more hydroxyl ions than hydrogen ions and the soil is described as alkaline; at values below pH 7 there are more hydrogen than hydroxyl ions and the soil is acid. The degree of acidity of a soil has a major influence on plant growth, soil organisms and the many chemical reactions that take place in the soil on a daily basis. As the ordinary units of measurement are inconveniently large the negative logarithm is adopted. It means that a soil that is at pH 5 is ten times more acid than one at pH 6, and one at pH 4 one hundred times more acid than that at pH 6.

Most soils have a pH between about 4 and 8. Strongly leached soils of wet climates will generally have a pH below 5.0, most agricultural soils will have a pH between 5.5 and 7.0 and soils of arid climates will usually have a pH above 7.0. In some circumstances, often associated with some form of pollution, the soil pH can fall below 4.0 and in some saline arid conditions it can rise above 8.0. Soils inherit much of their acidity from the parent rock below. The weathering of acidic parent rocks, such as granite, and rhyolite, will give rise to acid soils whereas the weathering of chalk or limestone rock will result in soils with a pH above 7.0, i.e. alkaline. Soils also become acid if they are in a strongly leaching climate, i.e. under high levels of rainfall. The other main soil component that influences the level of soil acidity is the organic matter. There are various mechanisms in the soil organic layer that can contribute to acidification, including respiration of roots and the decomposition of organic matter by soil microorganisms.

Many soils under a wet or moderately wet climate will be subject to leaching by rainfall, which itself is already moderately acid. However, this does not acidify soils as much and as quickly as might be expected because many soils have what is known as a buffering capacity - this is a natural in-built resistance to acidification. The soils with the largest buffering capacity are clay soils and those with good quality organic matter, which are able to store many nutrients on the edge of their particles. Water moving through the soil removes beneficial nutrients absorbed on the edges of the clay particles, leaving behind H+ ions, but the acidic environment also aids the weathering rocks and the further release of nutrients which then slows down the effects of any acidification. The buffering capacity is at its most resilient at a pH 5-0-6.5, the pH range favoured by much of the native flora and most agricultural and garden crops.