A model was developed to minimize the grower's costs for fertilization of Salix aimed for energy production, when a user-defined amount of organic waste products and commercial fertilizers are available. The fertilizers applied have to fulfil the nutrient demand of the crop, while the regulations on maximum heavy metal loading rates must not be exceeded. The model described is based on mixed integer programming and calculates the optimal amount of fertilizing products to apply in each year of a four year cutting cycle. The model is general and may be used for optimization of fertilization strategies for energy crops under a wide range of circumstances. Optimal fertilizing strategies were also studied for some typical cases in order to define the characteristics of the method and to study some general trends in the results. The results showed that the use of untreated municipal waste water was very economically favourable for fertilization of Salix in cases where this product is available and where conditions are suitable for a permanent irrigation system. Another important conclusion was that the nutrient demand of the Salix crop may be met by organic products, without exceeding the Swedish limits for maximum heavy metal application rates on agricultural land. A third conclusion was that products with low concentrations of nutrients, for example biogas sludge, are economically favourable for the grower when the transport distances are short. When the transport distances increase, more concentrated products such as commercial fertilizers become more competitive.
