Excessive nitrogen surpluses, the difference between inputs and removals by crops, can pose a threat to the environment, leading to pollution of water, air and soil. The integration of environmental concerns into agricultural policies and practices aims to reduce current and potential pollution. The monitoring of nitrogen surpluses from agriculture can be a useful tool to highlight those areas vulnerable to nutrient pollution. Fertile soils are rich in nutrients, essential components which play a key role in plant metabolism and growth. Crops take the nutrients they need from the soil, with the result that soils can become impoverished if there is no process to replace these nutrients.
Impoverished soils reduce crop yields, and ultimately the economic viability of the farm itself.
Traditionally, rotation of crops and regular fallow periods, together with spreading of animal manure, allowed the land to recover some of its fertility, but today the main method used to restore nutrients to soil and to increase crop yields is the application of mineral fertilizers. Nitrogen in commercial fertilizer is particularly soluble, to facilitate uptake by crops, and ease of storage and handling means that it can be easily applied at the times when plants most need it. Mineral fertilizer is now the major source of nutrients applied to soil in EU countries, although input of animal manure remains important, especially in regions of high livestock density. However excessive application of nutrients can pose a threat to the environment and in extreme cases to the fertility of the soil itself. Losses to the environment can be minimised if a reasoned fertilization is used, together with 'sustainable' agricultural practices, such as crop rotation, planting cover crops, and ploughing in crop residues.