Soil acidification and the importance of liming agricultural soils with particular reference to the United Kingdom

Abstract

Soil acidification is caused by a number of factors including acidic precipitation and the deposition from the atmosphere of acidifying gases or particles, such as sulphur dioxide, ammonia and nitric acid. The most important causes of soil acidification on agricultural land, however, are the application of ammonium-based fertilizers and urea, elemental S fertilizer and the growth of legumes. Acidification causes the loss of base cations, an increase in aluminium saturation and a decline in crop yields; severe acidification can cause nonreversible clay mineral dissolution and a reduction in cation exchange capacity, accompanied by structural deterioration. Soil acidity is ameliorated by applying lime or other acid-neutralizing materials. 'Liming' also reduces N₂O emissions, but this is more than offset by CO ₂ emissions from the lime as it neutralizes acidity. Because crop plants vary in their tolerance to acidity and plant nutrients have different optimal pH ranges, target soil pH values in the UK are set at 6.5 (5.8 in peaty soils) for cropped land and 6.0 (5.3 in peaty soils) for grassland. Agricultural lime products can be sold as 'EC Fertiliser Liming Materials' but, although vital for soil quality and agricultural production, liming tends to be strongly influenced by the economics of farming. Consequently, much less lime is being applied in the UK than required, and many arable and grassland soils are below optimum pH.

Keywords: Acid deposition; fertilizer; lime requirement; liming; soil acidification.

Figure 1

Examples of three of the buffering mechanisms in soils: carbonate/bicarbonate buffer in the soil under Broadbalk Wilderness, where acid deposition has been buffered by very large quantities of chalk applied in the 19th century; cation exchange buffer in the Park Grass unlimed ‘Nil’ treatment (no fertilizer or manure applied so acidification only from acid deposition); aluminium/manganese buffer in the Park Grass unlimed, ammonium sulphate fertilizer treatment that has experienced severe acidification.

Figure 2

Soil pH measured in samples taken from experimental treatments of the Long‐Term Liming experiments at Rothamsted and Woburn. Key: Rothamsted: 0 (X), 5 (■), 10 (▲) and 20 (♦) t/ha lime; (b) Woburn: 0 (X), 4.6 (■), 10.9 (▲) and 17.3 (♦) t/ha lime. The vertical arrows (↑) point to years when lime was applied.

Figure 3

Variable‐rate lime recommenations for fields of the ‘North Wyke Farm Platform’, a farm scale experiment at North Wyke, Devon, UK, at which basic operations and measurements are made and other experiments superimposed. (Robert Orr, private Communication).

Figure 4

The pH status of >200 000 UK arable and grassland soils as measured by the Professional Agriculture Analysis Group in the 2013/14 season (PAAG, 2014). The dashed vertical line marks the soil pH recommended in the Fertiliser Manual (RB209; Defra, 2010).