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Ashes to forest and soil

Ash use
Ashes to forest and soil

Logging residues (i.e. small trees, branches and tree tops) are suitable as biofuel. However, when these particularly nutrient-rich tree parts are harvested, important elements are lost from the forest ecosystem. Therefore in 1998, the Swedish Forest Agency starts to recommend that wood ash should be recycled to forest land from which logging residues have been or will be harvested.

Sweden has a rather new forest soil from the last icetime 10 000 years agor. The content of minerals are rather high compared with other parts of Europe. But the buffering capacity is low and acid rains has made problems with low pH for lakes and streams. The forests can stand the low pH. For a longtime sustainable forestry the minerals must be returned. But for the moment it is more important to return basic ions to prevent the acidification of the water streams and lakes. The conclusion of Sv EnergiAskor is that it is important to return ashes to forests but for the time beeing it more important to now create systems for the return of ashes than to return the ashes to exacly the place from their origin. That means that you can return ashes to the soils where the economical benenfit is as high as possible. The availble nitrogen is normally is the critical nutrient the growth rate for trees in Sweden. (Maybe it can also be the available P in SV Sweden). At mineral soils the high pH of ashes releases some of the mineralogical N which increases the growth rate. For mineral soil with a low growth rate it looks as the ashes make N a little less available so there is a tendency that the growth rate is lowered. If that is just for the fist years or not is not looked at. The nitrogen is there so it can be that there isn´t a lower growth rate for a longer  time.

For organic soils as peatlands there is a heavy deficiency of  minerals. There ashes will give an economical increase of the growthrate. One problem is that it is difficult to work with modern heavy harvesting equipment on unfrozen peatlands.

Wood ash should meet three basic requirements:

• It should contain all the macronutrients present in the logging residues (except nitrogen, which is eliminated during combustion).

• It should not cause an accumulation of metals or any other harmful substance in the soil. For this reason, the ash should originate from the combustion of non-contaminated wood only.

• It should not cause direct harm when spread. To be suitable for application, the ash must first
be processed in order to reduce the reactivity and solubility. Unprocessed ash is too reactive to
be spread, as it can ‘burn’ natural vegetation, especially mosses. If the ash is not processed, a rapid and harmful change in pH may also occur in nearby streams.

There are currently three main methods for processing wood ash:

• Self-hardening and crushing

• Compaction

• Granulation

In all three processes, the ash is first wet with water to initiate chemical hardening, but the amount of water required and the procedures that follow vary between the processes. When the ash has been properly processed, it can be spread either by ground spreader or helicopter.
However, ground application is considerably more common due to the lower cost. Wood ash application should be thoroughly planned in advance. Suitable sites for spreading include thinned stands where the ash spreader can use existing access roads and clear-felled areas with established ground vegetation. The suitable ash dose must also be determined. The maximum dose allowed in Sweden is 3 tonnes of wood ash (dry weight) per hectare, while lower doses are recommended for less fertile soils. However, in Finland higher doses are used to increase tree growth on drained peatland.




CR 100924


More information about ashes to forest and soil, manuals, reports, and Svenska EnergiAskors thoughts:

PM: Ashes to forest and soil


Experiments with ash to enhance tree growth on drained peatlands.


Distrubution of ash in thinning forests.


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