Release of mineral nutrients and heavy metals from wood and peat ash fertilizers: Field studies in Finnish forest soils

Two field trials were conducted to study the rate of release of nutrients and heavy metals from wood and peat ash fertilizers in forest soils. In the first experiment, ash particle residues were extracted from peat and mineral soils 3 and 5 years after spreading and analysed for changes in element concentrations. Dissolution of different ash fertilizer products was also estimated by measuring the changes over time in the recovery of added nutrients and heavy metals in the surface of two peat soils. The results showed that potassium (K), sodium (Na), boron and sulfur are easily released from wood ash, whereas heavy metals are highly insoluble in all types of ash fertilizer products. Granulated ash fertilizers were less soluble than powdered fertilizers and the products stabilized by self-hardening. Minor differences occurred in ash fertilizer dissolution between peat and mineral soils and soils of different fertility. The most significant difference between wood and peat ash concerning their dissolution was the low solubility of K and Na in peat ash. Given the very slow dissolution of ash fertilizers, long-term studies are needed to understand fully the effect of ash fertilization on nutrient and heavy metal cycling in forest ecosystems.