SMART (Simulation Model for Acidification's Regional Trends) is a soil acidification model developed to estimate the long term chemical changes in soil and soil water in response to changes in atmospheric deposition. This model simulates the concentrations of Al, divalent base cations (Ca + Mg), monovalent base cations (Na + K), NH4, SO4 and NO3 in the soil solution. In SMART most of the geochemical processes are included (weathering, cation exchange, sulphate absorption). However, only a very limited number of biological processes are taken into account.
Nutrient cycling processes are not included because the model is based on the assumption that the amount of organic matter is in an equilibrium state. However, net N-immobilisation is taken into account to include the effect of an increase in the N content in organic matter as a result of high N deposition.
Cation exchange, sulphate adsorption, dissolution of carbonates and Al hydroxides are treated as equilibrium reactions. Weathering of base cations and denitrification/nitrification are described as first-order reactions.