Holm, B. and Heinsoo, K.* 2014. Biogas Digestate Suitability for the Fertilisation of Young Salix Plants. Baltic Forestry 20(2): 263-271

We analysed the possibilities to use the digestate from pig slurry anaerobic digestion as a nutrient supply for Short Rotation Coppice Salix species. Our greenhouse experiment revealed that the total biomass (roots, cutting and shoots) of one-year-old Salix plants treated with moderate amounts (hereafter 0.5F) of digestate during the vegetative period was more than twice that of control (hereafter C) plants and this difference increased in time. After the first vegetative period both shoot and root biomass production of 0.5F plants were also significantly higher than those of plants supplied with mineral NPK fertiliser at optimal N load (hereafter MIN). Although the digestate application at optimal N load (hereafter F) harmed the plants by damaging the roots during the first vegetative period, they were able to recover and annual production was comparable with C plants during the following year. The MIN plants supplied with the same N load as F plants died back during winter despite the larger supply of K. The largest load (hereafter 2F) of digestate caused serious problems and was lethal to one-year-old plants. By the end of the experiment the pH of the growing substrates treated with digestate or mineral fertiliser was lower than in control pots. Less ammonium N was available but the K and P content increased in growing substrates following digestate application. Our results reveal that digestate can be an alternative fertiliser in SRC and the nutrients from digestate are more available for Salix plants than those from mineral fertiliser.

Key words: biogas digestate, biomass, fertilization, Salix, shoot growth rate