Usually there are vertical flows of water in the soil profile. Nitrate is easily soluble in water and is therefore transported between different layers as long as there is a driving force by water for transportation. Ammonium can either be immobile or mobile, as determined by the switch Ammonium mobility. The vertical nitrate flow from one layer to another is calculated as:
where qw is the water flow, θ(z) is the soil moisture content and ∆z is the depth of the soil layer.
Since the ammonium ions are rather immobile, the vertical flow of ammonium is calculated slightly differently:
where fN,ads is the adsorption fraction.