Use of Wood Ash at Hoddom and Kinmount Estate
11 March 2025At Hoddom and Kinmount Estate, the farm manager James Marshall has recently been applying wood fly ash to some of the land with an aim of reducing the use of inorganic fertiliser and lime.
What is wood fly ash?
The wood fly ash being applied is ‘waste’ from a biomass power plant in Northwest England that the Estate has previously provided willow to. Wood fly ash consists of fine particles of ash which rise with the flue gases following combustion of untreated wood. Although fly ash typically has little or no remaining organic matter, it does contain valuable plant nutrients. It has a good liming value and generally contains significant amounts of phosphate (P), potash (K) and magnesium (Mg), and useful concentrations of trace elements such as zinc.
Wood fly ash is regulated as waste therefore to be applied to agricultural land it must be applied under a Waste Management Licence Exemption (Paragraph 7) which is authorised by SEPA. Such authorisation comes with a set of rules that must be adhered to and includes regular soil and ash testing, as well as conditions imposed on its use. If pH, phosphate and potash levels are too high then wood fly ash cannot be applied.
Application
James started to apply wood fly ash last September to a block of older permanent pasture that had historically received little organic manure. The grassland had a low pH and had low levels of phosphate and potash. A greening effect is now visible on these fields. Over the past couple of months wood fly ash has been applied to a further three grassland blocks, which were also low in pH, phosphate and potash.
Prior to the application of the wood fly ash, nutrient budgeting was carried out thereby allowing James to obtain accurate lime, phosphate and potash requirements for the grassland. The ash has been applied to select fields based on these requirements. The balance of the nutrients required will either be met by the use of digestate or inorganic fertiliser.
Impact
By using wood fly ash, it has been calculated that James has been able to reduce the amount of inorganic phosphate applied by 5.7 tonnes, amount of inorganic potash by 3.8 tonnes, and the amount of lime by 281 tonnes. At current costs that is a saving of around £10k and a reduction in carbon dioxide equivalents (CO2e) of 132 tonnes.
Following the application of the wood fly ash there is expected to be improved grass growth and yields. This will be monitored over the growing season.
Going forward James will continue to analyse the soils to ensure indices are suitable for continued applications of wood fly ash, if the pH or P and K indices are too high, wood fly ash will not be applied.
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