Keywords: Liming, Acid Rain, Fertilisers, Lime, Lime loss, Lime requirement, Soil acidification.
Soil acidification resulting from acid deposition, microbial processes and root exudation is a natural process. However in the last 150 years, natural acidifying processes have been enhanced by man-made emissions, and agricultural land has received extra acidifying inputs from ammonium-based fertilisers and the greater utilisation of legumes. Whilst sulphur compounds were the main anthropogenic component of acid rain 20 years ago, nitrogen compounds now dominate. Current levels of air pollution mean that approximately 500 kg lime (CaCO3) /ha/yr are now required to prevent soil acidification merely from acid deposition, with an additional 500 kg/ha/yr for every 50 kg/ha/yr ammonium-N fertiliser applied, and 500 kg/ha/yr to alleviate acidity from N fixation by clover-based systems that typically supply about 300 kg N/ha/yr.
There is evidence of a decrease in the amounts of lime being applied to all farmland, and of a decrease in soil pH under grassland. If acidification is not prevented, crop yields will decline because of nutrient deficiencies and aluminium and manganese toxicities; also legumes may fail to nodulate. Soil degradation can occur at very acid (<4) soil pH values, with the release of aluminium and potentially toxic metals into the soil waters and their uptake by plants. Farmers and land managers must be convinced of the continuing need to apply lime to maintain optimum soil pH levels.
The lime requirement of land in England and Wales is now determined from look-up tables based on soil texture and pH; the use of chemical buffer methods has been discontinued. Models for calculating lime requirements have been developed and should be incorporated into the new Decision Support Systems being developed for farming. Lime standards are regulated through the Fertiliser Regulations and assessed by Neutralising Value and Sieve Analysis (particle size). It has not been possible to agree a European standard for grades of lime, however. The effectiveness of lime in the field is determined by the above, but also by hardness. Several measures of reactivity are being assessed for their ability to measure the effectiveness of lime in the field. Liming in the UK has evolved as a low-cost, low-technology operation but, despite the cost implications, new technologies and the Environmental Protection Act require a change to precision farming methods with lime viewed as a fertiliser.
Dr K.W.T. Goulding, Soil Science Department, IACR-Rothamsted, Harpenden, Herts. AL5 2JQ
B. Annis, OBE, NDA, Needham Chalks Ltd., Needham Market, Suffolk. IP6 8EL
36 pages, 5 figures, 9 tables, 49 refs.