16.5.1  Soil acidification

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As soils acidify, plant growth generally declines and there is a decrease in the range of plant species that may be grown. The activity of soil fauna and flora decreases also. The extent of acid soils and the degree to which plant growth is reduced by acid soil factors make these soils of considerable ecological and economic importance.

Soil pH (negative logarithm of the molar activity of hydrogen ions) has been used to define acid soils. Since pH is measured on a logarithmic scale, it is important to realise that a soil with pH 4 is 10 times more acid than a soil with pH 5! Acid soils may be classified as those soils with a pH of less than 7 (i.e. neutral pH). However, this has little value in practice, since most plant species will only show reduced growth at substantially lower pH. Thus, soils with a pH of less than 5.5 may be regarded as acid (i.e. those in which growth of many plant species is adversely affected).

Acid soils occupy about 30% of global land surfaces, and predominate in two major regions of the world: humid temperate forests and humid tropics and subtropics (von Uexküll and Mutert 1995). However, many soils in other areas are acid also. Tropical acid soils alone comprise approximately two billion hectares, or 14%, of the total ice-free area of the world. Approximately 40% of the world’s arable soils (i.e. those on which crops may be grown) are acid.

Eight regions of Australia have been identified in which agricultural production is reduced by acid soils (Figure 16.15). In southern Australia, these include the wheat and sheep belt of Western Australia, the southern portion of South Australia, cropping and grazing lands of southeastern South Australia along with the western, central and northeastern districts of Victoria. There are also about one million hectares of highly acidic soils in Tasmania. Further north in New South Wales, the cropping and grazing regions of the Riverina and southwestern slopes contain many acid soils, as do the tablelands and wet sub-tropical coastal farming regions of New South Wales and Queensland. Soils of the tropical wet coast of Queensland and adjacent intensively cropped and grazed areas are also acid to a considerable extent.


Figure 16.15 Eight areas around mainland Australia numbered to match major biogeographic regions, where soil acidification has serious implications for reduced growth of crops and pastures. All areas commonly receive sufficient rainfall for nitrate leaching, and include poorly buffered soils with a long agricultural history. (Based on Land and Water Resources Research and Development Corporation 1995)

Overall, acid soils are most common where rainfall exceeds 450 mm year–1 and more than 80 million hectares of the most productive agricultural land in Australia are acidic, with more than 40% of this land being highly acidic (Land and Water Resources Research and Development Corporation 1995). Acid sulphate soils of coastal areas may also acidify when drained for crop production or urban use, with consequent effects on estuarine aquatic life. This most noticeably results in fish kills, often following rains after a dry period during which oxidation of soil S has occurred.


Table 16.8

Acid soil infertility in Australia is now recognised as an insidious and invisible form of land degradation. Worldwide, soil nutrient decline and acidification are often the most easily visible forms of land degradation. A landmark Australian study showed that soils under subterranean clover pasture acidify, with a decrease from a soil pH of around 6.0 in virgin soils to pH 5.2 where pasture had been grown for more than 30 years (Table 16.8). Subsequent studies have shown that the acid addition rate over extensive areas of New South Wales ranges from near zero to 3–5 kmol H+ ha–1 year–1, with up to 20 kmol H+ ha–1 year–1 in some exploitative systems (Helyar et al. 1990). With an acid addition rate of 4 kmol H+ ha–1 year–1, there would be a decrease of about 1 pH unit in the surface 30 cm of a sandy loam within 30 years. This would take about 120 years in more strongly buffered clay soils.