What soil constraints should be included in crop and forest models?

Models of agro-ecosystems provide means of prediction beyond the bounds of experience or experimentation. Extrapolation may involve simulation of the expected behaviour for different climate and/or soil conditions, perhaps with alternative management, or for a longer time scale than observed experimentally. The issues to be addressed include those concerned with crop growth and profitability, effects on the soil resource, and consequences for the environment. The extent to which the model output is credible depends on how well the model represents those aspects of the system that are important for a particular application. In principle, any model can be enhanced to deal with additional factors, but more complex models become increasingly hungry for inputs, and increasingly difficult to test against actual data. There are judgements to be made as to whether the demands of model building/testing can be justified. The choice of what soil constraints should be included in models will depend on what the model is to be used for and the understanding of the processes that the model purports to represent. The dynamics of water and nitrogen are the soil features commonly represented in crop/forest models. Based particularly on experiences with the APSIM modelling framework, the paper reports developments aimed at dealing with erosion, soil structure, salinisation, acidification, and phosphorus as limitations to plant growth. Modelling the consequences of global change on crops and forests involves issues of productivity, climatic variability, “greenhouse” effects and sustainability. Discussion focuses on whether current models need to be enhanced by including other constraints in order to address such issues.