Wood Ash: Transforming a Waste Product into a Sustainable Soil Improver

Supervisors: Professor Graeme Paton, Dr Luke Beesley (James Hutton Institute), Dr Gareth Norton and Dr Rupert Hough (James Hutton Institute)

Background: The growth of the renewable energy sector has led to the development and installation of efficient biomass boilers, the feed stock for which is normally virgin biomass. In some instances wood from other sources is also used, i.e. waste wood. Woody biomass is a renewable source of energy, and as such facilities to generate heat from its use are eligible for government schemes and grants. The by-product of this form of energy generation is wood ash which has a number of very useful applications in agriculture. One of the main benefits is the effect wood ash has on increasing soil pH, therefore the ash can be applied as a useful soil conditioner. Wood ash is also a source of both macro and micronutrients. Therefore wood ash could be used as a sustainable alternative to liming and enable more strategic management of calcium/ magnesium ratios at the field scale. As biomass boilers continue to increase in popularity, there is a need to find a sustainable use for the associated end-product.

To date the majority of studies have focussed primarily on ash derived from virgin timber. In terms of sustainability, waste wood may provide an alternative feed stock that could reduce life cycle GHG emissions and energy inputs. There is also a need to effectively reduce the volume of this waste stream currently going to landfill. However, waste wood can have significant concentrations of potentially toxic substances associated with paints, preservatives and glues that may be undesirable in the agricultural food-chain. It may be that ash from waste wood could be blended with ash from virgin timber to produce a ‘risk permissible” soil amendment; some elements in ash from waste wood sources (ie Cu and Zn) may provide useful supplementary additions of nutrients to impoverish intensively cultivated soils. It is essential to comprehensively evaluate the nature of ash associated with waste wood feedstocks in relation to ash from virgin timber for use in these applications.

Hypothesis - sustainable application of mixed source wood ash will serve to reduce bio-energy waste products and enhance agricultural soil performance.

Project: The major aim of the project will be to assess the potential use of wood ash, from both virgin and waste wood, as a suitable amendment to agricultural soil for increasing pH and macro and micronutrient availability.

The research will focus on the effect the application of ash has to the soil chemical (pH, micronutrient availability and CEC) physical (bulk density and water retention, infiltration) and biological properties (microbial biomass / community composition, soil respiration) as well as the impact it has on crops (yield and macro and trace element composition). Ash will also be assessed for potentially toxic substances and their transfer to the edible portion of crops.

Experiments will be conducted using a range of UK crops including wheat, barley and potatoes and grass used for livestock grazing. Experiments will initially be conducted at pot scale at University of Aberdeen glasshouse/environmental controlled facilities and then up-scaled intelligently to field plot trials at the James Hutton Institutes experimental farm(s).

The final stage of the work will be to model the impact the application of ash as a soil amendment on crop quality. The aim of the modelling will be to develop a tool to predict micro-nutrient value (and potentially yield of crops) from empirical chemical analysis of the ash product and the receiving soil. This modelling would include both beneficial as well as potentially toxic elements, and thus indicate under which circumstances different ash types (virgin vs. waste wood vs. mixed ash product) could be safely applied to soils singly or repeatedly.
Research training value of project: Skills to be developed for the student include: practical glass house and laboratory skills especially in the use of analytical chemistry techniques such as ICP-MS and MP-AES, field trials, mathematical modelling, quantitative risk assessment; more generic transferable skills such as presentation and writing, teaching and mentoring, and stakeholder interaction.

The student will also benefit from being closely involved with a number of producers of renewable energy both large and small-scale. This will enable the student to gain insight into the industry including gaining a detailed understanding of feedstock and by-product (ash) handling at the different facilities.