Wood ash is not only a valuable fertiliser in forest ecosystems, it can also benefit agricultural soils, especially acid soil types.

Analysing the impact of wood ash (5 and 201 ha-1) on an Italian agricultural soil regarding its physicochemical, microbiological and biochemical properties, Perucci et al. (2008) observed increasing pH values and electrical conductivity as well as decreasing microbial biomass C in the first months after application, but no long­term effects of ash amendments were found. Enhanced crop production for barley (Hordeum vulgare L.) and canola oil seed (Brassica rapa L.) was monitored when Boralf soils in central Alberta were supplemented with wood ash (12.5 or 25 t ha-1) in combination with N fertiliser (Patterson et al. 2004a). Although wood ash was, moreover, found to positively influence canola seed oil content, it may impair oil quality owing to an increase in the concentration of glucosinolate (Patterson et al. 2004b).

Combining wood ash with N sources is an interesting option for designer com­posts or fertilisers. Admixture of 8 and 16% wood ash and organic wastes prior to composting did have positive effects on the composting process (temperature, microbial activity) and the quality of the final product (no increase in heavy metal concentrations, improved nutrient balance) (Kuba et al. 2008). In comparison with mineral and organic fertilisers, wood-ash-amended compost was superior for the recultivation of a Tyrolean ski run, increasing plant cover and soil microbial biomass and respiration (Kuba et al. 2008) (Fig. 1.2). Composts produced with 8% wood ash admixture fostered utilisation of C sources (polymers, carboxylic and amino acids, alcohol, and carbohydrates) in a MicroResp™ assay and led to a change in microbial community structure, whereas compost with 16% ash altered bacterial and fungal community composition, but did not enhance C utilisation (Bougnom and Insam 2009). Bougnom et al. (2009, 2010) demonstrated that compost produced with wood ash supplement (8 and 16%) may be a cheap alternative to liming in tropical areas, where many soils are characterised by a low pH. Wood ash alone also significantly increased pH and electrical conductivity in a tropical soil in Cameroon and was found to supply nutrients to the soil (Voundi Nkana et al. 2002). Besides raising soil pH, wood ash amendments (4 and 6 t ha-1)

on an acid soil in Nigeria improved maize grain yield (Mbah et al. 2010). As lime or artificial fertilisers are unaffordable for resource-poor farmers, wood ash may be an alternative for improving soil fertility in agricultural soils in the tropics (Voundi Nkana et al. 1998; Bougnom et al. 2010; see Chap. 7, Bougnom et al. 2011). Wood ash application of up to 41 ha-1 on tropical soil in Uganda was observed to enhance bean (Phaseolus vulgaris) and soybean (Glycine max. L) biomass, but led to higher Cu, Zn, Cd and Pb concentrations in edible plant parts (Mbaherekire et al. 2003). Analysing the effect of wood ash application as well as combined wood ash and compost amendments on soil microorganisms, Odlare and Pell (2009) revealed toxic effects of wood ash on potential denitrification in an arable soil on a short­term and a long-term basis. Compost was, however, able to mitigate these heavy — metal-related negative effects of the ash.