As the weather turns cooler, the heating season has already been underway in Latvia for some time. Centralised heat supply is provided using firewood, pellets, wood chips, as well as forestry and wood processing residues. When biomass is burned in specialised equipment or boilers, ash is produced as a by-product of the heat or electricity generated (bottom ash). Emissions of fine particles (fly ash) are 'captured' by filters installed in the furnace chimneys. Wood ash can also be a useful soil amendment.

How is ash produced, and what are its contents and properties?

Logging residues are left in the forest to cover the access paths intended for forest machinery. However, if the soil bearing capacity is sufficient, logging residues can also be used for the production of energy wood chips. Logging residues account for a proportionally smaller biomass of the total wood mass, yet this fraction has a higher concentration of nutrients compared to industrial wood. Wood ash constitutes approximately 1–3% of the burnt biomass mass, depending on the concentration of mineral elements within it and the amount of sand impurities.

Wood ash contains several plant nutrients: macro-elements such as potassium (K) and phosphorus (P), and micro-elements such as calcium (Ca), magnesium (Mg), manganese (Mn), sulphur (S), iron (Fe) and aluminium (Al), among others. The proportion of macro- and micro-elements in the ash depends on the tree species and the fraction of the timber burnt. Ash contains only a small amount of nitrogen (N<0.1%) because it forms gaseous substances during the combustion process. The properties of ash can vary, depending on the type of boiler, temperature, combustion process, the biomass fuel, and subsequently the storage conditions of the ash.

Before spreading wood ash, it is recommended that it be processed (stabilised) with water. Loose wood ash releases nutrients, especially potassium, very quickly. By processing wood ash, the solubility of these elements is reduced. Storing wood ash also changes its properties. In contact with atmospheric moisture and carbon dioxide (CO2), hydroxides, carbonates and bicarbonates are formed. Granulating wood ash also stabilises the pH. In Sweden and Finland, the use of granulated wood ash is recommended for enriching forest soil.

Why is ash necessary for a forest stand?

Wood ash returns nutrients required for tree growth to the forest stand, thereby compensating for the loss of nutrients that may occur as a result of logging. Potassium ensures cellular resistance to frost damage, and its deficiency causes the yellowing of needles from previous years. Calcium plays a role in cell membrane structure, and its concentration in needles increases with the age of the tree. Phosphorus plays an essential role in the physiological processes of trees that consume energy.

The composition and application of wood ash have been studied in the Nordic countries and the United States. It is known that wood ash has a positive effect on volume increment in coniferous stands on organic soils. In trials conducted in Sweden in 30- and 60-year-old pine and spruce stands on mineral soils, a positive combined effect of nitrogen mineral fertilisers and wood ash on growth was observed.

Wood ash is alkaline, and when spread, it raises the pH of the forest floor and, as it leaches, the soil pH as well. Therefore, the properties of wood ash as a liming material have also been studied in the Nordic countries to reduce soil acidification. Wood ash has a more diverse chemical composition than lime, but its liming effect is lower than that of lime.

How does it work in practice?

The use of wood ash for enriching forest soil is influenced not only by its properties but also by the costs of processing, transport and spreading. Although a large proportion of the wood ash currently produced in Latvia is disposed of in landfills, with the confirmation of its positive effect on tree growth patterns and the development of guidelines for the safe application of ash, wood ash may prospectively be used for enriching forest soil with nutrients.

Currently, wood ash that does not have waste status can be obtained from companies that have carried out the necessary analyses and registered it as a liming material, changing its status to 'fertiliser'. By registering ash as a liming agent, the manufacturer determines the Ca and Mg content, the neutralising capacity, and the concentration of heavy metals within it. Using ash with a known composition allows for calculations of the quantity required for soil improvement. The most frequently used dose of wood ash is 2 to 5 tonnes of dry matter per hectare. Both of the most popular forest certification systems in Latvia, PEFC and FSC, support the use of wood ash to improve tree growth conditions.

Returning wood ash to the forest stand is a simple example of the circular economy, demonstrating how all material obtained from nature can be used and recycled.

Useful to know!

At the end of 2020, in the VAAD Fertilisers and Substrates Register database (1) the status of 'fertiliser' had been applied to wood ash produced by a total of 24 companies registered in Latvia. This year, seven companies registered as liming material suppliers, twelve in the previous year, four in 2018, while one company has been offering wood ash for soil liming since 2015.

In accordance with the Cabinet of Ministers regulations No. 506 (2) wood ash qualifies for the status of 'other liming materials'. Wood ash intended for registration must meet specific quality requirements, namely, a neutralising capacity of at least 20%.

Currently, the Latvian State Forest Research Institute (LVMI) Silava is implementing research on the use of wood ash:

1. Research programme for improving tree growth conditions for 2016–2021 - http://www.silava.lv/23/section.aspx/View/181

2. Soil improvement with materials containing biological origin substances - http://www.silava.lv/23/section.aspx/View/250

3. Development of new technologies for the production of plant fertilisers from biogas plant fermentation residues – digestate – and wood chip cogeneration residues – wood ash - http://www.silava.lv/23/section.aspx/View/257

4. Establishing innovative White willow-perennial grass agroforestry systems using mixtures of wood ash and less in-demand peat fractions on improved marginal mineral soils - http://www.silava.lv/23/section.aspx/View/274
1. http://www.vaad.gov.lv/sakums/registri/meslosanas-lidzekli-un-meslosanas-plani.aspx
2. https://likumi.lv/ta/id/276480#piel3