Phosphorus modelling on agricultural fields trade-offs between applicability and complexity

Besides nitrogen, phosphorus is the second most important nutrient for plants, but has only little natural inputs. If it is not added regularly by fertilizers, be it mineral or organic, the soil gets depleted after a few years. As a consequence, crop growth is impeded as well as the microbial turnover of soil organic matter. However, many crop growth models do not account for phosphorus dynamics, although it might be a limiting factor.In a mechanistic soil process model such as BODIUM, crops are growing dynamically adapting to their boundary conditions and affect many other soil functions due to water and nutrient uptake, root exudation and biomass input to the soil. Moreover, microbial metabolism depends on the stoichiometry of soil organic matter and available nutrients, i.e. C/N/P ratios. A phosphorus component shall therefore enhance the nutrient cycle and improve the crop growth prediction, both in regard to yields as well as to feedbacks with other soil processes.Here, we want to present an extended BODIUM version including the representation of the P cycle and present first simulations with data from the Static Fertilization Experiment in Bad Lauchstdt. Beside stoichiometric considerations for all organic pools, the mineral dynamics are represented with only total and available P. These can easily be measured by standard procedures and are now mandatory in some countries, although with varying protocols. This is especially relevant for farmers, who are a declared target group of the model.