Impact of high-volume wood-fired boiler ash amendment on soil properties and nutrients

Forest application of boiler ash is fast becoming a popular alternative to landfilling. Boiler ash is a good source of calcium (Ca), magnesium (Mg), potassium (K), phosphorus (P), manganese (Mn), and zinc (Zn), but it may potentially increase soil pH and electrical conductivity. A monitoring study was conducted to determine the changes in soil properties and the availability and leachability of nutrients following the application of large quantities of boiler ash in a sandy soil (with a spodic horizon). Two application rates (900 and 1800 Mg ha− 1) and two application methods (surface and subsurface) were used in a three-hectare area, which was divided into 15 subplots. Soil samples from different depths were collected over a period of 21 months. Soil pH increased from 5.6 to above 9 and the electrical conductivity increased by up to 2 orders in all plots compared to the controls. The high alkalinity from the ash in the 1800 Mg ha− 1 rate depleted the spodic layer, and this was more pronounced in the subsurface treatments. Plant-available water was doubled to 12% in the 1800 Mg ha− 1 treatment and soil bulk density was reduced from 1500 kg m− 3 to 1200–1360 kg m− 3. Total and plant available macronutrients (Ca, Mg, K, and P) and micronutrients [iron (Fe), Mn, copper (Cu), and Zn)] increased substantially after ash application. Based on our study, it is highly recommended that ash application rates be lowered to agronomic rates, e.g., ∼10 Mg ha− 1, based on liming equivalence, to maximize the beneficial effects of boiler ash on soil.