Use of wood ash and anaerobic sludge for grassland fertilization: Effects on plants and microbes

Increasing the share of renewable energies is among the prime policies of the European Union. Among the options are the incineration of wood in combined heat and power plants as well as the anaerobic digestion of organic materials to obtain biogas. Both processes leave behind residual matter – wood ash and anaerobic sludge. These residual materials contain chemical elements with considerable fertilizer value, but can also contain heavy metals, xenobiotics and may emit odorous components. Wood ash is often deposited in waste dumps at considerable expense, and the increasing centralization of biogas plants results in problems for the distribution of the residual sludge, with not enough agricultural land being in the vicinity of the biogas plants. Wood ash has traditionally been used as fertilizer. Some studies have shown that grate (bottom) ashes may be used as fertilizer in agriculture and forestry (FRITZE et al., 2000). The ability of ashes to increase soil pH by oxides, hydroxides and carbonates of K, Mg and Ca is an advantage for the treatment of acidic soils (HOLZNER and OBERNBERGER, 1998; HOLZNER, 1999). DEMEYER et al. (2001) found a positive effect of ashes on soil texture, aeration, water holding capacity and cation exchange capacity. The gener-