Keywords: acid rain, fertilisers, lime, lime loss, lime requirement, soil acidification.
Acidity and alkalinity in rain, soil or any material are identified by measuring the pH, which has a scale of 0 to 14, with 7 equivalent to neutrality; values less than 7 indicate acidity and those greater than 7 alkalinity. UK agricultural soils usually have a pH in water of between 5 (unlimed mineral soils) and 7.5 (chalky or limestone soils). 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 (SO2), ammonia (NH3) and nitric acid (HNO3). However, acid deposition has been declining: that of sulphur (S) is now <5 kg ha-1 yr-1 and of nitrogen (N) no more than 25 kg ha-1 yr-1. The most important causes of soil acidification on agricultural land are the application of ammonium-based N fertilisers and urea, elemental S fertiliser and the growth of legumes, with crop growth and nutrient uptake and the mineralisation of soil organic matter making a small contribution.
If soils are not buffered by naturally-occurring chalk or limestone, acidification causes the loss of base cations, an increase in aluminium saturation and a decline in crop yields; severe acidification can cause non-reversible clay mineral dissolution and a reduction in CEC, accompanied by structural deterioration. Such weathering will not be reversible except over geological timescales and so represents a serious and costly degradation of soil quality.
Soil acidity is ameliorated by applying lime as chalk or limestone, or other acid-neutralising materials. The amount of lime required to neutralise soil acidity depends on the soil pH and soil type and is generally estimated from lookup tables; estimates of the amounts of lime needed to offset acid deposition, acidifying fertilisers and the growth of legumes are also available and can be up to 1 t ha-1 yr-1. Throughout Europe, each country has its own specifications for liming materials, but those for the UK are prescribed in the UK Fertiliser Regulations 1991. EC Regulation 463/2013 came into effect in 2013, which adds liming materials to the European Fertiliser Regulations so that agricultural lime products can be sold as ‘EC Fertiliser Liming Materials’, but this is not yet widely adopted, nor are measures of reactivity widely used. However, methods to apply lime more precisely with pelletised lime and using Variable Rate Application are gradually being taken up.
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 arable crops and 6.0 (5.3 in peaty soils) for grass. This optimises plant tolerance to acidity and nutrient availability, improves soil structure, helps to restore degraded soils, displaces aluminium and overall increases crop yields and agricultural productivity. The one negative consequence of liming is that the carbon dioxide emitted when lime neutralises acid soil has to be estimated in national greenhouse gas inventories. There are also conflicts between its impacts on productivity and plant biodiversity, especially in upland grassland. When used with other appropriate management practices, however, liming can benefit grassland biodiversity.
Keith W T Goulding, Sustainable Soils and Grassland Systems Department, Rothamsted Research, Harpenden, Hertfordshire, UK.
30 pages, 7 figures, 4 tables, 71 references