Water can leave the land through a process called leaching, and this is where the water moves directly down through the soil profile and into groundwater.
And that’s probably the largest way that nitrogen will leave a farm is when you have got water in excess of what is a called the field capacity of the soil at that site, which is the amount of water that a soil can normally hold without leaching – basically, water starting to move down though it.
As that water moves, it can carry different chemicals with it as well and in particular carries negatively charged chemicals with it. Positively charged chemicals are held by the soil because soil is slightly negatively charged. One of the important forms of nitrogen is called nitrate, and that is negatively charged and so is repelled by soil, and you get that moving down with the water to groundwater and then onto surface waters like streams.
Professor Louis Schipper, University of Waikato