Nutrient and cost optimization of fertilizing strategies for Salix including use of organic waste products

A model was developed to minimize the grower's costs for fertilization of S alix aimed for energy production, when a user-defined amount of organic was te products and commercial fertilizers are available. The fertilizers appli ed have to fulfil the nutrient demand of the crop, while the regulations on maximum heavy metal loading rates must not be exceeded. The model describe d is based on mixed integer programming (MIP) and calculates the optimal am ount of fertilizing products to apply in each year of a four year cutting c ycle. The model is general and may be used for optimization of fertilizatio n 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 munici pal 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 t he nutrient demand of the Salix crop may be met by organic products, withou t exceeding the Swedish limits for maximum heavy metal application rates on agricultural land. A third conclusion was that products with low concentra tions of nutrients, for example biogas sludge, are economically favourable for the grower when the transport distances are short. When the transport d istances increase, more concentrated products such as commercial fertilizer s become more competitive. (C) 1999 Elsevier Science Ltd. All rights reserv ed.