There is a lot of nitrogen contained in chemical compounds in a crop of plants growing in a field. Harvesting removes this nitrogen. In order to grow another crop, fertiliser is added to replace the nitrates and other nutrients which the growing crop removed from the soil. Nitrogen-containing ions are very soluble and can be washed or leached from the soil to accumulate in lakes and rivers.

The breakdown of sewage and waste from cattle and pigs also results in the addition of nitrate to lakes and rivers.

The addition of large amounts of nutrients such as nitrates and phosphates to areas of fresh water results in eutrophication. This is summarised in the flow chart below.