Nitrogen demand by each plant works by the potential-actual method . The shape of the S curve is determined by plant parameters . Next the N demand is reduced to account for the amount of nitrate in the layer and the amount of water taken up from the layer. Because the plant takes up most of its nitrogen passively in the water solution, N uptake is limited by water uptake.
Nitrogen fixation is simulated simply as a fraction of the daily nitrogen demand.
Any nitrogen fixation reduces the plant's N demand, though it cannot exceed it. This means that nitrogen-fixing plants cannot add nitrogen to the soil. To start the calculation of phosphorus demand, the optimal P uptake is calculated in the same way as the optimal N uptake. Because phosphorus is more closely associated with the mineral phase than is nitrogen, the supply of P available to the plant in each soil layer depends not on water uptake but on the root biomass in each layer and on the amount of labile P available. The plant's P uptake in response to the amount of labile P available is determined by an S curve .