Agriculture contributes negative and positive externalities to society, that is, beneficial and detrimental changes in human wellbeing to third parties for which they are not generally compensated or charged. Beneficial externalities include the creation of amenity and landscape and negative externalities include pollution of surface and groundwater. In so far as parts of agricultural subsidies compensate for beneficial externalities, they are said to be 'internalised' and should not be the subject of further policy measures. However, agricultural subsidies also add to the negative externalities by expanding output and encouraging environmentally detrimental farming practices. Comprehensive attempts to value these externalities in the UK and to compare them to the true value added of the agricultural sector are to be found in Hartridge and Pearce (2002) and Pretty et al. (2000). A particular feature of the negative externalities is the damage done by nutrient pollution and by pesticides. Nutrient pollution refers to water pollution mainly from nitrates and phosphorus, concentrations being elevated by leaching from soils of fertilisers and animal manure and slurry. A similar leaching process occurs with pesticides. Significant repositories for these leached pollutants are surface waters and groundwater.
