Acid Treatment of Animal Slurries: Potential and Limitations

Keywords: Gaseous emissions, ammonia, sulphuric acid, slurry fertilising value, soil quality and productivity, nitrogen, phosphorus, sulphur.

The application of animal manure to soil to supply nutrients to plants has received less attention with the increase in the use of mineral fertilisers. Nevertheless, the increased price of mineral fertilisers and the need to recycle animal manure has increased the interest of farmers in using animal manure as organic fertiliser. Nevertheless, the application to soil of animal manure, particularly slurry (liquid manure), has resulted in significant emissions of ammonia (NH₃) that impact negatively on public health and the environment and represent a loss of available nitrogen for plants.

Several solutions have been presented to minimise ammonia emissions, with acidification appearing as an obvious solution. Nevertheless, it is effectively only in Denmark that slurry acidification is being used at the farm scale. One of the reasons for such limited uptake is the lack of information about the impact of acidified slurry application on the soil, with most of the studies having been carried out in northern Europe where soils and climate are quite different from those in southern Europe.

In this paper, an overview of the potential for, and limitations of, acid treatment of animal slurries is presented. A compilation and review of the most relevant studies has been performed in order to fully understand the global impact of slurry acidification and assess the potential of acidified slurry application to soil as a Best Available Technique.

Briefly, acidification of slurry decreases the pH and also affects slurry characteristics, particularly in terms of nitrogen, phosphorus, micro elements and carbon solubility, and in plant availability. Emissions of other gases such as methane or hydrogen sulphide seems to be also positively affected by acidification even if not all studies are in agreement. Similarly, the effect of the application to soil of acidified slurry on crop production and diffuse pollution has been considered. In most cases, acidification of slurry led to increasing crop yields but it could also increase the risk of nitrate leaching. Repeated application of slurry acidified with sulphuric acid (most commonly used) led to a relatively low decrease of soil pH depending on soil characteristics namely its buffer capacity and organic matter content. Furthermore, a significant increase of electrical conductivity and sulphur content was observed in the same soils.

Acidification of slurry appears to be a promising treatment to improve the fertiliser value of slurry but more work still needs to be done to fully assess the long term effects on soil quality and to consider alternatives to the use of sulphuric acid as the acidifying agent.

David Fangueiro^a, Sonia Surgy^a, Irene Fraga^b, Ernesto Vasconcelos^a and João Coutinho^c.

^aLEAF, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal.

^bCITAB, University of Trás-os-Montes e Alto Douro, UTAD, Quinta dos Prados, 5000-801 Vila Real, Portugal.

^cChemistry Centre, University of Trás-os-Montes e Alto Douro, UTAD, Quinta dos Prados, 5000-801 Vila Real, Portugal.

24 pages, 7 figures, 4 tables, 39 references